Understanding infrared thermography reading 3 part 2 of 2

Infrared Thermal TestingReading III- SGuide-IRT Part 2 of 2My ASNT Level III Pre-Exam Preparatory Self Study Notes 29th May 2015

Charlie Chong/ Fion Zhang

Infrared applications


Infrared applications



Fion Zhang at Shanghai1st June 2015

http://meilishouxihu.blog.163.com/



Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/


Greek letter


IVONA TTS Capable.

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SGuide-IRTContentPart 1 of 2■ Chapter 1 - Introduction to Principles & Theory■ Chapter 2 - Materials and Their Properties■ Chapter 3 – Thermal Instrumentation

Part 2 of 2■ Chapter 4 – Operating Equipment and Understanding Results■ Chapter 5 – Applications■ Appendices A, B, C


Chapter 4Operating Equipment and Understanding Results


4.1 Temperature ChangesDistinguishing real temperature changes from apparent temperature changes is one of the biggest challenges facingthermographcrs. Thermal imaging instruments register temperature changes in response to changes in radiosity at the target surface when in many cases, there is no change in real surface temperature. To complicate matters further, external mechanisms can exaggerate these misleading readings. To combat this situation. thermographers should understand the len basic causes of apparent temperature change - some of which are only apparent and some of which are the result of real temperature changes at the target surface.

Causes of Apparent Temperature Changes Apparent temperature changes can be caused by difrerences in emisivity Ɛ, reflectivity ρ, transmissivity τ and target geometry G.


■ Emissivity Differences ∆τEmissivity differences at the target surface can change the target radiosity. even on an isothermal target. and may give the appearance of temperature variations on the thermogram. Frequently, these can be seen on painted metal surfaces where scratches expose bare metal that has a different emissivity than the paint.

■ Reflectivity Differences ∆ρ Reflectivity djfferences may become apparent when heat sources external to the target surface reflect off low emissivity target (low emissivity ≡ high reflectivity) surfaces into the instrument. These can be point sources or extended sources and they can add to or subtract from the apparent temperature reading as will be discussed later.


■ Transmissivity Differences ∆τTransmissivity differences can be caused by heat sources behind the target if the target is partly transparent in the infrared range. These will only be seen if the target transmissivity is high enough and the heat source is different enough in temperature from the target to contribute significantly to the total target radiosity.

■ Target Geometry Differences ∆DTarget geometry differences are caused by multiple reflections within recesses or concavities on the target surface. They are actually variations in effective emissivity caused by changes in surface configurations. An example of this is the apparent temperature gradient in the far corner of an enclosure that is at a uniform temperature. Geometric differences diminish as target surface emissivity approaches unity. (blackbody does not affected by G)


Causes of Real Temperature ChangesReal temperature changes may be caused by differcnces in mass transport (fluid flow), phase change (physical state). thermal capacitance, induced heating, energy conversion (friction. exothermic reactions and endothermic reactions), direct heat transfer by conduction, convection and radiation (thermal resistance) or a combination of two or more of these causes.

■ Mass Transport Differences (Fluid Flow)Mass transport differences are real temperatutc changes at the target surface caused by various forms of fluid flow. Free and forced convection are two examples of mass transport differences. Cool air exiting an air conditioning register will cause the register to become cooler. Hot water flowing within a pipe will cause the inside surface of the pipe to become warmer. (This will result in the outside of the pipe also becoming warmer.)


Mass Transport Differences (Fluid Flow)


Mass Transport Differences (Fluid Flow)


■ Phase Change Differences (Physical State)Phase change differenccs occur when materials change physical stale. An example of this is water evaporating off the surfaceof a building. As the water evaporates, it has a cooling effect on the entire surface. Thermal imaging equipment aimed at the building will register this cooling effect.

■ Thermal Capacitance Differences ∆Cp

Thermal capacitance differences cause temperature changes in transient conditions when one part of a target has a greater capacity to store heat than another. In the thermogram of a water tank. as shown in Figure 4.1. the water level inside the tank is apparent because of the contrast in temperature, which is caused by the difference in thermal capacitance between water and air. This real temperature change is also evident in roof surveys as illustrated in Chapter 5.


Figure 4.1: An indication of water level In a storage tank


Thermal Capacitance Differences ∆Cp

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■ Induced Heating Differences (by electromagnetic induction)Induced heating differences occur when ferrous metals are within a magnetic field. Depending on the orientation of the parts and the strength of the magnetic field, induced currents within the ferrous parts can cause substantial heating. An example of this is when an aluminum bolt in a structure is mistakenly replaced with a ferrous bolt. If the structure is within a magnetic field, the bolt may become hot. This induction effect is exploited in the thermographic location of steel reinforcing bars embedded in concrete structures. Here a magnetic field is introduced to the structure and the resultant warm spot on the thermogram indicate the presence of the reinforcing bars.


Induced Heating Differences

http://processmodeling.org/








■ Energy Conversion DifferencesEnergy conversion differences occur when energy is converted from one form to another. Friction (mechanical energy converted to thermal energy) is a commonly observed example of temperature changes because of energy conversion. Another is electrical energy converted to thermal energy, as illustrated in Figure 4.2, where the current carrying wire of a twisted pair generates heat revealing insulation discontinuities. Exothermic or endothermic reactions (chemical energy converted to thermal encrgy) are further examples. typified by the heating that accompanies the curing of polymers.

■ Direct Heat Transfer DifferencesDirect heal transfer differences are also commonly observed in thermographic survey programs. An example of this is shown in the direct transfer of thermal energy through the wall of a catalytic cracker reformer vessel as illuslrated in Figure 4.3. The differences in heat flow illustrate the differences in thermal resistance between good refractory material and degraded material.


Figure 4.2: The current carrying wire of a twisted pair generates heat that reveals insulation defects


Figure 4.3: Catalytic reformer vessel with insulation defects


Energy Conversion Differences


Infrared Thermogram Energy Conversion Differences






Direct Heat Transfer Differences


■ Combination of Heat Transfer MechanismsThermal images of operating equipment and systems will often exhibit heat flow by a combination of mechanisms working simultaneously. Figure 4.4 depicts the investigation into the thermal design of a new motorcycle engine. The thermal signature is a combination of fluid flow (in the cooling fins), exothermic reactions (within the cylinders) friction (at the piston rings and within the bearing) and thermal resistance (in the exhaust system).


Figure 4.3: Thermogram of a new motorcycle engine heat flow by a combination of mechanisms working simultaneously


Infrared Thermogram of a running motor


Image InterpretationA clearer understanding of the pitfalls possible in image interpretation helps the thermographer to perform the required tasks competently. As in the three modes of heat transfer (conduction, convection & radiation), these mechanisms frequently occur in combinations. Although the ability of the thermographer to clearly identify the causes of temperature change in a paticular target environment may be unnecessary when making measurements, it is absolutely essential for the correct and responsible interpretation of results. In situations where the thermographer is unfamiliar with the measurement environment, a knowledgeable facility representative should accompany the thermographer during the measurements or be available for consultation. By providing expert information concerning the processes taking place and the likely sources of temperature differences, the thermographer will be able to anticipate thermal behavior and better understand and interpret the thermographic results.


■ Spectral Considerations in Product and Process ApplicationsMany products, both simple and complex have complex spectral characteristics in the infrared region. Spectral filtering of the measuring instrument can exploit these complex spectral characteristics to measure and control product temperature without contact. For example, if it is necessary to measure the temperature of objects from 200 to 1000 °C (392 to 1832 °F) inside a heating chamber with a glass port , or inside a thin walled glass bell jar, an instrument operating in the 2 to 3 μm band will see through the glass and make the measurement easily. On the other hand, an instrument operating at wavelengths longer than 4.8 μm will measure the surface temperature of the glass.


Infrared Thermogram of Glass of Water - Spectral Considerations

2 to 3 μm band will see through the glass and make the measurement easily.

4.8 μm will measure the surface temperature of the glass.


Infrared Thermogram


Infrared Thermogram


Infrared Thermogram


Spectral characteristics are exploited in the monitoring of incandescent lamp temperatures during production as illustrated in Figures 4.5, 4.6 and 4.7. Figure 4.5 shows the spectral characteristics of the imaging radiometer as well as the transmission spectra of glass envelopes of various thicknesses.

Using a 2.35 μm band pass filter with the instrument allows the instrument to see through the glass and monitor the temperature of critical internal lamp components. Substituting a 4.8 μm high pass filter allows the instrument to monitor the glass envelope temperature.

Figures 4.6 and 4.7 are thermograms of the glass envelope and the internal lamp components respectively, recorded in immediate sequence. An important generic example of the need for spectral selectivity is in the measurement of plastics being formed into films and other configurations. Thin films of many plastics are virtually transparent to most infrared wavelengths, but they do emit at certain wavelengths. Polyethylene, polypropylene and other related materials have a very strong, though narrow, absorption band at 3.45 μm. Polyethylene film is formed at about 200 °C (392 °F) in the presence of heaters that radiate at a temperature near 700 °C ( 1292 °F).


Figure 4.5: Spectral selectivity for measuring the surface and internal temperatures of incandescent lamps


Figure 4.6: Surface temperature thermogram of an incandescent lamp


Figure 4.7: temperature thermogram of an incandescent lamp


Incandescent Lamp

2.35 μm band pass filter - Incandescent filament temperature

4.8 μm high pass filter - bulb surface (envelope)temperature


Figure 4.8 shows the transmission spectra of 40 μm ( 1.5 x 10-3 in.) thick polyethylene film and the narrow absorption band at 3.45 μm. The instrument selected for measuring the surface of the film has a broad band thermal detector and a 3.45 μm spike band pass filter. The filter makes the instrument blind to all energy outside of 3.45 μm and enables it to measure the temperature of the surface of the plastic film without being influenced by the hot process environment. Figure 4.9 shows a similar solution for 13 μm (5 x 104 in.) thick polyester (polyethylene terephthalate 聚对苯二甲酸乙二醇酯 ) film under about the same temperature conditions. Here the strong polyester absorption band from 7.7 to 8.2 μm dictates the placement of a 7.9 μm spike filter placed in front of the same broad band detector as that used in the polyethylene application.


Figure 4.8: Measuring temperature of polyethylene


Figure 4.9: Measuring temperature of polyester

7.7 to 8.2 μm


IR Filter


IR Filter


Using Line Scanners for Monitoring Continuous ProcessesContinuous processes are most often processes in constant and uniform motion. When this happens. an imaging system may not be required to cover the full process image. To monitor and control processes in motion, an infrared line scanner can be used, scanning normal to the process flow, to generate a thermal strip map of the product as it passes the measurement site line as illustrated in Figure 4.10. If more than one measurement site line is required. additional line scanners may be deployed.


Figure 4.10: Line scanner for continuous process monitoring


4.2 Infrared Thermographic Equipment OperationBecause of product performance advances and meticulous ( 小心翼翼的 ) human engineering on the part of manufacturers, infrared thermographic equipment is far easier to operate in the twenty-first century than it was in the 1990s. It is relatively simple for the novice ( 新手 ) thermographer to turn on the equipment. aim at a target and acquire an image. Consequently, it is also easier than ever to misinterpret findings.


Preparation of Equipment for OperationEven when using point sensing instruments. preparation for making measurements requires an instrument operation check. a battery status check and a simple calibration check. This preparation follows a simple checklist, which is a critical element in the successful field operation of thermal imaging equ ipment. Equipment preparation is crucial in field measurements because of time consumption, measurement scheduling and the availability of on-sile personnel.

A seemingly small oversight in equipment preparation can waste considerable time and money. Calibration against a known temperature referencc is required for all infrared measuring instrumcnts and is normally accomplished through radiation reference sources also known as blackbody simulators. These temperature controlled cavities or high emissivity surfaces that are designed to simulate a blackbody target at a specific temperature or over a specific temperature range with traceability to the National Institute for Standards and Technology (NIST). Factory calibration and traceability is provided by the manufacturer.


Bccause most quantitative thermographic instruments measure radiant energy values converted to temperature readings by a computer, calibration information is usually stored in the computer software and is identified with a specific instrument serial number. If a specific instrument calibration is not available in the software, the computer will usually default to a generic calibration for that class of instrument. In addition to a blackbody calibration, the software is usually provided with correction functions for ambient effects such as atmospheric attenuation as a function of working distance and for emissivity correction.

Default settings for these values are normally in effect unless the operator chooses to alter them. Checking calibration of a thermal imaging system in detail requires placing a blackbody reference source in front of the instrument so that ilt subtends a substantial area in the center of the displayed image (much greater than the instantaneous field of view). The correct measurement conditions must be set into the computer where applicable [example. working distance = 10 m (33 ft), ambient temperature = 25 °C (77 ° F), emissivity = 1, etc.] and the temperature reading compared to the reference source setting. The spot measurement software diagnostic should be used if available. The detailed calibration should include the widest range of temperatures possible.


