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Review -
Pyrometers
Review -
PyrometersManually operated / Automatic
Sim Soo Young
Previously on “Pyrometers” by (1/4)
• Simplest & oldest non-contact way of estimating the temperature of a radiating body
by observing its color
• radiation thermometer (e.g. infrared thermometer)
• radiation wavelengths: visible & infrared radiation bands
(0.4 ~ 20 μm)
• Simplest & oldest non-contact way of estimating the temperature of a radiating body
by observing its color
• radiation thermometer (e.g. infrared thermometer)
• radiation wavelengths: visible & infrared radiation bands
(0.4 ~ 20 μm)
Previously on “Pyrometers” by (2/4)
• Classification of pyrometers by wavelength and operating method
Previously on “Pyrometers” by (3/4)
• Manually operated pyrometers
- operator’s eye acts as a comparator
1) Disappearing filament pyrometers
2) Two-color pyrometers (= Ratio pyrometers)
• Manually operated pyrometers
- operator’s eye acts as a comparator
1) Disappearing filament pyrometers
2) Two-color pyrometers (= Ratio pyrometers)
Previously on “Pyrometers” by (4/4)
• Automatic pyrometers
- optical system concentrating the radiation on radiation detector
- radiation detector which may be either a thermal or a photoelectric sensor
- signal converter, conditioning the detector output signal before being displayed
- measuring part, which may have an additional analogue or digital output
1) Total radiation pyrometers 2) Photoelectric pyrometers
3) Two-wavelength pyrometers 4) Multi-wavelength pyrometers
• Automatic pyrometers
- optical system concentrating the radiation on radiation detector
- radiation detector which may be either a thermal or a photoelectric sensor
- signal converter, conditioning the detector output signal before being displayed
- measuring part, which may have an additional analogue or digital output
1) Total radiation pyrometers 2) Photoelectric pyrometers
3) Two-wavelength pyrometers 4) Multi-wavelength pyrometers
Manually Operated PyrometersManually Operated Pyrometers
1. Disappearing Filament Pyrometers
• History
The first pyrometer by the potter Josiah Wedgwood (1780s)
Modern pyrometers became available when the first disappearing filamentpyrometer was built by L.Holborn and F.Kurlbaum. (1901)
• History
The first pyrometer by the potter Josiah Wedgwood (1780s)
Modern pyrometers became available when the first disappearing filamentpyrometer was built by L.Holborn and F.Kurlbaum. (1901)
1. Disappearing Filament Pyrometers
• Principle of operation
The brightness of a lamp filament is changed by adjusting the lamp currentuntil the filament disappears against the background of the target.
Eye of the observer is the detector. à lower limit of temperature range ≈ 700˚C
1. Disappearing Filament Pyrometers
1. Disappearing Filament Pyrometers – Red filter
• Red filter (Scholl RG2, Jena 4512)
Comparison occurs at one wavelength è The effective wavelength, λe =0.65μm
λe is nearly constant at all measured temperatures. è (1300Kà3600K, Δ: 0.003μm)
Comparison occurs at one color è subjective estimation of color cannot influencethe measurement results.
1. Disappearing Filament Pyrometers – Red filter
• Red filter
Steepness of the curve W0,λ=0.65 = f(T) is greater than that of W0 = f(T).
The spectral radiance difference at λ=0.65 is greater than that of total radiance.
L0,λ = CW0,λ (Weichert, 1976) (L: spectral radiance, C: constant, W: radiant intensity)
• Reasons for the application of a red filter
1) Comparison takes place only at one wavelength è eliminating the influence ofsubjective color estimation by different observers
2) At λe = 0.65μm, the lowest possible temperature can be measured.
3) At λe = 0.65μm, pyrometer sensitivity is higher than for the total radiation.
4) Easy to produce good filters of λe = 0.65μm which are stable in time
5) The smallest color changes as a function of wavelength are observed.
