Principles of Level - Vass

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The Principles of

Level MeasurementRF capacitance, conductance, hydrostatic tank gauging, radar,and ultrasonics are the leading sensor technologies in liquidlevel tank measurement and control operations. Making thewisest selection for your own application requires a basicunderstanding of how these devices work.

Gabor Vass, Princo Instruments, Inc.

With the wide variety of approaches to level measurement and asmany as 163 suppliers offering one or more types of level-measuring

instrument, identifying the right one for your application can be verydifficult. In recent years, technologies that capitalized onmicroprocessor developments have stood out from the pack. Forexample, the tried-and-true technique of measuring the head of a liquidhas gained new life thanks to smart differential pressure (DP)transmitters. Todays local level-measuring instruments can includediagnostics as well as configuration and process data that can becommunicated over a network to remote monitoring and controlinstrumentation. One model even provides local PID control. Some ofthe most commonly used liquid-level measurement methods are:

RF capacitance

Conductance (conductivity)

Hydrostatic head/tankgauging

Radar

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Ultrasonic

Before you can decide whichone is right for yourapplication, however, youneed to understand how eachworks and the theory behindit. (Each method has its ownabbreviations, so you mayfind the sidebar,Abbreviations for CommonFlow Sensing Terminology,,a useful reference during thediscussions that follow.)

RF Capacitance

RF (radio frequency)

technology uses the electricalcharacteristics of a capacitor,in several differentconfigurations, for levelmeasurement. Commonlyreferred to as RF capacitanceor simply RF, the method issuited for detecting the levelof liquids, slurries, granulars,or interfaces contained in avessel. Designs are available

for measuring process level ata specific point, at multiplepoints, or continuously overthe entire vessel height. Radiofrequencies for all types rangefrom 30 kHz to 1 MHz.

Capacitance Measurement Theory. All RF level systems make use ofenhancements of the same capacitance-measuring technique, and thesame basic theory underlies them all. An electrical capacitance (theability to store an electrical charge) exists between two conductorsseparated by a distance, d, as shown in Figure 1. The first conductor

can be the vessel wall (plate 1), and the second can be a measurementprobe or electrode (plate 2). The two conductors have an effective area,A, normal to each other. Between the conductors is an insulatingmediumthe nonconducting material involved in the levelmeasurement.

The amount of capacitance here is determined not only by the spacingand area of the conductors, but also by the electrical characteristic(relative dielectric constant, K) of the insulating material. The value of

Photo 1. This view of a typical RFcapacitance probe shows the electronicchassis enlarged to twice the size of itshousing.


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K affects the charge storage capacity of the system: The higher the K,the more charge it can build up. Dry air has a K of 1.0. Liquids andsolids have considerably higher values, as shown in Table 1.

The capacitance for the basic capacitor arrangement shown in Figure 1can be computed from the equation:

C = E (K A/d) (1)

where:

C = capacitance in picofarads (pF)

E = a constant known as theabsolute permittivity of freespace

K = relative dielectric constant of the insulating material

Abbreviations for Common Flow Sensing

TerminologyAbbreviations Term Related Technology

AAMCFMCW

FMGWRHHTGI RFK RFLTPDPPTR RFRF RF

TTTDR

AdmittanceAmplitude modulatedCapacitanceFrequency-modulatedcontinuous waveFrequency modulated

Guided wave radarHead or hydrostatic headHydrostatic tank gaugingImpedanceRelative dielectricconstantLevel transmitterPressureDifferential pressurePressure transmitterResistanceRadio frequency

Temperature transmitterTime-domainreflectometer

RF capacitanceRadar or microwaveRF capacitanceRadar or microwave

Radar or microwaveRadar or microwaveHydrostatic headgauging

Hydrostatic headgaugingcapacitancecapacitanceHydrostatic headgaugingHydrostatic headgaugingHydrostatic headgaugingHydrostatic head

gaugingcapacitancecapacitanceHydrostatic headgaugingRadar or microwave




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A = effective area of theconductors

d = distance between theconductors

To apply this formula to alevel-measuring system, youmust assume that the processmaterial is insulating, which,of course, is not always true.A bare, conductive, sensingelectrode (probe) is inserteddown into a tank (see Figure2,) to act as one conductor ofthe capacitor. The metal wall of the tank acts as the other. If the tank isnonmetallic, a conductive ground reference must be inserted into thetank to act as the other capacitor conductor.