If the instrument is out of cal ibration. it may be possible to recalibrate it under celtain conditions. (Refer to the operator's handbook.) Otherwise, it may be necessary to return it to the factory for recalibration.

A detailed calibration check should be made at least every six months. Periodic calibration spot checks should also be performed. Ideally, calibration checks should be done before and after each field measurement mission and can be accomplished by means of a high quality radiation thermometer and high emissivity sample targets. To perform a spot check. place the target in front of the instrument. Set emissivity the same for both instruments and measure the apparent temperature simultaneously with the imager and the radialion thermometer. Spot checks should be run at a few temperatures covering the range of temperatures anticipated for the specific measurement mission. Because the fields of view and spectral ranges of the two instruments may not match, exact correlation may not be possible.The errors should be repeatable from day to day, however, and the procedure will provide a high degree of confidence in the results of the measurement mission.


Transfer caljbration using a radiation reference source in the field is effective where extremely accurate measurements are required within a narrow range of temperatures. Typically, instrument calibrations are performed over a broad range of temperatures. with certain maximum allowable errors occurring at temperatures within this broad range. The transfer calibration can optimize accuracy over a limited range. The procedure requires introducing a radiation reference source into the total field of view along with the target of interest with the reference set very close to the temperature range of interest. Using the diagnostic software to measure the apparent temperature differences between the reference and various points of the target of interest should provide improved accuracy. The equipment checklist used in preparation for a day of field measurements helps ensure that there will be no surprises on site. A standard checklist should be prepared to include all items in the thermographic equipment inventory. These should include instrument spare lenses, tripods, harnesses, transport cases, carts, batteries, chargers, liquid or gaseous cryogenic coolant, safety gear, special accessories, film, diskettes, spare fuses, tool kits, data sheets, operator manuals, calibration data, radiation reference sources, interconnecting cables, accessory cables and special fixtures.


The batteries mentioned on the mission checklist should be fully charged batteries. It is the thermographer's responsi bility to ensure that there is a comfortable surplus of battery power available for each field measurement session. The fact that batteries become discharged more rapidly in cold weather also must be considered when preparing for field measurements.


Procedures for Checking Critical Instrument Performauce ParametersThere are established procedures for checking the critical performance parameters di scussed in Chapter 3. The parameters that are most important to most measurement programs are:

1. Thermal resolution or minimum resolvable temperature difference (MRTD). 2. Imaging spatial resolution or instantaneous field of view (lFOV). and 3. Measurement spat ialresolution (IFOVmeas).

Comments on item 1■ Thermal resolution or ■ minimum resolvable temperature difference (MRTD) or ■ noise equivalent temperature difference (NETD).

the above describe the same phenomenon.


Comments:■ Thermal resolution or ■ minimum resolvable temperature difference (MRTD) or ■ noise equivalent temperature difference (NETD).


Recalling!

Temperature sensitivity is also called: thermal resolution

or minimum resolvable temperature difference (MRTD) or

noise equivalent temperature difference (NETD).

for my ASNT exam


Figure 4.11 : Test configuration for minimum resolvable temperature difference measurement


■ Thermal Resolutionthermal resolution can be measured using a procedure developed for military evaluation of night vision systems. This procedure uses standard resolution targets as illustrated in Figure 4.11 and is described as follows:

1. Set up the test pattern such that ΔT exceeds the manufacturer's specification for minimum resolvable temperature difference.

2. Determine the spatial frequency It of the target in cycles per milli-radian as follows:

a. the number of radians equals the bar width W divided by the distance d to the target (example: 2 mm at 1 m = 2 mRad); and

b. the spatial frequency, It = 1 cycle / (1 bar + 1 space) = 1 / (W + S). (If W = 2 mRad and S = 2 mRad, then It = 1/(2 + 2) = 0.25 cycles per milli-radian).


3. Reduce the ΔT until the image is just lost (note ΔTH) . Raise ΔT until the image is just reacquired (note ΔTC) then:

ΔT = ABS(ΔTH) + ABS(ΔTC)2

4. Then change distances or use different size bar targets to plot minimum resolvable temperature difference for other spatial frequencies.


Figure 4.12: Test configuration for modulation transfer function measurement


■ Imaging Spatial ResolutionImaging spatial resolution of scanning imagers can be ensured using another procedure that stems from military night vision evaluation protocol and uses the same standard bar target. The procedure measures the modulation transfer function (MTF), a measure of imaging spatial resolution. Modulation is a measure of radiance contrast and is expressed:

Modulation = Vmax – Vmin

Vmax + Vmin

where:V = the voltage analogue of the instantaneous radiance measured.


Modulation transfer is the ratio of the modulation in the observed image to that in the actual object. For any system the modulation transfer function will vary with scan angle and background and will almost always be different when measured along the high speed scanning direction than it is when measured normal to it. For this reason. a methodology was established and accepted by manufacturers and users alike to measure the modulation transfer function of a scanning imager and, thereby to verify the spatial resolution for imaging (night vision) purposes.


A sample setup is illustrated in Figure 4.12 for a system where the instantaneous field of view is specified at 2.0 mRad using the same setup as illustrated in Figure 4.10. The procedure is as follows:

1. Set ΔT (where ΔT = T2 – T1) to at least 10x the manufacturer's specified minimum resolvable temperature difference (MRTD).

2. Select distance to simulate the manufacturer's specified imaging spatial resolution. The bar width W represents one resolution element. For example. instantaneous field of view can be calculated where bar width W= 2 mm and distance d = 1 m.

IFOV = W/d = 2mm/1m

where: d= distances to target, W=bar width


3. Display imager's horizontal line scan through the center of the bar target.4. Calculate the modulation transfer function:

Modulation = Vmax – Vmin

Vmax + Vmin

where:MTF = modulation transfer function (a ratio).Vmax = maximum measured voltage V.Vmin = minimum measured voltage V.


5. If the modulation transfer function (MTF) = 0.35* or greater. the imager meets the imaging spatial resolution specification. (If the signal representing the horizontal scan line is not accessible, consult the manufacturer for an alternate means by which modulation transfer function can be verified. In a digital image, the gray level may replace the Voltage value. Note: There are disagreements among users and manufacturers regarding the acceptable minimum value of modulation transfer function to verify imaging spatial resolution with values varying between 0.35 and 0.5, depending on the manufacturer and the purpose of the instrument.)


■ Measurement Spatial ResolutionMeasurement spatial resolution (IFOVmeas) can be measured using a procedure that measures the slit response function (SRF) of the imaging system. This procedure was developed by instrument manufacturers and is generally accepted throughout the industry. In this technique, a single variable slit is placed in front of a blackbody source and the slit width is varied until the resultant signal approaches the signal of the blackbody reference. Because there are other errors in the optics and the 100 percent level of slit response function is approached rather slowly, the slit width at which the slit response function reaches 0.9 is usually accepted as the measurement spatial resolution. Again, there are some disagreements as to whether 0.9 or 0.95 should be considered acceptable. The test can establish whether the imager meets the manufacturer's specifications for measurement spatial resolution. The test configuration for slit response function determination is illustrated in Figure 4.13.


Figure 4.13: Test configuration for slit response function measurement


1. Set ΔT (where ΔT = T2 – T1) to at least 10x the manufacturer's specified minimum resolvable temperature difference (MRTD).

2. Select distance and slit width to simulate the manufacturer's specified measurement spatial resolution. The bar width W (mm) represents one resolution element. For example, for a 3 mRad measurement spatial resolution, if d= 1m, W = (1.0 x 0.003) = 3 mm.

3. Display imager's horizontal line scan through the center of the bar target.


4. Open slit until Vmeas = Vmax.

5. Close slit until Vmeas = 90% of Vmax and measure slit width (W).

6. Compute: IFOVmeas = W·d-1. This should be equal to or smaller than the manufacturer 's imaging spatial resolution specification.

Again, if the signal representing the horizontal scan line is not accessible, consult the manufacturer for an alternate means by which measurement spatial resolution can be verified.


Common Mistakes in Instrument OperationRemembering a few key cautions regarding proper equipment application

can help the thermographer to avoid some common mistakes. The following guidelines should be observed.

1. Select an instrument appropriate to the measurement application in accordance with the guidelines reviewed in Chapter 3.

2. Leam and memorize the startup procedure. 3. Leam and memorize the default values. 4. Set or use the correct emissivi ty and be particularly cautious with

emissivity settings below 0.5. 5. Make sure the target to be measured is larger than the measurement

spatial resolution of the instrument. (FOV? or IFOV?)6. Aim the instrument as close to normal (perpendicular) with the target

surface as possible. 7. Check for reflections off the target surface ρ and either avoid or

compensate for them. 8. Keep sensors or sensing heads as far away as possible from very hot

targets.


■ Learning the Startup ProcedureLearning the startup procedure thoroughly is essential, particularly for thermographers who operate several different models of thermographic and thermal sensing equipment. Efficient startup lets the data gathering process begin with no unnecessary delays; it saves valuable on-site time and inspires confidence of facility personnel. A quick review of the operator's manual and a short dry run before leaving home base is usually all that is required.


■ Memorizing the Default ValuesMemorizing the default values provided in the operator's manual is another important contribution to time efficiency and cost effectiveness. These include default values for several important variables in the measurement such as emissivity, ambient (background) temperature, distance from sensor to target, temperature scale (degrees Fahrenheit or Celsius), lens selection and relative humidity. It is important to remember that the instrument's data processing software automatically uses these values to compute target temperature unless the thermographer changes these values to match the actual measurement conditions. Typical default values are 1 m (3 ft) distance to target, emissivity of 1.0 and background temperature of 25 °C (77 °F). Failure to correct for these can result in substantially erroneous results, if, for example the target is known to be 10 m (33 ft) away is known to have an effective emissivity of approximately 0.7 and is reflecting an ambient background temperature of 10 °C (50 °F). By memorizing the default values, the thermographer will know when it is necessary to change them and when time can be saved by using them unchanged without referring to a menu.


■ Setting the Correct Effective EmissivitySetting the correct effective emissivity is critical in making temperature measurements. Table 2.2 can be used as a guide when obtaining absolute temperature values is not critical. When measurement accuracy is important. it is always better to directly determine the effective emissivity of the surface to be measured using the actual instrument to be used in the measurement and under similar operating conditions. This is because emissivity may vary with temperature, surface characteristics and measuremenl spectral band and may even vary among samples of the same material. There are several methods that may be used to quickly estimate target effective emissivity. One known as the reference emitter technique can be used to detennine the emissivity setting needed for a particular target material. The determination uses the same instrument that will be used for the actual measurement. The procedure is illustrated in Figure 4.14 and is described as follows:


Table 2.2: Normal spectral emissivities of common materials


Figure 4.14: Test configuration for the determination of effective emissivity using the reference emitter method.

#2 This area was painted, taped or conditioned with material of known emissivity.

#4; Set instrument emissivity using the known emissivity value and observed the material temperature

#5: Adjust emissivity to obtained the sample temperature obtained in #4

Non conditionedSurface


1. Prepare a sample of the material large enough to contain several spot sizes or instantaneous fields of view of the instrument. A 100 mm x 100 mm (4.0 in. x 4.0 in.) sample may be big enough.

2. Spray half of the target sample with flat black (light absorbing) paint, cover it with black masking tape or use some other substance of known high emissivity.

3. Heat the sample to a uniform temperature as close as possible to the temperature at which actual measurements will be made.

4. Make certain that the value for background temperature has been properly entered. Then set the instrument emissivity control to the known emissivity of the coating and measure the temperature of the coated area with the instrument. Record the reading.

5. Immediately point to the uncoated area and adjust the emissivity set until the reading obtained in step 4 is repeated. This is the emissivity value that should be selected in measuring the temperature of this material with this instrument.


■ Measuring and Reporting Temperature Accurately - Filling the Instantaneous Field of View IFOVmeas

If true temperature measurement of a spot on a target is required, the spot must completely fill the instrument’s measurement spatial resolution (IFOVmeas). lf it does not. some useful information about the target can still be learned. but an accurate reading of target temperature cannot be obtained. The simple expression. D= α∙d, can be used to compute measurement spot size D at the target plane from a working distance “d” where α is taken to be the manufacturer's published value for measurement spatial resolution.

For example. if the target spot to be measured is 5 cm (2 in.) and the calculated spot size D is 10 cm (4 in .), move the instrument closer to the target or use a higher magnification lens if either is possible. If not, expect the reading to be affected by the temperature of the scene behind the target. Also. be sure to allow for aiming errors and instrument imperfections. An extra 30 percent should be enough.


■ Aiming Normal to the TargetAiming normal (perpendicular) to the target surface or as close as possible to normal is important because the effective emissivity of a target surface is partially dependent on the surface texture. It stands to reason, then, that if the surface is viewed at a skimming angle, the apparent texture will change, the effective emissivity will change greatly and the measurement will be affected by misleading reflections. These can result in cold errors as well as hot errors.