1. Disappearing Filament Pyrometers – Red filter
• Reasons for the application of a red filter
1) Comparison takes place only at one wavelength è eliminating the influence ofsubjective color estimation by different observers
2) At λe = 0.65μm, the lowest possible temperature can be measured.
3) At λe = 0.65μm, pyrometer sensitivity is higher than for the total radiation.
4) Easy to produce good filters of λe = 0.65μm which are stable in time
5) The smallest color changes as a function of wavelength are observed.
1. Disappearing Filament Pyrometers - Indicator
• Scale defining equation for black bodies
(Wien’s law)
(Weichert, 1976)
(L: spectral radiance, W: radiant intensity, λ: wavelength, Tt: true temperature)
• Black body: physiological feeling of brightness through a red filter of spectral transmissivity (τλ)
(Vλ: relative spectral sensitivity of a standard human eye)
• Filament: physiological feeling of brightness through a red filter of spectral transmissivity (τλ)
1. Disappearing Filament Pyrometers - Indicator
(εf λ : spectral emissivity of the filament)
• Filament: physiological feeling of brightness through a red filter of spectral transmissivity (τλ)
• The brightness of the filament and of the target are equal & λ=λe
1. Disappearing Filament Pyrometers - Indicator
(Tf: filament temperature, I: lamp current)
• Direct calibration of the ammeter of the pyrometer in temperature units.è Radiance temperature of a target at the wavelength, λe
• Scale divisions of the temperature scale is not linear.
1. Disappearing Filament Pyrometers - Indicator
• For a non-black body…. How to calibrate? (Equation)
(ελe: non-black body of spectral emissivity, Ti: pyrometer readings of black body)
(black body)C2: 1.4388 x 10-9 m•Kλe: 0.65μm
(non-black body)
1. Disappearing Filament Pyrometers - Indicator
• For a non-black body…. How to calibrate? (Diagram)
1. Disappearing Filament Pyrometers – Grey filter
• The tungsten filament can only be used up to 1400˚C.
• Dark deposit on the glass è changing the lamp characteristic
• To extend the pyrometer measurement up to 2000˚C a grey filter is used.
• A grey filter: reduce the target radiance.
1. Disappearing Filament Pyrometers – Grey filter
• For a black body
(w/o grey filter)
(w/ grey filter, τ'λe: spectral transmissivity)
Denote:
A: Radiance reducing factor of the grey filter
With grey filter, it’s possible to calibrate the pyrometer above the maximumfilament temperature.
1. Disappearing Filament Pyrometers – Applications
• Typical applications
- Comparison measurement in calibration of total radiation pyrometers
- Temperature measurement of small size targets ( 0.1 mm)
- Temperature measurement in research laboratories
- Comparison measurement of temperature of non-black bodies
- Measurement of temperature uniformity inside furnace chambers
• The application of disappearing filament pyrometers in industry has become less frequent.
• Typical applications
- Comparison measurement in calibration of total radiation pyrometers
- Temperature measurement of small size targets ( 0.1 mm)
- Temperature measurement in research laboratories
- Comparison measurement of temperature of non-black bodies
- Measurement of temperature uniformity inside furnace chambers
• The application of disappearing filament pyrometers in industry has become less frequent.
2. Two-Color Pyrometers
• General information
= Ratio pyrometer
The ratio of spectral radiances at 2 wavelengths is estimated by the human eye
The observer adjusts the filter position so that the target appears to be grey.
Target temperature increases è the percentage of green ↑, red ↓
• General information
= Ratio pyrometer
The ratio of spectral radiances at 2 wavelengths is estimated by the human eye
The observer adjusts the filter position so that the target appears to be grey.
Target temperature increases è the percentage of green ↑, red ↓
Color temperature
Measurement range: 1200 to 2000˚C
Error: ± 20 to 30 ˚C
2. Two-Color Pyrometers
• Scale defining equation (For a black body and a grey body)
(W: Spectral radiant intensity, ελ1 : emissity)
(For grey body, ελ is constant.)