With the tank empty, the insulating medium between the twoconductors is air. With the tank full, the insulating material is theprocess liquid or solid. As the level rises in the tank to start coveringthe probe, some of the insulating effect from air changes into that fromthe process material, producing a change in capacitance between thesensing probe and ground. This capacitance is meas ured to provide adirect, linear meas urement of tank level.

As shown in Figure 2, theelectrode sensor, or probe,

connects directly to an RF leveltransmitter, which is mountedoutside the tank. In one design,with the probe mountedvertically, the system can beused for both continuous levelmeasurement and simultaneousmultipoint level control.Alternatively, for point levelmeasurement, one or moreprobes can be installedhorizontally through the side of

the tank; Figure 2 shows thistype being used as a high-levelalarm. Photo 1 shows a typical probe assembly with an enlarged viewof the microprocessor-based transmitter that fits in the housing; in use,its digital indicator faces up. Trans mission of the level-measurementsignal can take several forms, as can the in strument that receives thesignal at either a local or a remote location.

Referring to Figure 2, the transmitter output is 420 mA DC plus

Figure 1. Basic capacitors all sharethe same principle of operation.

TABLE 1

Dielectric Constants ofSample Substances

SubstanceIsopropyl alcoholKeroseneKynarMineral oilPure waterSandSugar

Teflon

Value18.31.88.02.1804.03.0

2.0




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optional HART Protocol for remote diagnostics, range change, drycalibration, and so on. The instrument receiving the signal can be adistributed control system (DCS), a programmable logic controller(PLC), a Pentium III PC, or a strip or circular chart recorder.

When the process material isconductive, the sensing probeis covered with an insulatingsheath such as Teflon orKynar. The insulated probeacts as one plate of thecapacitor, and the conductiveprocess material acts as theother. The latter, beingconductive, connectselectrically to the groundedmetallic tank. The insulatingmedium or dielectric for this

application is the probessheath. As the level ofconductive process materialchanges, a proportional change in capacitance occurs. Note that thismeasurement is unaffected by changes in the temperature or exactcomposition of the process material.

RF Impedance or RF Admittance. When another electricalcharacteristic, impe dance, enters the picture, the result is furtherrefinements in RF level measurement. Offering improved reliabilityand a wider range of uses, these variations of the basic RF system are

called RF admittance or RF impedance. In RF or AC circuits, impedance, Z, is defined as the total opposition to current flow:

Z = R + 1/ j 2 p f C (2)

where:

R = resistance in ohms

j = square root of minus 1 (1)

p = the constant 3.1416

f = measurement frequency (radio frequency for RF measurement)

C = capacitance in picofarads

An RF impedance level-sensing instrument measures this totalimpedance rather than just the capacitance. Some level-meas uringsystems are referred to as RF admittance types. Admittance, A, isdefined as a measure of how readily RF or AC current will flow in a

Figure 2. In the RF capacitancemethod of liquid level measurement,

the electrode sensor connects directlyto an RF transmitter outside the tank.




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circuit and is therefore the reciprocal of impedance (A = 1/Z). Thus,there is no basic difference between the RF impedance and RFadmittance as a level-measurement technology.

In some cases, the process material tends to build up a coating on thelevel-sensing probe. In such cases, which are not uncommon in levelapplications, a significant meas urement error can occur because theinstrument measures extra capacitance and resistance from the coatingbuildup. As a result, the sensor reports a higher, and incorrect, levelinstead of the actual tank level.

Note that the equation forimpedance includesresistance, R. The RFimpedance method can beprovided with specificcircuitry capable of measuringthe resistance and capacitance

components from the coatingand the capacitive componentdue to the actual processmaterial level. The circuitry isdesigned to solve amathematical relationshipelectronically, thereby producing a 420 mA current output that isproportional only to the actual level of the proc ess material. It isvirtually unaffected by any buildup of coating on the sensing probe,enabling an RF system to continue functioning reliably and accurately.

Conductance

The conductance method of liquid level measurement is based on theelectrical conductance of the measured material, which is usually aliquid that can conduct a current with a low-voltage source (normally


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One of the oldest and mostcommon methods ofmeasuring liquid level is tomeasure the pressure exertedby a column (or head) ofliquid in the vessel. The basicrelationships are:

P = mHd

or:

H = mP/d (3)

where, in consistent units:

P = pressure

m = a constant

H = head

d = density

P is commonly expressed in pounds per square inch; H, in feet; and d,in pounds per cubic feet; but any combination of units can be used, solong as the m factor is suitably adjusted.