A safe rule is to view the target at an angle within 30 degrees of normal (perpendicular). If the target emissivity is very high this can be increased to as high as a 60 degree angle if necessary.

Note: where: θ angle from normal θ = 30ºθ = 60º where Ɛ is high


■ Recognizing and Avoiding Reflections from External SourcesRecognizing and avoiding reflections from external sources is an important acquired skill for the thermographer. If there is a concentrated source of radiant energy (point source) in a position to reflect off the target surface and into the instrument, steps should be taken to avoid misleading results. There is the greatest likelihood of errors due to point source reflections when the (1) target emissivity is low, (2) the target is cooler than its surroundings or (3) the target surface is curved or irregularly shaped.

It should be noted that, although most errors due to reflections are from sources hotter than the target, reflective errors from cold sources can also occur and should not be discounted. A common source of reflective error is the reflection of the cold sky off glass or other reflective surfaces. If a temperature anomaly is caused by a point source reflection, it can be identified by moving the instrument and pointing it at the target from several different directions. If the anomaly appears to move (changes/ varies) with the instrument, it is a point source reflection. Once identified. the effect can be eliminated by changing the viewing angle, by blocking the line of sight to the source or by doing both.


Errors due to the reflection of an extended source, however, cannot be eliminated in this manner. The ambient instrument background (what the instrument sees reflected off the target surface) is the most commonly encountered example of an extended source reflection.

Errors due to extended source reflections are more likely when the target emissivity is low or when the target is cooler than its surroundings. Most instrument menus include a provision for entering the ambient background temperature if it is different from the default setting. The system will automatically correct the temperature reading. This will also work if the ambient background is an extended source such as a large boiler. In this situation, substituting the boiler's surface temperature for the background ambient setting will correct the temperature reading.

Keywords:Reflection due to Point sourceReflection due to Extended source


■ Measuring the Appropriate Background Temperature Using the InstrumentA technique commonly used by thermographers to determine an appropriate setting for "ambient background temperature" requires a piece of aluminum foil large enough to fill the total field of view of the instrument. First. crush the foil into a ball and then flatten it so that it simulates a diffuse reflecting surface. Next, place the foil so that it fills the instrument's total field of view and reflects the ambient background into the instrument. Allow the foil to come to thermal equilibrium. With the instrument's emissivity Ɛ set to 1.00, measure the apparent temperature of the foil. Use this apparent temperature reading as the ambient background temperature setting.


■ Avoiding Radiant Heat Damage to the InstrumentAvoiding radiant heat damage to the instrument is always important. Unless an infrared sensing or imaging instrument is specifically selected or equipped for continuous operation in close proximity to a very hot target. it may be damaged by extensive thermal radiation from the target. A good rule for the thermographer to follow is "don't leave the instrument sensing head in a location where you could not keep your hand without suffering discomfort.“ Accessories such as heat shields and environmental enclosures are available from manufacturers for use when exposure to direct radiant heat is unavoidable. These accessories should be used to protect the instrument when appropriate.


■ Temperature Differences Between Similar MaterialsParticularly in electrical applications, it is critical to measure and report temperature differences between similar components with similar surface materials. such as the fuses on different phases of the same supply. Strict observance of the procedures regarding the use of the correct (1) effective emissivity value, (2) filling the measurement spatial resolution, (3) using the correct background temperature, (4) setting and using the correct viewing angles θ will ensure that these differences are measured and reported correctly


4.3 Safety and HealthSafety and health considerations are critical to successful thermography programs as well as to the welfare of the thermographer and client personnel. Strict adherence to applicable codes is the responsibility of the thermographer. It is essenlial that the basics of these regulations be understood.


■ Liquid and Compressed GasesSome instruments in the field use liquid or compressed gases for detector

cooling. The handling of these materials can be hazardous and it is the thermographer's responsibility to learn safe practices and to adhere to them. In general, these procedures are included in the safety regulations for each facility. They can also be found in the operator's manuals for these instruments. Some instruments use liquid nitrogen LN as a detector coolant Liquid nitrogen is not very hazardous but some safety precautions should be observed. The following four guidelincs for using and storing liquid nitrogen are taken from the AGEMA Model 782 Operator's Handbook:

1. Never store the liquid in sealed containers. Liquid nitrogen and similar cryogenic liquids are always stored in Dewar flasks or the equivalent insulated containers, with loosely fitting covers that allow the gas to vent without building up dangerous pressures,

2. Never come into direct contact with liquid nitrogen. Serious frost bite injury (similar to a bum) can result if the liquid is allowed to splash in to the eyes or onto the skin.


3. Always re place the filler cap after filling to avoid the risk of spillage and condensation.

4. It is advisable to transfer some of the liquid from the storage Dewar to a smaller vessel (that is a vacuum jug) to effect more convenient filling and minimize spillage. Slowly pour a small amount into the instrument's liquid nitrogen chamber and wait until boiling ceases. This ensures that the chamber is at the same temperature as the liquid and minimizing splashing and spillage. Fill the chamber completely and replace the filler cap.“


Dewar Flask for LN Loosely fitting covers that allow the gas to vent without building up dangerous pressures


■ BatteriesProcedures for the handling of batteries and their safe disposal must also be followed by the thermographer. In general these procedures are included in the safety regulations for each facility. They can also often be found in the instrument operator's manuals. Generally, instructions for the safe disposal of batteries are provided in the literature accompanying the batteries. In the absence of such instructions, exhausted batteries should be considered as hazardous waste and handled accordingly.


■ Electrical SafetyFailure to recognize and observe electrical safety regulations can result in electrical shock and irrepairable damage to the human body. Electrical current flowing through the heart, even as small as a few milli-ampere can disrupt normal heart functions and cause severe trauma and some times death. In addition. body tissue can be severely and permanently damaged. Shock hazards are proportional to equipment operating voltage levels and distance from the hazard. Voltage levels as low as 60 V causing current to flow through the chest area with low skin resistance can be lethal. Examples of electric shock current thresholds and typical electrical contact resistances are given in Table 4.1 .

Safety practices are important as well. One good safety rule to follow is to never touch electrical contacts unless qualified to do so. areing can also be lethal and even low voltage equipment may produce killing areas. It is important that only trained personnel wearing are protective gear be permitted to approach energized equipment. Spectators should not approach at all.


Safety codes have been developed that specify the minimum distances to be maintained from live equipment and, in addition, protective clothing and devices (face shield, protective clothing and insulated gloves) are required in all facilities. Although the codes may vary from facility to facility, they all spell out the safety rules to which thermographers are expected to adhere. Examples of National Electrical Safety (NES) codes currently being observed in facilities in the United States and Canada that specify the minimum clearance zone from operating high voltage equipment in terms of voltage and distance are described in Table 4.2. thermographers must be aware of the safety regulations in force and know the recommended protective clothing. It is recommended that the applicable safety guidelines set forth in the following documents be reviewed:

1. National Fire Protection Association NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplace, 1995, and 2. National Fire Protection Association NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, 1994.


Table 4.1: Electric shock current thresholds and skin contact resistances


Table 4.2: Examples of specified clearance distances from high voltage equipment


4.4 Record KeepingKeeping thorough and detailed records is very imponant to the thermographer, particularly when performing a comprehensive program of thermographic facility surveys. As discussed in Chapter 3, most equipment manufacturers sell software that provides a filing system to maintain records of all images and accompanying data and comprehensive report preparation software for timely and comprehensive reporting of the findings of infrared surveys and other measurement missions. Although recording the actual findings is the basic reason for record keeping, support records are also important. These records include equipment status history as well as personnel qualification documentation.


Records of surveys should be documented to include:

1. day, date, location, identification of test site and equipment or components inspected;

2. thermographer's identification and qualifications; 3. equipment used and calibration history (when last calibrated. when last

spot check was made, etc.); 4. what was inspected, what was not inspected and why; 5. visual test reports of cracking, etc. with photographs if appropriate; 6. other observations noted by the inspector. such as noise and aroma; 7. backup video tapes of the entire measurement survey; and 8. specific mention of any critical findings.

All images should be maintained as files for future reference and trending. Reports may be tailored to include only those items considered significantbut records should be maintained for all measurements. Maintenance and repair records of all equipment and accessories should also be kept. Easily accessible and easily understood notes and records are a measure of the competence and professionalism of the thermographer and lead to credibility in the eyes of management, whatever the industry or discipline.


Easily accessible and easily understood notes and records are a measure of the competence and professionalism of the thermographer and lead to credibility in the eyes of management, whatever the industry or discipline.


Chapter 4Review Questions

Q&A1. b

2. d

3. b

4. a

5. b

6. a

7. c

8. a

9. d

10. d


Q1. Apparent but not real temperature changes recorded by an infrared instrument can be due to:a. emissivity, reflectivity and mass transport differences.b. emissivity, reflectivity and geometric differences.c. thermal capacitance, reflectivity and geometric differences.d. thermal capacitance, mass transport and emittance differences.

Q2. Apparent temperature changes recorded by an infrared instrument that are, in fact real temperature changes can be due to:a. emissivity, reflectivity and mass transport differences.b. emissivity. reflectivity and geometric differences.c. thermal capacitance, reflectivity and geometric differences.d. thermal capacitance, mass transport and energy convertion.


Q3. Sun glints 闪耀 cause false indications of temperature changes. In this respect, they are similar to:a. solar heating.b. emissivity artifacts.c. resistive heating.d. mass transport.

Q4. The lower the temperature of a target to be measured, the more imponant it is to:a. correct for ambient reflections.b. fill the instrument 's measurement spatial resolution with the target.c. use a cooled detector.d. keep batteries fully charged.


Q5. The higher the temperature of a target to be measured, the less important it is to:a. fill the instrument's measurement spatial resolulion with the target.b. correct for ambient reflections.c. correct for atmospheric absorption in the measurement path.d. keep batteries fully charged.

Q6. Placing a blackbody reference source next to a distant target will usually help correct for:a. the effect of atmospheric absorption in the measurement path .b. ambient reflections off the target surface.c. target surface emissivity artifacts.d. point source reflections.


Q7. To make an effective infrared temperature measurement, the angle between the target surface and the instrument's line of sight should be: a. always 90 degrees (perpendicular).b. any angle providing the target always fills the measurement spatial resolution of the instrument.c. as close at possible to 90 degrees but not less than 60 degrees.d. anywhere between 30 degrees and 45 degrees.

Q8. If a target does not fill the measurement spatial resolution of the measuring instrument at a convenient measurement distance, it may be necessary to:a. use a higher magnification lens or move in closer,b. place a blackbody reference next to the target.c. use the instrument 's electronic zoom feature,d. use more than one isothenn to make the measurement.


Q9. Differential thermography can be very useful because it:a. tends to minimize the effects of surface emissivity artifacts.b. tends to emphasize only those areas where temperature changes occur.c. helps record changes for thermal trending purposes.d. is all of the above.

Q10. When planning a measurement mission, it is important to remember that batteries:a. may never reach fu ll charge.b. are about the least reliable element at a thermographer's disposal.c. lose their charge more rapidly in cold weather.d. are all of the above.


Chapter 5Applications


5.1 Overview of ApplicationsBecause temperature is, by far, the most measured and recorded parameter in industry, it is no surprise that applications for temperature measurement and thermography are found in virtually every aspect of every industry. Because of the widespread applicability of thermal sensing and thermography, attempting to classify applications into formal categories meets with considerable overlap. Because of this ambiguity, and because the thermal principles of investigation involved should be well known by the qualified thermographer, applications are presented in this chapter by thermal principles of investigation categories, as set forth in the Infrared Thermal Testing Method, Level III Topical Outline contained in Recommended Practice No. SNT-TC-JA (1996) as follows:

1. exothermic and endothermic investigations,2. friction investigations,3. fluid flow investigations,4. thermal resistance investigations and5. thermal capacitance investigations.


5.2 Exothermic and Endothermic InvestigationsAn exothermic process is one that releases heat and exhibits a temperature increase. An endothermic process is one that absorbs heat and exhibits a temperature decrease. The link between these methods is that the investigator does not need to apply any thermal stimulation. The relevant thermal pattern exists in the subject because of another process performed on (or within) the subject. Applications for thermograpgy in this area are commonly found in electrical and electronic diagnostics, chemical processes such as the application of foam-in-place insulation, fire detection, night vision and surveillance, animal studies, heating and cooling systems and other areas where thermal energy is released or absorbed.

Keywords:the investigator does not need to apply any thermal stimulation.


Electrical ApplicationsElectrical findings represent the primary use of infrared thermography in facilities and utilities. They also represent the most straightforward application of the equipment. The most common electrical findings are caused by high electrical resistance, short circuits, open circuits, inductive currents and energized grounds. Much of the routine scanning is done qualitatively, but quantitative thermography is required in many instances to estimate true temperature rises. Specifically in electrical applications, the flow of current through a conductor generates heat in direct proportion to the power dissipated. This is directly proportional to the electrical resistance and to the square of the current (P = I2R) and is commonly called I2R loss. A poor connection or, in some cases, a defective component, will have an increased resistance, resulting in a temperature increase and, consequently, a temperature rise in the area of the discontinuity.