The ratio of spectral radiant intensities is a function of the temperature
Target temperature increases è the ratio of Wλ1/ Wλ2 decreases.
2. Two-Color Pyrometers
RED
GREEN
Reduction in the precision of temperature measurement with increasing target temperature
The upper limit of the pyrometer: 2200˚C The lower limit of the pyrometer: 700 ˚C
GREEN
2. Two-Color Pyrometers
• Scale defining equation (For a non-grey body)
Non-black and non-grey bodies : selectively radiating bodiesè wavelength dependence of their spectral emissivityè ελ1 ≠ ελ2
Manually operated two-color pyrometers are now being replaced by automatic ones.
Automatic PyrometersAutomatic Pyrometers
1. Optical Systems
• To reach a sufficiently high measurement precision…è lenses, light-guides, or mirrors
• Lenses should be….
1) High transmission factor over a wide wavelength range2) High mechanical strength3) High working temperature4) Good resistance to atmospheric and chemical influences5) Good resistance to abrasion6) Good resistance to rapid temperature variations
1. Optical Systems – lenses or mirrors
• In pyrometry, the upper cut-off wavelength of incident radiation is important.è determines the lowest temperature which the pyrometer can measure.
• Quartz- High mechanical and chemical resistance- Withstand rapid temperature variations
• KRS-5- Most commonly used material for lensesof low temperature pyrometers
- starting from -50˚C
• Silicon- Sometimes replaces KRS-5 - The Ardometer pyrometer(Siemens AG)
• Quartz- High mechanical and chemical resistance- Withstand rapid temperature variations
• KRS-5- Most commonly used material for lensesof low temperature pyrometers
- starting from -50˚C
• Silicon- Sometimes replaces KRS-5 - The Ardometer pyrometer(Siemens AG)
• Mirrors: the lowest measured temperatures, where no lenses may be applied .• Metals with high reflection factor (e.g. Gold)
1. Optical Systems – light guides
• When the objects are too small• When pyrometer would be endangered by excessive temperatures
èLight guides replace lenses.
• Absorption along the rod, imperfect reflection from the rod walls andreflection losses è some of the transmitted energy is lost.
• the angle of incidence > the critical angle
• Materials: Artificial sapphire(Al2O3) or quartz(SiO2)• Solid rod or flexible stranded fibreoptic cable of thin fibres
• When the objects are too small• When pyrometer would be endangered by excessive temperatures
èLight guides replace lenses.
• Absorption along the rod, imperfect reflection from the rod walls andreflection losses è some of the transmitted energy is lost.
• the angle of incidence > the critical angle
• Materials: Artificial sapphire(Al2O3) or quartz(SiO2)• Solid rod or flexible stranded fibreoptic cable of thin fibres
2. Radiation Detectors – thermal radiation detector
• Used in <total radiation pyrometers>• Detector is heated by incident radiation.
• Detector should be….
1) High sensitivity (output signal / incident radiation power)2) Time stable properties3) High resistance to shocks and vibrations4) Low thermal inertia5) Output signal independent of the pyrometer position6) High output signal-to-noise ratio7) High emissivity8) Sensitivity independent of wavelength
• Thermopile:most commonly used, the reference junctions at the pyrometer housing temperature• Thermistor and metal bolometers:used in AC bridge circuits è easy amplification of the output signals• Pyroelectric detectors:
low temperature radiation pyrometers, high sensitivity but complicated construction of pyrometer
• Used in <total radiation pyrometers>• Detector is heated by incident radiation.
• Detector should be….