The density of a liquid varies with temperature. For the highest

precision in level measurement, the density must therefore becompensated for or expressed with relation to the actual temperature ofthe measured liquid. This is the case with hydrostatic tank gauging(HTG) described below.

For decades, DP-type instrumentslong before the DP cellwereused to measure liquid level. Orifice meters, originally designed tomeasure differential pressure across an orifice in a pipeline, readilyadapted to level measurement. Todays smart DP transmitters adaptequally well to level measurements and use the same basic principlesas their precursors. With open vessels (those not under pressure or a

vacuum), a pipe at or near the bottom of the vessel connects only to thehigh-pressure side of the meter body and the low-pressure side is opento the atmosphere. If the vessel is pressurized or under vacuum, thelow side of the meter has a pipe connection near the top of the vessel,so that the instrument responds only to changes in the head of liquid(see Figure 4).

DP transmitters are used extensively in the process industries today. Infact, newer smart transmitters and conventional 4 20 mA signals forcommunications to remote DCSs, PLCs, or other systems have actually

Figure 4. The hydrostatic head, ordifferential pressure, method can addmeasurements (at left) for hydrostatictank gauging (HTG).




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resulted in a revival of this technology. Problems with dirty liquidsand the expense of piping on new installations, however, have openedthe door for yet newer, alternative methods.

Hydrostatic Tank Gauging. One growing, specialized application forsystems that involve hydrostatic measurements is hydrostatic tankgauging (HTG). It is an emerging standard way to accurately gaugeliquid inventory and to monitor transfers in tank farms and similarmultiple-tank storage facilities. HTG systems can provide accurateinformation on tank level, mass, density, and volume of the contents inevery tank. These values can also be networked digitally for multipleremote access by computer from a safe area.

Figure 4 shows a simplifiedsystem that incorporates onlyone pressure transmitter (PT)with a temperature transmitter(TT) and makes novel use of a

level transmitter (LT) to detectaccumulation of water at thebottom of a tank. Mass(weight) of the tanks contentscan be calculated from thehydrostatic head (measured byPT) multiplied by the tankarea (obtained from a lookuptable). The liquidstemperature-densityrelationship can be used to

calculate the volume andlevel, provided the tank is notunder pressure. Data fed into a computer system make it possible forall calculations to be automatic, with results continuously available formonitoring and accounting purposes.

The level transmitter, with its probe installed at an angle into thebottom portion of the tank, is an innovative way to detect accumulationof water, separated from oil, and to control withdrawal of product only.Moreover, by measuring the water-oil interface level, the LT providesa means of correcting precisely for the water level, which wouldincorrectly be measured as product.

Though the DP transmitter is most commonly used to measurehydrostatic pressure for level measurement, other methods should bementioned. One newer system uses a pressure transmitter in the formof a stainless steel probe that looks much like a thermometer bulb. Theprobe is simply lowered into the tank toward the bottom, supported byplastic tubing or cable that carries wiring to a meter mountedexternally on or near the tank. The meter displays the level data andcan transmit the information to another receiver for remote monitoring,

Figure 5. Radar (microwave) levelmeasurement can use either of twotypes of antenna construction at the

top of vessel.




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recording, and control.

Another newer hydrostatic measuring device is a dry-cell transducerthat is said to prevent the pressure cell oils from contaminating theprocess fluid. It incorporates special ceramic and stainless steeldiaphragms and is apparently used in much the same way as a DPtransmitter.

Radar or Microwave

Radar methods of level measurement are sometimes referred to asmicrowave types. Both use electromagnetic waves, typically in themicrowave X-band (10 GHz) range. This technology is being adaptedand refined for level measurement, so you should check out the latestofferings. Most applications have been designed for continuous levelmeasurement.

Basically, all types operate on the principle of beaming microwaves

downward from a sensor located on top of the vessel. The sensorreceives back a portion of the energy that is reflected off the surface ofthe measured medium. Travel time for the signal (called the time offlight) is used to determine level. For continuous level meas urement,there are two main types of noninvasive systems, as well as oneinvasive type that uses a cable or rod as a wave guide and extendsdown into the tanks contents to near its bottom.

One type of noninvasive system uses a technology called frequency-modulated continuous wave (FMCW). From an electronic module ontop of the tank, a sensor oscillator sends down a linear frequency

sweep, at a fixed bandwidth and sweep time. The reflected radar signalis delayed in proportion to the distance to the level surface. Itsfrequency is different from that of the transmitted signal, and the twosignals blend into a new frequency proportional to distance. That newfrequency is converted into a very accurate measure of liquid level.