Alas!

Electrical Applications is NOT

“thermal resistance investigations”

for my ASNT exam


High electrical resistance is the most common cause of thermal hot spots in electrical equipment and power lines. When the line current is relatively constant and resistance is higher than it should be, additional power is dissipated and a thermal anomaly occurs. This is frequently dangerous and always costly in terms of valuable waits lost, unwanted heat and accelerated aging of equipment, which results in premature replacement of equipment.

Typical examples of resistive heating include loose connections, corroded connections, missing or broken conductor strands and undersized conductors. Figure 5.1 is an example of excessive heating caused by high resistance at a connection (upper center) because of deterioration of the connection. The connection appears to be more than 5 °C (9 °F) warmer than the adjacent connections. In power lines and switchyards, hot connections caused by deterioration are the most common findings that are associated with potential failures. Short circuits are another cause of electrical failure. When they occur in a power line, they usually are extremely brief in duration and have immediate and disastrous results. Within an operating component, however. the shorted section will cause excessive current to flow with resultant heating, and this frequently can be detected and diagnosed using thermographic equipment.


Figure 5.1: Excessive heating due to a defective electrical connection

The connection appears to be more than 5 °C (9 °F) warmer than the adjacent connections.


Excessive heating due to a defective electrical connection


One example of this would be shorted sections of a current transformer winding causing the transformer to appear hotter than normal and/or hotter than other similar devices. Similar problems can occur within power supplies and within rotating equipment such as motors and generators. Open circuits do not generally show up as hot spots and are often overlooked by inexperienced thermographers as indications of potential problems. An operating element running cooler than normal may indicate that the element is open and inoperative. A common problem with inverters, for example, is blown (open) capacitors that appear cool. Power supplies, resistor or integrated circuit chips that are open and inoperative will usually be cooler than normal, although the malfunction may cause excessive heating elsewhere in the operating clement.


Inductive currents flowing with in ferrous components or elements that are within the magnetic field of large equipment (i.e., the main generator in a power plant) can cause excessive heating. Warm areas can appear in motor frames and structural elements and several examples have been documented where steel bolts have been inappropriately used to replace nonferrous bolts in framework supporting large rotating machinery. Heat caused by inductive heating does not always lead to failure, but should be documented by the conscientious thermographer. Energized grounds occur in plants and facilities, sometimes as the result of partial insulation breakdown in an operating element. These findings are, in many cases, considered life safety situations. Because an energized ground connection is usually extremely hot , there is seldom difficulty identifying it thermographically. The problem is tracing the cause. which may be elusive. The ground connection may also be carrying induced currents because of a breakdown of an element in close proximity. Most often the diagnosis requires considerable input from knowledgeable facilities personnel.


When starting new thermography programs, it is necessary to establish guidelines to determine how much temperature deviation from normal constitutes an electrical problem. There is no simple standard because there are so many factors, including ambient variations, that can influence temperature. With this caution in mind, it is reasonable to set forth guidelines to assess the severity of findings based on common sense and experience as well as on temperature readings. Most facilities have rule-of-thumb systems whereby they classify the potential severity of a finding based on temperature rise and known load conditions.


Moisture in AirframesThe detection of moisture in airframe sections can be accomplished thermographically because of the endothermic process that takes place when there is moisture ingress in an airbomc structure and this water freezes. When thermal images are taken immediately after the aircraft lands. the skin above the sections where moisture has entered show up as cool spots on the thermogram, as seen in Figure 5.2. (does the heat capacitance qualified as endothermic process?)

In chemical processes, an example of an exothermic reaction is the installation of foam-in-place polyurethane insulation. As the liquid chemicals are released into the cavity, they solidify into a foam and release heat. This heat is conducted into the walls of the cavity wherever the foam is produced. As a result. the cavity walls are uniformly heated by a successful blow. A thermographic investigation can evaluate the effectiveness of this process by mapping the uniformity of the temperature distribution on the outside walls cool spots would indicate sections where the foam had not migrated.


Figure 5.2: Water ingress in an aircraft section


Process Control and Product MonitoringFor many years, infrared sensors have been used for quantitative surface temperature monitoring of products and processes. When measurement of one point in the process, or even a number of points, is not considered adequate to characterize the process for successful monitoring or control, infrared thermography can be used, The most significant aspect of this approach is that it is unique and unprecedented. Infrared point sensors are used, when appropriate, in place of conventional temperature sensors. Infrared scanners and imagers, however, are the only practical means to acquire a high resolution thermal map of an entire surface in real time (at or near television display rates). Full surface thermal process control was not a viable option until the integration of computers and image processing software with thermal scanners.


Line Scanners or Imagers for Mapping of Continuous ProcessesFull image process control can be defined as using an infrared thermal image as a model against which to compare, and thereby control, part or all of the thermal surface characteristic of a product or process. If the process is moving at a uniform, predictable rate, a thermal image can be produccd by a line scanner scanning normal to the motion of the process as illustrated in Chapter 4. Figure 4.10. The control method is similar to thaI used in point sensing applications, although far broader in scope. The scanner or imager is first used to characterize the thermal map of the product under ideal conditions to produce, digitize and store a criterion image - what the ideal thermal distribution would be if the process resulted in perfectly acceptable products as designed. During the actual process, the thermal map, or any critical portion of the map, is constantly compared to the stored criterion image model by means of image subtraction and/or statistical analysis techniques.


The differences produced by this comparison are used to adjust or correct the settings of the process mechanisms that govern the heat applied, or to alarm and automatically reset the process. Figure 5.3 shows the evaluation of a web process used on the outside of drywall construction material.

The thermogram clearly shows excessive heat on the right edge of the material, a condition that can cause the paper to become brittle. The information derived from. The thermogram is used to correct the temperature distribution, thus resulting in a more acceptable product. Although the image is not of an automatically controlled process. it would be possible to close the loop to maintain ideal temperature distribution automatically.


Spectral Considerations in Product and Process ApplicationsMany products. both simple and complex, have complex spectral characteristics in the infrared region. Spectral filtering of the measuring instrument can exploit these complex spectral characteristics to measure and control product temperature without contact. A good example of the exploitation of spectral characteristics in the monitoring of incandescent lamp temperatures during production. An important generic example of the need for spectral selectivity is in the measurement of plastics being formed into films and other configurations. Several examples of this exploitation are illustrated in the detailed discussion of spectral considerations in Chapter 4.


Night Vision, Seareh, Surveillance, Security and Fire DetectionThe level of heat given off by the human body makes it readily detectable to thermographic instruments. Similarly. exothermic actions of engines and moving vehicles make them good targets for infrared surveillance applications. Night vision, seareh, surveillance and security applications are, with very few exceptions, qualitative applications of infrared thermography. They provide the user with the capability to see through an atmospheric path in total darkness. The clarity of the image is of critical importance and temperature measurement is not required. Ideally, in these applications. the objective is to display (and sometimes to record) an image that has the very best spatial resolution at the longest possible range under the most adverse atmospheric conditions. An example of a typical surveillance image is the thermogram of a helicopter taken at night, shown in Figure 5.4.


Figure 5.4: Thermogram of helicopter taken at night


Thermogram of helicopter taken at night


Thermogram of Jet


Aircraft Under IR Trap


Aircraft Under IR Trap


Instruments used for these applications evolved from military programs based on the need to detect and identify tactical targets through atmosphere in the dark and in bad weather. For this reason, they generally operate in the 8 to 12 μm spectral window where the atmosphere has very little absorption. Exceptions to this generality are infrared seeking and homing sensors that are sensitive to specific target emission signatures. such as rocket engine plumes. These instruments usually operate somewhere in the 3 to 5 μm region. The same qualitative instruments can be readily adapted to fire detection applications. From the ground or the air, they provide the capability of detecting incipient fires and unextinguished portions of forest fires. The 8 to 12 μm spectral region over which they operate also provides improved visibility (less absorption loss) through smoke and fog.


8 to 12 μm spectral region over which they operate also provides improved visibility (less absorption loss) through smoke and fog.


8 to 12 μm spectral region over which they operate also provides improved visibility (less absorption loss) through smoke and fog.


8 to 12 μm spectral region for AstrologyStellar cluster and star-forming region M 17


Animal StudiesBody heat allows infrared thermographic studies of animals to be made. Inflammation raises the temperature of infected, diseased or traumatized portions of the body, as illustrated in Figure 5.5. This shows the thermal contrast between a bruised equine foreleg (left) and a normal foreleg (right).


Figure 5.5: Injured equine foreleg (left) appears warmer than a normalforeleg


Injured equine foreleg (left) appears warmer than a normal foreleg

http://www.thermomed.org/en/veterinarymedicine.html


5.3 Friction InvestigationsFriction generates heat as energy conversion from mechanical energy to thermal energy. Sliding friction is a force that acts on one body sliding over another. The maximum force of friction that one body is capable of exerting over another is directly proportional to the normal, or perpendicular force with which the bodies are pressed together. This proportionality is called the coefficient of friction and the equation for sliding friction is:

f = μ N

where:f = the maximum force of friction.μ = the coefficient of friction.N = the normal force with which the two bodies are pressed together.


Work energy is expended by frictional force and converted to stored heat. This stored heat is then conducted. convected and radiated to the surroundings. which can be sensed and measured using thermal instruments. The heating and resultant damage from excessive friction is one of the most common types of mechanical failure detectable by infrared thermography. Many of the mechanical failures located by thermography occur in rotating machinery. Problems caused by friction include worn, contaminated or poorly lubricated bearings and couplings and misaligned shafts. Typical findings occur in motor bearings such as that shown in Figure 5.6 where the temperature imbalance on a blower fan is because of uneven friction as seen through the end screen. The apparent temperature on the tower section is about 10 °C (18 °F) warmer than the upper section. Friction investigations applicable to thermography also include air turbulence flow studies in aircraft and spacecraft modeling, machine gear and belt temperature monitoring and effectiveness studies for the cooling of machine tools.


Figure 5.6: Overhead Motor Bearing (bottom)


5.4 Fluid Flow InvestigationsFor successful fluid flow investigations to be performed, a temperature higher or lower than ambient must be induced into the fluid paths. Often, this condition already exists but somelimes the investigator must artificially introduce such a fluid. Fluid flow applications include piping, valves, heat exchangers, cooling towers, effluent mapping and ocean mapping. In predictive maintenance and plant condition monitoring, many pipe blockage and leakage conditions can be detected using infrared thermography. Ideally, the condition is simple to detect if the valve or pipe section is not covered with insulating material, and if the temperature of the fluid conducted by the valve or pipe section is sufficicnlly hotter or cooler than ambient. when conditions are not ideal, blockages or leakages may be difficult or impossible to detect. Adverse conditions include pipes or valves covered with heavy insulating jackets, particularly those covered with low emissivity metal cladding. Under most measurement conditions, a closed valve will have a distinct temperature gradient across it and a leaky valve will not. For example, when a hot fluid is blocked by a closed valve the temperature difference gradient can be observed thermographically.


Steam traps are special valves that automatically cycle open and closed to remove condensate from sections of steam process lines. If the thermographer has prior knowledge of their normal operation, steam traps can usually be observed thermographically to determine if they are operating properly. Without this prior knowledge, using infrared thermography for steam trap diagnostics may be confusing and misleading. In the image sequence shown in Figure 5.7, the various operating conditions of the valve (top) result in clearly detectable thermal pattern changes. The thermal appearance of the steam trap (bottom) remains essentially the same in all three images Blockage of any fluid transfer line can be simple to detect thermographically if the fluid temperature is sufficiently hotter or cooter than ambient. If not, there are more sophisticated approaches that have had documented success. For example, the injection of uniform transient heat will often result in transient temperature differentials at the blockage site because of the difference in thermal capacity between the fluid (in liquid form) and the solid blockage. Heat injection techniques are discussed in greater detail in subsequent sections.


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5.5 Thermal Resistance InvestigationsThermal resistance studies are involved in any thermographic application where the conductive flow of thermal energy is affected by variations in thermal resistance that exhibits a variation in effectjve temperature at the target surface. Applications include building and vessel envelope studies. furnaces, refractory linings, hazardous heat leaks and a wide variety of materials testing applications.


■ Building Insulation and Other FactorsAs previously discussed, the conductive heat flow through a laminar structure is related to both the temperature difference from one side of the structure to the other and the aggregate thermal resistance of the materials encountered. The higher the thermal resistance (insulating properties), the less heat will flow; therefore, when steady state heat flow can be established, mapping the temperature on the outside of a structure and knowing the thickness and the inside temperature, permits the determination of the insulation properties.