1) High sensitivity (output signal / incident radiation power)2) Time stable properties3) High resistance to shocks and vibrations4) Low thermal inertia5) Output signal independent of the pyrometer position6) High output signal-to-noise ratio7) High emissivity8) Sensitivity independent of wavelength
• Thermopile:most commonly used, the reference junctions at the pyrometer housing temperature• Thermistor and metal bolometers:used in AC bridge circuits è easy amplification of the output signals• Pyroelectric detectors:
low temperature radiation pyrometers, high sensitivity but complicated construction of pyrometer
2. Radiation Detectors – thermal radiation detector
Thermopile Thermistor Pyroelectricdetector
2. Radiation Detectors – photoelectric detector
• Used in <two/multi-wavelength pyrometers>
• Photoconductors:incident radiation à captured incident photons à photoelectrons à current
• Photodiodes:conductivity s proportional to the intensity of the radiation
• Photovoltaic cells:generated voltage is a logarithmic function of the incident radiation
• Vacuum photocells:incident radiation à emission of electrons from a metallic photocathode in a vacuum glass
• Photovoltaic cells:generated voltage is a logarithmic function of the incident radiation
• Used in <two/multi-wavelength pyrometers>
• Photoconductors:incident radiation à captured incident photons à photoelectrons à current
• Photodiodes:conductivity s proportional to the intensity of the radiation
• Photovoltaic cells:generated voltage is a logarithmic function of the incident radiation
• Vacuum photocells:incident radiation à emission of electrons from a metallic photocathode in a vacuum glass
• Photovoltaic cells:generated voltage is a logarithmic function of the incident radiation
2. Radiation Detectors – photoelectric detector
Photoconductors Photodiodes Photovoltaic cells
Vacuum photocells Photomultipliers
2. Radiation Detectors – photoelectric detector
• Properties of detectors
PbS, CdS : Photoconductors
Si, Ge: Photodiodes
InSb: Photovoltaic cells
3. Total Radiation Pyrometers
• The temperature of a body is determined by the thermal radiation,which it emits over a large range of wavelengths.
• This radiation is concentrated onto a thermal radiation detector
3. Total Radiation Pyrometers
• Scale defining equation (For a black body)
(Ap: Plate area)
(K2: Heat transfer coefficient by convection and conduction)
when the plate is in the thermal steady-state…
• Scale defining equation (For a non-black body)
3. Total Radiation Pyrometers
simplify
• Equation • Diagram
3. Total Radiation Pyrometers
• Influence of housing temperature
to make the readings independent of the housing temperature,the difference between Tp and TH should be equal.
• Influence of target distance
the whole field of view should be filled by the target area
• Influence of target distance
the whole field of view should be filled by the target area
• Extension of measurement range
with grey filter è weakening the radiant flux coming from the object
(Tt: reading of pyrometer with grey filter,T’t: measured temperature,
τ1: filter transmission factor)
4. Photoelectric Pyrometers
• Measurement of rapidly changing temperatures
Total radiation pyrometers: 1 ms ~ 15 msPhotoelectric pyrometers: 1 ~ 2 μs
Photoelectric PyrometerwithDirect radiant flux
Photoelectric PyrometerwithDirect radiant flux
Photoelectric PyrometerwithModulated radiant flux
4. Photoelectric Pyrometers
• Scale defining equation (For a black body)
the output signal of the photoelectric radiation detectors is proportionalto the number of photons (N)
(Warnke, 1972)
(in a narrow temperature range)
(IT: output current, B: constant, T: black body temperature,n: 5~12)
• Scale defining equation (For a non-black body)
4. Photoelectric Pyrometers
(Reynolds, 1961)
Band emissivity is never precisely known.Band emissivity is never precisely known.
in practical, (Worthing, 1941)
5. Two-Wavelength Pyrometers
• Automatic two-color pyrometers
• The eye of observer is replaced by a photoelectric detector.
Rotating disk
Light guideLight guideMirror
6. Multi-Wavelength Pyrometers
• Used to measure the temperature of non-grey bodies of low emissivity
• Good precision of the method in the temperature range up to about 1700˚C
• Splitting the incoming radiation : light guide systems, filters, and prisms
Thank you.Any questions?Thank you.Any questions?
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