The sensor outputs afrequency-modulated (FM)signal that varies from 0 to~200 Hz as the distanceranges from 0 to 200 ft (60m). An advantage of thistechnique is that the level-measurement signals are FMrather than AM, affording thesame advantages that radiowaves offer. Most tank noiseis in the AM range and doesnot affect the FM signals.

The second noninvasive

Figure 6. In continuous ultrasoniclevel measurement, a transducermounted at the top of the tank sendsbursts of waves downward onto a




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technology, pulsed radar orpulsed time-of-flight, operateson a principle very similar to that of the ultrasonic pulse meth od. Theradar pulse is aimed at the liquids surface and the transit time of thepulses re turn is used to calculate level. Because pulse radar is lowerpower than FMCW, its performance can be affected by obstructions inthe tank as well as foam and low-dielectric materials (K < 2).

Antennas for the noninvasive methods come in two designs: parabolicdish and cone. Sche matically, Figure 5 shows that the parabolic dishantenna tends to direct the signals over a wider area while the conetends to confine the signals in a narrower downward path. The choiceof one or the other, and its diameter, depends on application factorssuch as tank obstructions that may serve as reflectors, the presence offoam, and turbulence of the measured fluid.

Guided-wave radar (GWR) isan invasive method that uses a

rod or cable to guide the microwave as it passes down fromthe sensor into the materialbeing measured and all theway to the bottom of thevessel. The basis for GWR istime-domain reflectometry(TDR), which has been usedfor years to locate breaks inlong lengths of cable that areunderground or in building

walls. A TDR generatordevelops more than 200,000pulses of electromagneticenergy that travel down thewaveguide and back. Thedielectric of the measuredfluid causes a change inimpedance that in turndevelops a wave reflection.Transit time of pulses downand back is used as a measureof level.

The waveguide affords a highly efficient path for pulse travel so thatdegradation of the signal is minimized. Thus, extremely low dielectricmaterials (K < 1.7 vs. K = 80 for water) can be effectively measured.Further, because the pulse signals are channeled by the guide,turbulence, foams, or tank obstructions should not affect the measurement. GWR can handle varying specific gravity and media buildupor coatings. It is an invasive method, though, and the probe or guidemay be damaged by the blade of an agitator or the corrosiveness of the

material to determine its level.

Figure 7. Not every levelmeasurement technique is suitable fora given application.

Figure 8. The initial cost for fivecontinuous and point level-measurement technologies varies.




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material being measured.

Ultrasonic and Sonic

Both ultrasonic and sonic level instruments operate on the basicprinciple of using sound waves to determine fluid level. The frequency

range for ultrasonic methods is ~20200 kHz, and sonic types use afrequency of 10 kHz. As shown in Figure 6, a top-of-tank mountedtransducer directs waves downward in bursts onto the surface of thematerial whose level is to be measured. Echoes of these waves returnto the transducer, which performs calculations to convert the distanceof wave travel into a measure of level in the tank. A piezoelectriccrystal inside the transducer converts electrical pulses into soundenergy that travels in the form of a wave at the established frequencyand at a constant speed in a given medium. The medium is normally airover the materials surface but it could be a blanket of nitrogen orsome other vapor. The sound waves are emitted in bursts and receivedback at the transducer as echoes. The instrument measures the time for

the bursts to travel down to the reflecting surface and return. This timewill be proportional to the distance from the transducer to the surfaceand can be used to determine the level of fluid in the tank. For practicalapplications of this method, you must consider a number of factors. Afew key points are:

The speed of sound through the medium (usually air) varies with themediums temperature. The transducer may contain a temperaturesensor to compensate for changes in operating temperature that wouldalter the speed of sound and hence the distance calculation thatdetermines an accurate level measurement.

The presence of heavy foam on the surface of the material can act asa sound absorbent. In some cases, the absorption may be sufficient topreclude use of the ultrasonic technique.

Extreme turbulence of the liquid can cause fluctuating readings. Useof a damping adjustment in the instrument or a response delay mayhelp overcome this problem.

To enhance performance where foam or other factors affect the wavetravel to and from the liquid surface, some models can have a beamguide attached to the transducer.