The measurement of conductive heat flow for insulation assessment is only one factor; however, in practical heat loss determination, other factors such as air infiltration and exfiltration, chimney effects. and thermal short circuits or bypasses can be serious enough to completely negate the benefits of good insulation. Thermographers have learned to consider the total structure when evaluating the results of thermographic surveys and to recognize and isolate thermal patterns typically associated with air flow as well as those caused by insulation deficiencies.


Figures 5.8 and 5.9 illustrate these distinct pattern differences. Figure 5.8 shows the distinct patterns caused by insulation deficiencies on the thermogram of an exterior wall of a structure heated rom within, whereas Figure 5.9. taken of a diffe rent structure under similar conditions. illustrates the effects of air exfiltration. It should be noted that most structural applications of thermography focus on qualitative features, such as thermal patterns and thermal anomalies. rather than quantitative temperature measurements. The only refe rence to temperature measurements was the stipulation in ANSUASHRAE 101- 1981 that. forthe inspection to be valid, "there shou ld be a minimum (difference) of 10 °C ( 18 °F) between the inside and outside surface tempera tures of the building for at least three hours prior to the survey." This stipulation was made presumably to establish quasi-steady state heat now thereby avoiding any misleading patterns because of struclUral differences in heat capacity and rendering images. which more rel iably represent only resistance di fferences. This standard has been superseded by ASTM C- I06O and ASTM C-1 155.


Figures 5.8 and 5.9 illustrate these distinct pattern differences. Figure 5.8 shows the distinct patterns caused by insulation deficiencies on the thermogram of an exterior wall of a structure heated rom within, whereas Figure 5.9. taken of a diffe rent structure under similar conditions. illustrates the effects of air exfiltration. It should be noted that most structural applications of thermography focus on qualitative features, such as thermal patterns and thermal anomalies. rather than quantitative temperature measurements. The only refe rence to temperature measurements was the stipulation in ANSUASHRAE 101- 1981 that. forthe inspection to be valid, "there shou ld be a minimum (difference) of 10 °C ( 18 °F) between the inside and outside surface tempera tures of the building for at least three hours prior to the survey." This stipulation was made presumably to establish quasi-steady state heat now thereby avoiding any misleading patterns because of struclUral differences in heat capacity and rendering images. which more rel iably represent only resistance di fferences. This standard has been superseded by ASTM C-106O and ASTM C-1155.


Figure 5.8: Example of missing insulation


Figure 5.9: Example of air exfiltration


Example of air exfiltration


Example of air exfiltration


■ Industrial Roof Moisture DetectionThermal resistance is commonly used to detect industrial roof moisture when there has not been adequate isolation (solar heating) to use the approach based on thermal capacitance. Roof moisture detection by thermal resistance requires that there be a minimum of 10 °C (18 °F) difference between interior and exterior surface temperature for at least 24 h before the survey. This approach is conducted at night with all surfaces clean and dry and with little or no wind (no greater than 15 mph). This approach is based on heat loss rather than solar gain. Saturated roof sections are better heat conductors (poorer insulators) with lower thermal resistance than dry sections, and the temperature difference between the interior and exterior will cause heat to be conducted more rapidly through wet sections than dry sections. Warmer areas on the exterior surface, therefore, indicate water saturation. Because there is a temperature differential between the interior and exterior, this approach is subject to artifacts caused by air flow and thermal conduction through the roof. For validity. the thermographer should be accompanied by supporting intrusive evidence such as roof core samples or by another non intrusive test such as electric capacitance or neutron backscatter.


■ Refractory SystemsIndustrial structures particularly refractory structures, readily lend themselves to thermographic investigations. Damage or wear to a refractory structure invariably results in the breakdown of thermal resistance. Heat escapes through the worn or damaged sections and can be seen on the thermogram. An example of this is illustrated ill Figure 5.10 where the slight vertical crack in the center of the stack results in a distinct temperature increase.


Figure 5.10: heat escaping from a worn refractory structure


Refractory Thermogram


Refractory Thermogram


■ Subsurface Discontinuity Detection in MaterialsSubsurface discontinuity detection in materials is characterized by steady state heat flow. which may be unstimulated or stimulated. Unstimulated steady state heat flow uses process heat such as that produced by buildings. HVAC systems etc. Stimulated steady state heat flow requires the addition of a source of (steady) heat or cold to establish sufficient heat flow through material.


■ The Unstimulated Measurement Approach to Infrared Materials Flaw DetectionThe unstimulated measurement approach uses the available heat flowing through the test sample. This occurs when products are being inspected during manufacture and the process being monitored produces or can be made to produce, the desired characteristic thermal pattern on the product surface. It occurs in injection molding, casting and drawing of products. An example of the unstimulated approach is illustrated in Figure 5. 11. On the left, areas of severe refractory breakdown in a boiler wall appear as the result of differences in heat flow because of the heat inherent in the boiler.


Figure 5.11: An example of passive IRNDT- a refractory break down in boiler


■ The Stimulated Measurement Approach to Infrared Materials Flaw DetectionWhen the desired characteristic thermal pattern on the product surface cannot be made to occur, or when the material samples or products are to be evaluated after manufacture, the stimulated, or thermal injection, approach is necessary. The stimulated approach can also involve thermal extraction, or the removal of heat from the sample, by introducing some form of cooling. Devices used for heat injection or extraction include the sun, air blowers, flood lamps, flash lamps, lasers, refrigerants, hot and cold water, chemical reactions, thermoelectric devices and mechanical heat sinks. In order for the stimulated approach to be effective, it requires the generation of a controlled flow of thermal energy across the Structure of the sample material under test. This is accompanied by thermographic monitoring of one of the surfaces (or sometimes both) of the sample, and the seareh for the anomalies in the thermal patterns so produced that will indicate a defect in accordance with established accept-reject criteria.


The equipment necessary to perform infrared materials discontinuity detection must include thermographic scanning instrumentation and the means to handle the test samples and to generate and control the injection or extraction of thermal energy to or from the samples. These can include hot and cold air blowers, liquid immersion baths. heat lamps. controlled refrigerants. electric current, scanned lasers and induction heating. The goal is to maximize the normal thermal flow, minimize the lateral thermal now (along the material surface), cause no permanent damage to the test samples, minimize and carefully meter the test time and generate the most uniform thermal pattern possible across the surface of the test sample. Because the source of energy is finite in dimension, the generation of a uniform thermal pattern on the sample surface is often difficult to accomplish. Using a personal computer with appropriate diagnostic software. a thermographer has access to numerous image manipulation routines including keyboard controlled image manipulation and subtraction. This image subtraction capability can be quite effective in compensating for limitations in heating pattern uniformity.


Figure 5.12 illustrates a typical infrared materials discontinuity detection configurat ion using the active (heat injection) method under computer control. When uniform heat is applied to one surface of a laminar test sample and an infrared scanner views the opposite surface, two types of defects are detectable. A metal occlusion within the structure has a higher thermal conductivity than the ply material and results in a warm (white) spot on the scanned surface. A void within the structure has a lower thermal conductivity than the ply material and results in a cool (dark) spot on the scanncd surface. The computer software can be used. when necessary, to nornlalize the effective temperature pattern before thermal insertion and to regulate the timing and intensity of the heat source. Available software also facilitates the precision timing and recording of test sequences so that they can be repeated with consistency.


Figure 5.12: Example of active (heat injection) IRNDT for occlusion and voiddetection


Material surface characteristics, as in any other thermographic application. are critical to test effectiveness. Materials with high and uniform surface emissivity are ideally suited for evaluation by infrared materials discontinuity detection. When evaluating samples with low or nonuniform emissivity, the thermographer has several alternatives. The first is to apply a removable, thin, high ernissivity coating, Another is to use an image subtraction routine as previously discussed in Chaptcr 3. This greatly reduces emissivity artifacts without affecting the material. Most materials successfully evaluated by infrared materials discontinuity detection are composed of layers of metals. plastics, composites or combinations of all three. The surfaces may be metal or plastic and the core structure may be solid, amorphous or geometrically configured (i.e. a honeycomb structure). Assembled layered sections (i.e. aircraft lapped sections) are also tested thermographically. The surfaces of the test amples facing the scanner are usually uniform in appearance and finish, although emissivity is low and surface scratches are frequently present.

Keywords:Emissivity artifact (Reflectivity)


Typical failure modes of the material samples are(1) voids between layers, (2) disbonds between layers, (3) impurities or foreign material in the laminar interfaces and (4) significam irregularities (damage) to the geometric core structure. Typical defects in assembled sections are loose or damaged welds and rivets and erosion/corrosion between sections. often accompanied by material loss and thinning.

Establishing test protocol involves determining acceptability of each part to be evaluated in terms of minimum size of void to be detected, minimum area of disbond that can be said to constitute a defect and any other void or disbond characteristic that is deemed significant. For this it is necessary to use known acceptable and known defective samples. Ideally, the defective samples furnished should include known defects of each classification and in the minimum sizes required to be detected and identified. When ideal defective samples are not available. it becomes necessary to synthesize flaws to simulate the minimum defects.


Selecting the infrared scanning system requires matching thermographic equipment performance capabilities to test criteria. To be effective, thermographic equipment used should offer resolution, sensitivity and versatility somewhat beyond that envisioned to be necessary to detect and identify the defects, the thermographer expects to encounter. The most critical of the scanner performance characteristics are (1) minimum resolvable temperature, (2) spatial resolution and (3) scan speed.

Figure 5.13 is an example of stimulated thermography that is ideal for the thermographer. Here the deicing mechanism on the wing of a DC·9 aircraft is evaluated. The deicing system also serves as the energizing source and the thermogram indicates areas that are not being heated as cool spots. The warm rings represent the instantaneous effect of the deicing mechanism.


Figure 5.13: Test of aircraft deicing element showing unheated areas on the wing of a DC-9


DC - 9


DC - 9


■ Stimulated Thermography Using Pulsed or Thermal Wave InjectionOne of the earliest applications of infrared materials discontinuity detection, performed as carly as 1970 was the detection of flaws in aircraft structures. This application continues to be an important one and most major airframe manufacturers have on going in-house infrared materials discontinuity detection programs. Innovations in heat injection techniques (i.e. the introduction of high intensity short-duration thermal pulses) have resulted in improved capability for detecting small and buried naws. These, coupled with the imroduclion of high speed focal plane array imagers and improvements in computer enhancement techniques for isolating and analyzing thermographic patterns and data, have had an important effect on image understanding and discontinuity recognition. The thermal wave technique is illustrated in Figure 5.14. Here, high intensity xenon flash lamps are used to irradiate the target surface with short duration pulses (on the order of milliseconds) of thermal energy. In many ways, this pulsed heating is similar to using the sun's heating cycle for the detection of underground voids. as previously discussed.


Figure 5.14: Conceptual sketch of thermal wave imaging


Xenon Lamp


In this case. however, the heat pulses and the detection intervals are thousands of times faster. While the surface cools, the heat is conducted into the material at a uniform rate until it reaches a thermal barrier or discontinuity, such as a flaw. At this time the temperature at the surface is lower than that at the discontinuity site, and a portion of the heat is conducted back to the surface, simulating a thermal echo. The time it takes from the generation of the pulse to the reheating at the surface, then, is an indication of the depth of the discontinuity. The behavior of the thermal energy moving through the material is similar in many ways to that of a wave of energy propagating through the material and being eflected back to the surface. For this reason the term thermal wave imaging has been adopted by some thermographers to deseribe the process. By using diagnostic software to time-gate the return thermal images, they can estimate the depths of flaws as well as their size and location, often with excellent precision. The term time resolved infrared radiometry is also used to describe the technique of selecting the image that best indicates the detected discontinuity.


Figure 5.15 illustrates a result of thermal wave injection and computer enhanced image analysis. The subject is erosion/corrosion damage in an aircraft skin lap joint. The high speed time-gating of images is essential because of the extremely high thermal diffusivity of the aluminum material. Within the past five years, time resolved infrared thermography has been successful to some extent in locating wall thinning because of erosion and corrosion in pipes and boiler tubes in utilities. Figure 5.16 is a time-resolved thermogram illustrating the resu lts of flash heating of a boiler tube section. The high lighted areas indicate maximum thinning.

Keywords:thermal wave imagingtime resolved infrared radiometry


Figure 5.15: Erosion/corrosion damage in a 737 aircraft lap joints elevated areas indicate erosion/corrosion, depressed areas are rivets


Figure 5.16: Time-resolved thermal Image of boiler wall section showing wall thinning due to corrosion


737 aircraft


5.6 Thermal Capacitance InvestigationsSeveral seemingly diverse applications have in common the fact that data sample timing is critical to accurate detection and analysis. These applications are those that are investigated on the basis of (usually nonhomogeneous) thermal capacitance and/or thermal diffusivity. Thermal capacitance and thermal diffusivity are discussed in Chapter 2.

Industrial Roof Moisture DetectionAs in most buildings and infrastructure applications. flat roof surveys are concerned with detection and identification of thermal patterns rather than quantitative measurements. These patterns are indications of subsurface moisture that is typically absorbed in the insulation. One approach to making these measurements depends on solar heating (insolation). This approach is conducted at night with all surface clean and dry and little or no wind (no greater Ihan 15 mph).


When there has been adequate solar heating of the roof during the day before the survey, stored thermal energy will cause water- saturated sections, with their higher thermal capacitance, to store more heal. At night. the roof radiates thermal energy to the cold sky. At some time during the night, the dry sections, with less stored heat, appear cool. The saturated sections appear warmer and the thermographer can easily locate and identify them. This procedure is particularly effective even when there is no temperature difference between the interior and exterior of the building. Unlike the thermal resistance approach previously discussed, this approach, illustrated in Figure 5.17 is subject to few thermal artifacts due to vent pipes, exhaust fans, etc.

In 1990, ASTM C1153-90, Standard Practice for the Location of Wet Insulation in Roofing Systems Using Infrared Imaging was released by the American Society for Testing and Materials. It outlines the minimum criteria for an acceptable infrared roof moisture survey and clearly stipulates the requirement for both dry and wet core samples. It also defines the minimum performance specifications of thermal sensing and imaging equipment used to perform thermographic surveys.


Figure 5.17: Thermogram with roof with moisture saturation.


Liquid Level DetectionThermal capacitance difference also allows thermographic detection of the liquid levels in storage tanks and other containers. In the thermogram of a fuel tank at night. shown in Figure 5.18. the fuel level is clearly evident because the fuel has a higher thermal capacitance than the air above it. The heat stored through solar absorption during the day maintains a higher temperature on the tank walls up to the fill level. Conversely. if the entire tank had cooled, the liquid would warm later than the air and the wall below the fill level would appear cooler than above.


Figure 5.18: Fuel level in a storage tank


Unstimulated and Stimulated Approaches to Infrared Materials Flaw DetectionMaterials discontinuity detection based on thermal capacitance differences is similar to that based on thermal resistance differences in that a stimulated approach may be used when the desired characteristic thermal pattern on the product surface cannot be madc to occur, or when the material samples or products are to be evaluated after manufacture. As previously discussed, this can involve thermal injection in a variety of forms, but it can also involve thermal extraction, or the removal of heat from the sample by introducing some form of cooling.


Underground Void DetectionThe detection of underground voids is based, for the most pan, on the difference in thermal capacitance between solid earth and the air cavities formed by buried tanks, eroded sewers and storm drains and improperly filled excavations. Typical programs to detect underground voids are performed using the sun as a basic source of thermal energy. During the day, the heat from the sun penetrates the earth and heats both the earth and the voids. The voids have a lower thermal capacitance and store less heat than the surrounding earth. On the subsequent thermographer they appear as cool areas. As in roof surveys, apparent findings are usually confirmed by means of other disciplines. Ground penetrating radar has come into use as a confirming discipline for thermographic underground void detection.


Subsurface Discontinuity Detection in MaterialsSubsurface discontinuity detection in materials is characterized by nonsteady (varying) heat flow through the subject, which can be unstimulated or stimulated. Unstimulated nonsteady heat flow uses (unsteady) process heat or a cool down after process heating. Stimulated nonsteady heat flow depends on the use of an (unsteady) source of heating or cooling.


Chapter 5Review Questions

Q&A1. b

2. b

3. c

4. a

5. b

6. a

7. b

8. c

9. d

10. a

11. d


1. A major area of infrared nondestructive material testing is based on the fact that:a. a good structural bond normalizes emittance artifacts.b. uniform structural continuity provides predictable thermal continuity.c. a structural void is a good thermal bond.d. thermal imagers can be made to measure temperature with great accuracy.

2. When analyzing a thermographic image. it is usually possible to distinguish between an overload condition and a loose connection because:a. a loose connection will appear cool compared to its surroundings.b. a loose connection will appear warmer than the wires on either side.c. an overload will cause a sharper thermal gradient.d. one side of a loose connection will appear much warmer than the other.


3. The most significant advantage of thermal wave imaging over conventional step stimulation methods of infrared/thermal materials testing is that:a. it can find smaller voids.b. it is simpler to implement.c. it can providc better information regarding the depth of a discontinuity.d. it provides images with better spatial resolution.

4. The diagnostics involved in thermography of electrical switchgear most frequently involves:a. exothermic invesligations.b. thermal resistance investigations.c. security investigations.d. fluid flow investigations.


5. The diagnostics involved in detection of moisture in flat roofs most frequently involve:a. exothermic and endothermic investigations.b. thermal resistance and thermal capacitance investigations.c. friction investigations.d. fluid flow investigations.

6. In time resolved thermography (thermal wave imaging) applied to materials nondestructive testing. the time of the return signal from a void or disband is most closely related to the:a. depth of the discontinuity.b. size of the discontinuity.c. amplitude of the heating pulse.d. spectral characteristics of the heating pulse.


7. In the process monitoring of thin film plastics successful thermographic measurement is most closely related to:a. correcting the instrument for background reflections.b. matching the spectral characteristics of the instrument to those of the target material.c. optimizing the speed of response of the measuring instrument.d. optimizing the spatial resolution of the measuring instrument.

8. The unstimulated approach to infrared nondestructive testing can usually be used when evaluating the condition of refractory linings of vessels because:a. refractory materials have high effective emissivities.b. refractory materials have high reflectivity in the infrared.c. a strong. uniform source of heat usually exists within the vessel.d. infrared focal plane imagers are available for these applications.


9. Thermography has been successfully applied to some veterinary medicine applications because, in most cases:a. healthy animals are hotter than sick animals.b. the emissivity of animal hides is high.c. animals have higher body temperatures than humans.d. infection and trauma usually cause the affected portion of the body to become warmer.

10. The use of thermography for the detection of moisture infiltration in airframes is made possible by a combination of thermal capacitance differences and:a. an endothermic effect that causes the infiltrated portions to appear cooler.b. an exothermic effect that causes the infiltrated portions to appear warmer.c. increased friction between the air flow and the infiltrated sections.d. reduced friction between the air flow and the infiltrated sections.


11. Sulface thermal patterns can often reveal:a. subsurface material defects.b. delaminations within a structurc.c. impurities within a material sample.d. all of the above.


Appendix AGlossaryThe following are explanations and definitions of terms commonly encountered by the infrared thermographer Many of these terms have multiple definitions and the one provided is the one most applicable to infrared thermography. NOTE: In some cases, the "textbook" definition of a term is replaced by one more explicitly dealing with the practice o/infrared thermography.


1. Absolute zero - The temperature that is zero on the Kelvin or º Rankine temperature scales. The temperature at which no molecular motion takes place in a material.

2. Absorptivity, α (absorptance) - The proportion (as a fraction of 1) of the radiant energy impinging on a material's surface that is absorbed into the material. For a blackbody, this is unity (1.0). Technically, absorptivity is the internal absorptance per unit path length. In tthermography, the two terms are often used interchangeably.

3. Accuracy (of measurement) - The maximum deviation, expressed in percent of scale or in degrees celsius or degrees fahrenheit. that the reading of an instrument will deviate from an acceptable standard reference, normally traceable to the National Institute for Standards and Technology (N IST).

4. Ambient operating range - Range of ambient temperatures over which an instrument is designed to operate within published performance specifications.

5. Ambient temperature - Temperature of the air in the vicinity of the target (target ambient) or the instrument (instrument ambient)

6. Ambient temperature compensation - Correction built into an instrument to provide automatic compensation in the measurement for variations in instrument ambient temperature.


Ambient temperature - Temperature of the air in the vicinity of the target (target ambient) or the instrument (instrument ambient)

instrument ambient

target ambient


7. Anomaly - An irregularity, such as a thermal anomaly on an otherwise isothermal surface; any indication that deviates from what is expected.

8. Apparent temperature - The target surface temperature indicated by an infrared point sensor, line scanner or imager.

9. Artifact ~ A product of artificial character because of extraneous agency; an error caused by an uncompensated anomaly. In thermography, an emissivity artifact simulates a change in surface temperature but is not a real change.

10.Atmospheric windows (infrared) ~ The spectral intervals within the infrared spectrum in which the atmosphere transmits radiant energy well (atmospheric absorption is a minimum). These are roughly defined as 2 to 5 μm and 8 to 14 μm .

11.Background temperature, instrument - Apparent ambient temperature of the scene behind and surrounding the instrument as viewed from the target. The reflection of this background may appear in the image and affect the temperature measurement. Most quantitative thermal sensing and imaging instruments provide a means for correcting measurements for this reflection. (See Figure A- 1.)

12.Background temperature, target - Apparent ambient temperature of the scene (1) behind and (2) surrounding the instrument, as viewed from the instrument. When the FOV of a point sensing instrument is larger than the target, the target background temperature will affect the instrument reading. (See Figure A-1 .)


Figure A-1

Apparent ambient temperature of the scene (1) behind and (2) surrounding the instrument


Background Temperature

Instrument Background

Target Background


13.Blackbody, blackbody radiator - A perfect emitter; an object that absorbs all the radiant energy impinging on it at all wavelengths and reflects and transmits none. A surface with emissivity of unity (1.0) at all wavelengths.

14.Bolometer. infrared ~ A type of thermal infrared detector. 15.Calibration - Checking and/or adjusting an instrument such that its readings agree

with a standard . 16.Calibration check - A routine check of an instrument against a reference to ensure

that the instrument has not deviated from calibration since its last use. 17.Calibration accuracy - The accuracy to which a calibration is performed. usually

based on the accuracy and sensitivity of the instruments and references used in the calibration.

18.Calibration source, infrared - A blackbody or other target of known temperature and effective emissivity used as in calibration reference.

19.Capacitance, thermal - This term is used to describe heat capacity in terms of an electrical analog, where toss of heat in analogous to loss of charge on a capacitor. Structures with high thermal capacitance change temperature more slowly than those with low thermal capacitance.

20.Capacity, heat - The heat capacity of a material or structure describes its ability to store heat. It is the product of the specific heat (cp) and the density (ρ) of the material. This means that denser materials generally will have higher heat capacities than porous materials.


Capacity, heat (Volumetric Heat Capacity) - The heat capacity of a material or structure describes its ability to

store heat. It is the product of the specific heat (cp) and the density (ρ) of the material. This means that denser materials generally will have higher heat capacities than porous materials.

Heat Capacity volumetric = Cp x ρfor my ASNT exam


Alas!

Heat Capacity Volumetric

=

Cp∙ρ

for my ASNT exam


21.Celsius (Centigrade) - A temperature scale based on 0 °C as the freezing point of water and 100 °C as the boi ling point of water at standard atmospheric pressure; a relative scale related to the Kelvin scale [ 0 °C = 273.12 K; 1ºC (ΔT): 1 K (ΔT) ].

22.Color - A ternl sometimes used to deline wavelength or spectral interval, as in two-color radiometry (meaning a method that measures in two spectral intervals); also used conventionally (visual color) as a means of displaying a thermal image, as in color thermogram.

23.Colored body - See nongraybody. 24.Conduction - The only mode of heat now in solids, but can also take place in

liquids and gases. It occurs as the result of (1) atomic vibrations (in solids) and (2) molecular collisions (in liquids and gases) whereby energy is transferred from locations of higher temperature to locations of lower temperature.

25.Conductivity, thermal, (k) - A material property defining the relative capability to carry heat by conduction in a static temperature gradient. Conductivity varies Slightly with temperature in solids and liquids and with temperature and pressure in gases. It is high for metals (copper has a k of 380 W/m∙°C) and low for porous materials (concrete has a k of 1.0 W/m∙°C) and gases.

26.Convection - The form of heat transfer that takes place in a moving medium and is almost always associated with transfer between a solid (surface) and a moving fluid (such as air). whereby energy is transferred from higher temperature sites to lower temperature sites.


27.Detector, infrared - A transducer element that converts incoming infrared radiant energy impinging on its sensitive surface to a usefu l electrical signal.

28.Diffuse reflector - A surface that reflects a portion of the incident radiation in such a manner that the reflected radiation is equal in all directions. A mirror is not a diffuse reflector.

29.Diffusivity, thermal, (α) - (Note: same symbol as absorptivity may be confusing.) The ratio of conductivity (k) to the product of density (ρ) and specilic heat (Cp)[ α = k/ρ∙Cp cm2 s-1 ]. The ability of a material to distribute thermal energy after a change in heat input. A body with a high diffusivity will reach a uniform temperature distribution faster than a body with lower diffusivity.

30.D* (detectivity star) - Sensitivity figure of merit of an infrared detector - detectivity expressed inversely so that higher D* indicate better performance; taken at specific test conditions of chopping frequency and information bandwidth and displayed as a function of spectral wavelength.