Ultrasonic or sonic methods can also be used for point levelmeasurement, although it is a relatively expensive solution. Anultrasonic gap technique is an alternative way to measure point levelwith low-viscosity liquids. A transmit crystal is activated on one sideof a measurement gap and a receive crystal listens on the oppositeside. The signal from the receive crystal is analyzed for the presence orabsence of tank contents in the meas urement gap. These noncontactdevices are available in models that can convert readings into 420 mA




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outputs to DCSs, PLCs, or other remote controls.

Selecting the Best Method

Figures 7 and 8 summarize some guidelines that will help you selectthe right level measurement method for your application. Remember,

however, that initial cost is only one considerationa low initial costmay be far outweighed by high maintenance costs or loss of accuracyover time.

Suppliers often provide recommendations if you specify your needs,usually by filling out a form. Five types of information commonlydefine the level-measuring instrument or system needed:

Process material. Give the generic name of the material, such as a 5%sodium hydroxide solution.

Material characteristics. Specify whether you need to measure a

liquid, slurry, solid, interface, granular, or powder. Give values of thematerials dielectric constant, K, conductivity in microsiemens percentimeter (mS/cm), viscosity in centipoise (cP), and density in pounds

per cubit foot (lb./ft.3). Also describe consistency in such terms aswatery, oily, like a batter, or like molasses. If this informationis not available, send the supplier a sample for evaluation.

Process information. Give values of the normal temperature andpressure, as well as the minimum and maximum. If turbulence ispresent, indicate its degree as light, medium, or heavy. Describe vesselmaterial: Is it metallic, nonmetallic, or lined? Give materials of

construction of wetted materials, for example 316 stainless, Kynar,Teflon, or other. Describe area classification: nonhazardous, hazardous(list them), or corrosive (list them too).

Vessel function. Describe the main function of the vessel, such assump, reactor, storage, water separation at bottom, and so on. Provide aschematic diagram showing the vessel size and shape, the probemounting and location, 0% and 100% of level, and the presence of anagitator or other internal obstruction.

Power requirements. Specify from the following: 115 VAC, 230

VAC, 24 VAC, or loop- powered (24 VAC, two-wire type).

With a firm grasp of the principles underlying the methods, you shouldbe able to intelligently choose among the options the supplier offersyou.

For Further Reading

Bacon, J.M. June 1996. The changing world of level measurement,




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InTech.

Boyes, W. Feb. 1999. The Changing State of the Art of LevelMeasurement, Flow Control.

Carsella, B. Dec. 1998. Popular level-gauging methods, Chemical

Processing.

Considine, D.M. 1993. Fluid Level Systems, Process/IndustrialInstruments & Control Handbook. 4th Ed. New York, McGraw-Hill:4.130-4.136.

Gillum, D.R. 1995. Industrial Pressure, Level, and DensityMeasurement, ISA Resources for Measurement and Control Series.Research Triangle Park, NC, Instru ment Society of America.

Johnson, D. Nov. 1998. Process Instru mentations Utility Infielder, Control Engi neering.

Koeneman, D.W. July 2000. Evaluate the Options for MeasuringProcess Levels, Chemi cal Engineering.

Level Measurement. 1995. Instrument Engineers Handbook:Process Measure ments and Analysis, B.E. Liptak, Ed., 3rd Ed., Vol. 2.Radnor, PA, Chilton Book Co.:269-397.

Level Measurement and Control. Apr. 1999. Measurements &Control:142-161.

Level Measurement Systems. 1995. Omega Complete Flow andLevel Measure ment Handbook and Encyclopedia. Vol. 29, Stamford,CT, Omega Engineering Inc.

Level measurement, tank gauging sectors grow, diversify, Apr.1999. Control Engi neering:13.

Owen, T. Feb. 1999. Advanced Elec tronics Overcome MeasurementBarriers, Control.

Parker, S. 1999. Selecting a level device based on application needs,

Chemical Proc essing, 1999 Fluid Flow Manual:75-80.

Paul, B.O. Feb. 1999. Seventeen Level Sensing Methods, ChemicalProcessing.

Ramirez, R.C. Oct. 1999. Microwaves calm down black liquorrecovery, InTech:50-53.

RF Level Measurement Handbook. 1999. Princo Instruments Inc.




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Gabor Vass is National Sales and Marketing Manager, PrincoInstruments, Inc., Level Controls and Density Measurement Division,1020 Industrial Blvd., Southampton, PA 18966; 800-221-9237, fax215-355-7766, [email protected], http://www.princoin%20struments.com/.

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