31.Display resolution, thermal - The precision with which an instrument displays its assigned measurement parameter (temperature). usually expressed in degrees, tenths of degrees, hundredths of degrees. etc.

32.Effective emissivity (Ɛ*) - The measured emissivity value of a particular surface under existing measurement conditions (rather than the generic tabulated value for the surface material) that can be used to correct a specific measuring instrument to provide a correct temperature measurement.


33. Effusivity, thermal (e) - A measure of the resistance of a material to temperature change:

e =(kρCp) ½ cm2 ºC-1 S1/2

where:k = thermal conductivityρ = bulk densityCp = specific heat

Comments: compare diffusivityα = (k/ρ)∙Cp cm2 s-1


Thermal EffusivityIn Thermodynamics, the thermal effusivity of a material is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.

e = (kρCp)½ cm2 ºC-1 S1/2

Here, k is the thermal conductivity, ρ is the density and Cp is the specific heat capacity. The product of ρ and Cp is known as the volumetric heat capacity.

A material's thermal effusivity is a measure of its ability to exchange thermal energy with its surroundings. If two semi-infinite bodies initially at temperatures T1 and T2 are brought in perfect thermal contact, the temperature at the contact surface Tm will be given by their relative effusivities.

This expression is valid for all times for semi-infinite bodies in perfect thermal contact. It is also a good first guess for the initial contact temperature for finite bodies.


34. Emissivity (Ɛ) - The ratio of a target surface's radiance to that of a blackbody at the same temperature, viewed from the same angle and over the same spectral interval; a generic lookup value for a material. Values range from 0 to 1.0.

35. EMI/RFI noise - Disturbances to electrical signals caused by electromagnetic interference (EMI) or radio frequency interference (RFI). In thermography, this may cause noise patterns to appear on the display.

36. Environmental rating - A rating given an operating unit (typically an electrical or mechanical enclosure) to indicate the limits of the environmental conditions under which the unit will function reliably and within published performance specifications.

37. Exitance, radiant (also called radiosity) - Total infrared energy (radiant flux) leaving a target surface. This is composed of radiated. reflected and transmitted components. Only the radiated component is related to target surface temperature.

38. Fahrenheit - A temperature scale based on 32 ºF as the freezing point of water and 212 ºF as the boiling point of water at standard atmospheric pressure; a relative scale related to the Rankine scale [ 0 ºF = 459.67 ºR; 1 ºF (ΔT) = 1 ºR (ΔT) ].

39. Field of view (FOV) - The angular subtense (expressed in angular degrees or radians per side if rectangular, and angular degrees or radians if circular) over which an instrument will integrate all incoming radian energy. In a radiation thermometer this denotes the target spot size; in a scanner or imager this denotes the scan angle or picture size or total field of view.


Alas!

Exitance = Rodiosity

for my ASNT exam


40.Fiber optic, infrared - A flexible fiber made of a material that transmits infrared energy, used for making noncontact temperature measurements when there is not a direct line of sight between the instrument and the target.

41.Filter, spectral - An optical element, usually transmissive, used to restrict the spectral band of energy received by an instrument's detector.

42.Focal plane array (FPA) - A linear or two-dimensional matrix of detector elements, typically used at the focal plane of an instrument. In thermography, rectangular FPAs are used in staring (nonscanning) infrared imagers. These are called infrared focal plane array imagers.

43.Focal point - The point at which the instrument optics image the infrared detector at the target plane. In a radiation thermometer, this is where the spot size is the smallest. In a scanner or imager, this is where the instantaneous field of view ([FOV) is smallest.

44.Foreground temptrature (see instrument ambient background) - Temperature of the scene behind and surrounding the instrument as viewed from the target. (See Figure A-1.)


45.Frame repetition rate - The time it takes an infrared imager to scan (update) every thermogram picture element (pixel); in frames per second.

46.Full scale - The span between the minimum value and the maximum value Ihat any instrument is capable of measuring. In a thermometer, this would be the span between the highest and lowest temperature that can be measured.

47.Graybody - An radiating object whose emissivity is a constant value less than unity ( 1.0), over a specific spectral range.

48.Hertz (Hz) – A unit of measurement of signal frequency; 1 Hz = 1 cycle per second. Image, infrared - See Thermogram.

49. Imager, infrared - An infrared instrument that collects the infrared radiant energy from a target surface and produces an image in monochrome (black and white) or color, where the gray shades or color hues correspond respectively to target exitance.

50. Image display tone - Gray shade or color hue on a thermogram. 51. Image processing, thermal - Analysis of thermal images, usually by computer;

enhancing the image to prepare it for computer or visual analysis. In the case of an infrared image or thermogram, this could include temperature scaling, spot temperature measurements, thermal profiles, image manipulation, subtraction and storage.


52. Imaging radiometer - An infrared thermal imager that provides quantitative thermal images.

53. Indium Antimonide (InSb) - A material from which fast, sensitive photo detector used in infrared scanners and imagers are made. Such detectors usually requiring cooling while in operation. Operation is in the short wave band (2 to 5 μm).

54. Inertia, thermal - See thermal effusivity. 55. Infrared - The infrared spectrum is loosely defined as that portion of the

electromagnetic continuum extending from the red visible (0.75 μm) to about 1000 μm (1mm).Because of instrument design considerations and the infrared transmission characteristics of the atmosphere, however. most infrared measurements are made between 0.75 and 20 μm.

56. Infrared focal plane array (lRFPA) - A linear or two-dimensional matrix of individual infrared detector elements, typically used as a detector in an infrared imaging instrument.

57. Infrared radiation thermometer - An instrument that converts incoming infrared radiant energy from a spot on a target surface to a measurement value that can be related to the temperature of that spot.

58. Infrared thermal imager - An instrument or system that converts incoming infrared radiant energy from a target surface to a thermal map or thermogram, on which color hues or gray shades can be related to the temperature distribution on that surface.


Thermal InertiaThermal inertia is a term commonly used by scientists and engineers modelling heat transfers and is a bulk material property related to thermal conductivity and volumetric heat capacity. For example, this material has a high thermal inertia, or thermal inertia plays an important role in this system, which means that dynamic effects are prevalent in a model, so that a steady-state calculation will yield inaccurate results. The term is a scientific analogy, and is not directly related to the mass-and-velocity term used in mechanics, where inertia is that which limits the acceleration of an object. In a similar way, thermal inertia is a measure of the thermal mass and the velocity of the thermal wave which controls the surface temperature of a material. In heat transfer, a higher value of the volumetric heat capacity means a longer time for the system to reach equilibrium.

The thermal inertia of a material is defined as the square root of the product of the material's bulk thermal conductivity and volumetric heat capacity, where the latter is the product of density and specific heat capacity:

e = I = √(kρCp) See also Thermal effusivity

k = is thermal conductivity, with unit [W m−1 K−1]ρ = is density, with unit [kg m−3]Cp = is specific heat capacity, with unit [J kg−1 K−1]e, I = has SI units of thermal inertia of [J m−2 K−1 s−1/2].

http://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia


59. Instantaneous field of Fiew (lFOV) - The angular subtense (expressed in angular degrees or radians per side if rectangular and angular degrees or radians if round) over which an instrument will integrate all incoming radiant energy; the projection of the detector at the target plane. In a radiation thermometer this denotes the target spot size; in a line scanner or imager it representS one resolution clement in a scan line or a thermogram and is a measure of spatial resolution. (D=α∙d)

60. IRFPA imager or camera – An infrared imaging instrument that incorporates a two-dimensional infrared focal plane array and produces a thermogram without mechanical scanning.

61. Isotherm - A pattern superimposed on a thermogram or on a line scan that includes or highlights all points that have the same apparent temperature Kelvin - Absolute temperature scale related to the celsius (or Centigrade) relative scale. The Kelvin unit is equal to 1 °C; 0 Kelvin = - 273.16 °C; the degree sign and the word degrees are not used in describing Kelvin temperatures.


Instantaneous field of View (lFOV)

D=σ∙dIFOV ratio = d/D or 1/σ

(care on unit used!)

for my ASNT exam


62. Laser pyrometer - An infrared radiation thermometer that projects a laser beam to the target, uses the reflected laser energy to compute target effective emissivity and automatically computcs target temperature (assuming that the target is a diffuse reflector) - not to be confused with laser-aided aiming devices on some radiation thermometer.

63. Line scan rate - The number of target lines scanned by an infrared scanner or imager in one second.

64. Line scanner, infrared - An instrument that scans an infrared field of view FOV along a straight line at the target plane to collect infrared radiant energy from a line on the target surface, usually done by incorporating one scanning element within the instrument. If the target (such as a sheet or web process) moves at a fixed rate normal to the line scan direction, the result can be displayed as a thermogram.

65. Measurement spatial resolution, lFOVmeas - The smallest target spot size on which an infrared imager can produce a measurcment, expressed in terms of angular subtense (mRad per side). The slit response function (SRF) test is used to measure measurement spatial resolution / IFOVmeas.

66. Medium, transmitting medium – The composition of the measurement path between a target surface and the measuring instrument through which the radiant energy propagates. This can be vacuum, gaseous (such as air), solid, liquid or any combination of these.


Laser pyrometer - Laser pyrometer - An infrared radiation thermometer that projects a laser beam to the target, uses the reflected laser energy to compute target effective emissivity and automatically computcs target temperature (assuming that the target is a diffuse reflector) - not to be confused with laser-aided aiming devices on some radiation thermometer.

Further reading on this subject is necessary.


67. Mercury cadmium telluride MCT (HgCdTe) - A material used for fast, sensitive infrared photodetectors used in infrared sensors, scanners and imagers that requires cooled operation. Operation is in the long wave length region (8 to 12 μm).

68. Micron (micrometer) (μ or μm) - One millionth of a meter; a unit used to express wavelength in the infrared.

69. Milliradian (mRad) - One thousandth of a radian (1 radian = 180/π); a unit used to express instrument angular field of view

70. Minimum resolvable temperature (difference), MRT(D) - thermal resolution; thermal sensitivity - the smallest temperature difference that an instrument can clearly distinguish out of the noise, taking into account characteristics of the display and the subjective interpretation of the operator.

71. Modulation - In general, the changes in one wave train caused by another; in thermal scanning and imaging, image luminant contrast; (Lmax - Lmin)/(Lmax + Lmin).

72. Modulation Transfer Function (MTF) - A measure of the ability of an imaging system to reproduce the image of a target. A formalized procedure is used to measure modulation transfer function; It assesses the spatial resolution of a scanning or imaging system as a function of distance to the targe!.


Figure 3.2: Response Curves of Various Infrared Detectors


73. Noise equivalent temperature (difference), NET(D) - The temperature difference that is just equal to the noise signal; a measure of thermal resolution, but not taking into account characteristics of the display and the subjective interpretation of the operator.

74. NIST, NlST traceability - The National Institute of Standards and Technology (formerly NBS). Traceability to NIST is a means of ensuring that reference standards remain valid and their calibration remains current.

75. Nongraybody - A radiating object that does not have a spectral radiation distribution similar to a blackbody and can be partly transparent to infrared (transmits infrared energy at certain wavelengths); also called a colored body. Glass and plastic films are examples of nongraybodies. The emissivity of a colored body has a spectral dependence.

76. Objective lens - The primary lens of an optical system, On an infrared instrument, usually the interchangeable lens that denotes the total field of view.

77. Opaque - Impervious to radiant energy. In thermography, an opaque material is one that does not transmit thermal infrared energy, (τ = 0).

78. Optical element, infrared - Any element that collects, transmits restricts or reflects infrared energy as part of an infrared sensing or imaging instrument.

79. Peak hold - A feature of an instrument whereby an output signal is maintained at the peak instantaneous measurement for a specified duration.


Compare:Minimum resolvable temperature (difference), MRT(D) - thermal resolution; thermal sensitivity - the smallest temperature difference that an instrument can clearly distinguish out of the noise, taking into account characteristics of the display and the subjective interpretation of the operator.

Noise equivalent temperature (difference), NET(D) - The temperature difference that is just equal to the noise signal; a measure of thermal resolution, but not taking into account characteristics of the display and the subjective interpretation of the operator..


80. Photodetector (photon detector) - A type of infrared detector that has fast response (on the order of microseconds), limited spectral response and usually requires cooled operation: photooctectors are used in infrared radiation thermometer. scanners and imagers, because, unlike thermal detector, direct photon interaction obviates 防止 external heating of the detector for the signal to be sensed.

81. Pyroelectric detector - A type of thermal infrared detector that acts as a current source with its output proportional to the rate of change of its temperature.

82. Pyroelectric vidicon (PEV), also called pyrovidicon - A video camera tube with its receiving elemen! fabricated of pyroelectric material and sensitive to wavelengths from about 2 to 20 μm; used in infrared thermal viewers.

83. Pyrometer - Any instrument used for temperature measurement. (1) A radiation or brightness pyrometer measures visible energy and relates it to brightness or color temperature. (2) An infrared pyrometer measures infrared radiation and relates it to target surface temperature.

84. Radian - An angle equal to 180 degrees/π or 57.29578 angular degrees. 85. Radiation, thermal - The mode of heat flow that occurs by emission and

absorption of electromagnetic radialion. propagating at the speed of light and unlike conductive and convective heal flow, capable of propagating across a vacuum; the form of heat transfer that allows infrared tthermography to work because infrared energy travels from the target to the detector by radiation.


86.Radiation rererenee source - A blackbody or other target of known temperature and effective emi ssivity used as a reference 10 obtain optimum measurement accuracy. ideally. traceable to NIST.

87.Radiation thermometer – See infrared radiation thermometer. 88.Radiosity – See Exitance, thermal. 89.Rankine - Absolute temperature scale related to the fahrenheit relative scale. The

Rankine unit is equal to 1ºF; 0 Rankine = - 459.72 ºF ; the degree sign and the word degrees is not used in describing Rankine temperatures.

90.Ratio pyrometer - An infrared thermometer that uses the ratio of incoming infrared radiant energy at two narrowly separated wavelengths to detennine a target's temperature independent of target emittance; this assumes graybody conditions and is normally limited to relatively hot targets (above about 149 ºC, 300 ºF).

91.Reference junction - In a thermocouple. the junction of the dissimilar metals that is not the measurement junction. This is normally maintained at a constant reference temperature.

92.Reflectivity, (reflectance) (ρ) - The ratio of the total energy reflected from a surface to total incidence on that surface; ρ = 1 - Ɛ - τ; for a perfect mirror this approaches 1.0; for a blackbody the reflectivity ρ is 0. Technically, reflectivity is the ratio of the intensity of the reflected radiation to the total radiation and reflectance is the ratio of the reflected flux to the incident flux. In tthermography, the two terms are often used interchangeably. (only subtraction where is the division, ratio?)


93. Relative humidity - The ratio (in percent) of the water vapor content in the air to the maximum content possible at that temperature and pressure.

94. Repeatability - The capability of an instrument to exactly repeat a reading on an unvarying target over a short or long term time interval. For thermal measurcments, expressed in ±degrees or a percentage of full scale.

95. Resistance, thermal (R) – A measure of a material's resistance to the flow of thermal energy, inversely proportional to its thermal conductivity, k. (1/R = k)

96. Response time - The time it takes for an instrument output signal or display to respond to a temperature step change at the target; expressed in seconds. (typically, to 95 percent of the final value and approximately equal to 5 time constants)

97. Resistance temperature detector (RTD) – a sensor that measures temperature by a change in resistance as a funct ion of temperature.

98. Sample hold - A feature of an instrument whereby an output signal is maintained at an instantaneous measurement value for a specified duration after a trigger or until an external reset is applied.

99. Scan angle - For a line scanner, the total angular scan possible at the target plane, typically 90 degrees.

100. Scan position accuracy - For a line scanner. the precision with which instantaneous position along the scan line can be set or measured.


101. Sector - For a line scanner, a portion of the total scan angle over which measurement is made at the target plane.

102. Seebeck effect - The phenomenon that explains the operation of thermocouples; that in a closed electrical circuit made up of two junctions of dissimilar metal conductors, a direct current will flow as long as the two junctions are at different temperatures, The phenomenon is reversible: if the temperatures at the two junctions are reversed. the flow of current reverses.

103. Sensitivity - See minimum resolvable temperature (difference), MRT(D). 104. Setpoint - Any temperature setting at which an activating signal or closure can be

preset so that. when the measured temperature reaches the setpoint, a control signal, pulse or relay closure is generated.

105. Shock - A sudden application of force, for a specific time duration; also the temporary or permanent damage to a system as a result of a shock.

106. Signal processing - Manipulation of temperature signal or image data for purposes of enhancing or controlling a process. Examples for (1) infrared radiation thermometer are peak hold, valley hold, sample hold and averaging. Examples for (2) infrared scanners and (3) infrared imagers are usually referred to as image processing and include isotherm enhancement. image averaging, alignment, image subtraction and image filtering.

107. Slit response function - A measure of the measurement spatial resolution (IFOVmeas) of an infrared scanner or imager.


108. Spatial resolution - The spot size in terms of working distance. In an infrared radiation thermometer this is expressed in milliradians or as a ratio (DId) of the target spot size (containing 95 percent of the radiant energy, according to common usage) to the working distance. In scanners and imagers it is most often expressed in milliradians.

109. Spectral response - The spectral wavelength interval over which an instrument or sensor responds to infrared radiant energy, expressed in micrometers (}lm) - also, the relative manner (spectral response eUlVe) in which it responds over that intelVal.

110. Specular (('Occtor - A smooth refl ecting surface that reflects all incident radiant energy at an angle complementary (equal around the nomlal) to the angle of incidence, A mirror is a specular refl ector.

111. Spot - The instantaneous size (diameter unless otherwise specified) of the area at the target plane that is being measured by the instrument. In infrared thermometry, this is specifi ed by most manufacturers to contain 95 percent of the radiant energy of an infin itely large target of the same temperature and emissivity.

112. Storage operating range - 1be temperature extremes over which an instrument can be stored and. subsequently, operate within published performance specifications.


113. Subtense, angular - The angular diameter of an optical system or subsystem, expressed in angular degrees or mRad. In thermography, the angle over which a sensing instrument collects radiant energy.

114. Target - The object surface to be measured or imaged. 115. Temperature - A measure of the thermal energy contained by an object; the

degree of hotness or coldness of an object measurable by any of a number or relative scales; heat is defined as thermal energy in transit and flows from objects of higher temperature to objects of lower temperature.

116. Temperature conversion – Convening from one temperature scale to another; the relationships are: Celsius = (Fahrenheit -32) x 5/9, Fahrenheit = 9/5 x Celsius + 32, 1 °C (ΔT) = 5/9 ºF (Δ.T), 0 °C = 273.12 Kelvin: 0 ºF = 459.67 Rankine.

117. Temperature measurement drift - A reading change (error), with time of a target with non-varying temperature that may be caused by a combination of (1) ambient changes, (2) line voltage changes and (3) instrument characteristics.

118. Temperature resolution - See minimum resolvable temperature (difference), MRT(D),


119. Thermal detector, infrared - A type of infrared detector that changes electrical characteristics as a function of temperature; typically. thermal detectors have slow response, (on the order of milliseconds) broad spectral response and usually operate at room temperature: thermal detectors are commonly used in infrared radiation thermometers and in some imagers. (See Photodetector ≡ photon detector)

120. Thermal viewer - A non-measuring thermal imager that produces qualitative thermal images related to thermal radiant distribution over the target surface.


121. Thermal wave imaging - A term used to describe an active technique for infrared nondestructive material testing in which the sample is stimulated with pulses of thermal energy and where the timebased returned thermal images are processed to determine discontinuity depth and severity; also called pulse stimulated imaging.

122. Thermistor - A temperature detector. usually a semiconductor, whose electrical resistivity decreases predictably and nonlinearly with increasing temperature.

123. Thermistor bolometer, infrared - A thermistor so configured as to collect radiant infrared energy; a type of thermal infrared detector.


124. Thermocouple - A device for measuring temperature based on the fact that opposite junctions between certain dissimilar metals develop an electrical potential when placed at diffcrent temperatures; typical thermocouple types are;

J iron/constantanK chromeValumelT copper/constantanE chromel/constantanR platinumlplatinum-30 percent rhodiumS platinumlplatinum- I0 percent rhodiumB platinum-6 percent rhodium/platinum-30 percent rhodiumG tungsten/tungsten-26 percent rheniumC tungsten-5 percent rhenium/tungsten-26 percent rheniumD tungsten-3 percent rheniumltungsten-25 percent rhenium


125. Thermogram - A thermal map or image of a target where the gray tones or color hues correspond to the distribution of infrared thermal radiant energy over the surface of the target (qualitative thermogram); when correctly processed and corrected, a graphic representation of surface temperature distribution (quantitative thermogram).

126. Thermograph - Another word used to describe an infrared thermal imager. 127. Thermometer - Any device used for measuring temperature. 128. Thermopile - A device constructed by the arrangement of thermocouples in

series to add the thermoelectric voltage. A radiation thermopile is a thermopile with junctions so arranged as to collect infrared radiant energy from a target, a type of thermal infrared detector.

129. Time constant - The time it takes for any sensing element to respond to 63.2 percent of a step change at the target being sensed. In infrared sensing and thermography, the time constant of a detector is a limiting factor in instrumcnt performance, as it relates to response time. (?)

130. Total field of view (TFOV) - In imagers, the total solid angle scanned, usually rectangular in cross section. (TFOV=FOV?)

131. Transducer - Any device that can convert energy from one form to another. In thermography, an infrared detector is a transducer that converts infrared radiant energy to some useful electrical quantity.


132. Transfer calibration - A technique for correcting a temperature measurement or a thermogram for various errors by placing a radiation reference standard adjacent to the larget.

133. Transfer standard - A precision radiometric measurement instrument with NTST traceable calibration used to calibrate radiation reference sources.

134. Transmissivity, (transmiUance) (τ) - The proportion of infrared radiant energy impinging on an object's surface, for any given spectral interval thai is transmitted through the object. (τ = 1 – Ɛ - ρ) For a blackbody. transmissivity τ = O. Transmissivity is the internal transmittance per unit thickness of a non-diffusing material.

135. Two-color pyrometer - See ratio pyrometer. 136. Unity - One (1.0). 137. Valley hold - A feature of an instrument whereby an output signal is maintained at

the lowest inslantaneous measurement for a specified duration; opposite of peak hold.

138. Working distance – The distance from the target to the instrument, usually to the primary optic.

139. Zone - In line scanners. a scanned area created by the transverse linear motion of the product or process under a measurement sector of the scanner.


Appendix BCost Benefit Determination


The sample worksheet at the end of this description provides a protocol for estimating cost benefits of any finding. Following the EPRI M&D Center Guidelines for cost benefit detcnnination. the benefits of detecting a failure mechanism at work on a system or component before failure are quantified in tcnns of probable dollars saved. To do this, the costs of eliminating the failure mechanism in a timely fashion are compared to the likely costs incurred if the failure mechanism was not corrected and the component or system failed. The approach used in the analysis considers three possible failure scenarios:

1. worst case (catastrophic failure),2. possible case (moderate failure). and3. probable case (minor failure - the failure most likely to occur).

The following three calculations are used to estimate failure scenarios:

1. estimate the percentage likelihood out of 100 percent of each of the three scenarios occurring - with the sum of the three percentages equal to 100 percent;

2. multiply the projected cost of each of the three scenarios by its estimated percent likelihood - the sum of these three products is the weighted estimated savings by not having to do any of them; and

3. estimate the cost benefit by comparing the actual cost of the timely service or repair to thc wcighted estimated savings.


Although the calculations are quite straightforward. the effective use of the guidelines is far from trivial because filling in the blanks can be a challenge. Here your historical database can be of substantial help. Of equal importance is a thorough knowledge of the criticality of the component or system to the operation of the facility to project the nature and extent of each of the failure scenarios. If your knowledge in this area is limited, rely on appropriate facility personnel for the information.

The historical database can help you estimate the percent likelihood of each scenario. as well as the associated costs. When preparing cost estimates. remember to include man hours, transportation of parts and equipment, cost of rcplacemem parts and equipment and damage to adjacent equipment. Avoided maintenance may also be included. Another factor in cost benefit determination that is worth considering is the long term savings in excess power that would have been consumed by components and systems restored to optimum operational efficiency by timely service or repair. Overheated componems invariably draw more current than they should either through direct I2R loss or because of excess friction or other inefficiencies. These kilowatts of power lost represent lost revenue – for every every hour that the situation is not corrected, kilowatt hours are lost in the form of dollars that cannot be billed to customers.


These dollars lost are in proportion to the square of the excess current and can be calculated for an electrical component if you know the excess current, I, and the resistance,

1. ΔP(W) = (ΔI)2R

Then divide ΔP by 1000 to convert to kilowatts and multiply by the average rate charged for a kilowatt hour. This will tell you how much every hour of non optimum operation is costing the facility,

2. Dollars lost = (lost KWH) x (Dollars/KWH).

In a rotating component, if you know the rated power consumption (watts) and the rated current, you can calculate the effective resistance and proceed as in step 1 above.


Appendix CCommonly Used Infrared Specifications and Standards


Appendix C, Commonly Used Infrared Specifications and Standards






End Of Reading


Peach – 我爱桃子


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NASA EDDY CURRENT TESTING RQA/M 1-5330 .17


■ωσμ∙Ωπ∆º≠δ≤>ηθφФρ|β≠Ɛ ʋ ∠ λ α ρτ√


Good Luck


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Charlie Chong/ Fion Zhang https://www.yumpu.com/en/browse/user/charliechong

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