The Principles of
RF capacitance, conductance, hydrostatic tank gauging, radar, and ultrasonics are the leading sensor technologies in liquid level tank measurement and control operations. Making the wisest selection for your own application requires a basic understanding of how these devices work.
Gabor Vass, Princo Instruments, Inc.
With the wide variety of approaches to level measurement and as many as 163 suppliers offering one or more types of level-measuring instrument, identifying the right one for your application can be very difficult. In recent years, technologies that capitalized on microprocessor developments have stood out from the pack. For example, the tried-and-true technique of measuring the head of a liquid has gained new life thanks to smart differential pressure (DP) transmitters. Todays local level-measuring instruments can include diagnostics as well as configuration and process data that can be communicated over a network to remote monitoring and control instrumentation. One model even provides local PID control. Some of the most commonly used liquid-level measurement methods are:
Hydrostatic head/tank gauging
Before you can decide which one is right for your application, however, you need to understand how each works and the theory behind it. (Each method has its own abbreviations, so you may find the sidebar, Abbreviations for Common Flow Sensing Terminology,, a useful reference during the discussions that follow.)
RF (radio frequency) technology uses the electrical characteristics of a capacitor, in several different configurations, for level measurement. Commonly referred to as RF capacitance or simply RF, the method is suited for detecting the level of liquids, slurries, granulars, or interfaces contained in a vessel. Designs are available for measuring process level at a specific point, at multiple points, or continuously over the entire vessel height. Radio frequencies for all types range from 30 kHz to 1 MHz.
Capacitance Measurement Theory. All RF level systems make use of enhancements of the same capacitance-measuring technique, and the same basic theory underlies them all. An electrical capacitance (the ability to store an electrical charge) exists between two conductors separated 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 measurement probe or electrode (plate 2). The two conductors have an effective area, A, normal to each other. Between the conductors is an insulating mediumthe nonconducting material involved in the level measurement.
The amount of capacitance here is determined not only by the spacing and area of the conductors, but also by the electrical characteristic (relative dielectric constant, K) of the insulating material. The value of 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 and solids have considerably higher values, as shown in Table 1.
The capacitance for the basic capacitor arrangement shown in Figure 1 can be computed from the equation:
C = E (K A/d) (1)
C = capacitance in picofarads (pF)
E = a constant known as the absolute permittivity of free space
K = relative dielectric constant of the insulating material
A = effective area of the conductors
d = distance between the conductors
To apply this formula to a level-measuring system, you must assume that the process material is insulating, which, of course, is not always true. A bare, conductive, sensing electrode (probe) is inserted down into a tank (see Figure 2,) to act as one conductor of the capacitor. The metal wall of the tank acts as the other. If the tank is nonmetallic, a conductive ground reference must be inserted into the tank to act as the other capacitor conductor.
With the tank empty, the insulating medium between the two conductors is air. With the tank full, the insulating material is the process liquid or solid. As the level rises in the tank to start covering the probe, some of the insulating effect from air changes into that from the process material, producing a change in capacitance between the sensing probe and ground. This capacitance is meas ured to provide a direct, linear meas urement of tank level.
Referring to Figure 2, the transmitter output is 420 mA DC plus optional HART Protocol for remote diagnostics, range change, dry calibration, and so on. The instrument receiving the signal can be a distributed control system (DCS), a programmable logic controller (PLC), a Pentium III PC, or a strip or circular chart recorder.
When the process material is conductive, the sensing probe is covered with an insulating sheath such as Teflon or Kynar. The insulated probe acts as one plate of the capacitor, and the conductive process material acts as the other. The latter, being conductive, connects electrically to the grounded metallic tank. The insulating medium or dielectric for this application is the probes sheath. As the level of conductive process material changes, a proportional change in capacitance occurs. Note that this measurement is unaffected by changes in the temperature or exact composition of the process material.
RF Impedance or RF Admittance. When another electrical characteristic, impe dance, enters the picture, the result is further refinements in RF level measurement. Offering improved reliability and a wider range of uses, these variations of the basic RF system are called RF admittance or RF impedance. In RF or AC circuits, impe dance, Z, is defined as the total opposition to current flow:
Z = R + 1/ j 2 p f C (2)
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 total impedance rather than just the capacitance. Some level-meas uring systems are referred to as RF admittance types. Admittance, A, is defined as a measure of how readily RF or AC current will flow in a circuit and is therefore the reciprocal of impedance (A = 1/Z). Thus, there is no basic difference between the RF impedance and RF admittance as a level-measurement technology.
In some cases, the process material tends to build up a coating on the level-sensing probe. In such cases, which are not uncommon in level applications, a significant meas urement error can occur because the instrument measures extra capacitance and resistance from the coating buildup. As a result, the sensor reports a higher, and incorrect, level instead of the actual tank level.
Note that the equation for impedance includes resistance, R. The RF impedance method can be provided with specific circuitry capable of measuring the resistance and capacitance components from the coating and the capacitive component due to the actual process material level. The circuitry is designed to solve a mathematical relationship electronically, thereby producing a 420 mA current output that is proportional only to the actual level of the proc ess material. It is virtually unaffected by any buildup of coating on the sensing probe, enabling an RF system to continue functioning reliably and accurately.
The conductance method of liquid level measurement is based on the electrical conductance of the measured material, which is usually a liquid that can conduct a current with a low-voltage source (normally <20 V). Hence the method is also referred to as a conductivity system. Conductance is a relatively low-cost, simple method to detect and control level in a vessel.
One common way to set up an electrical circuit is to use a dual-tip probe that eliminates the need for grounding a metal tank. Such probes are generally used for point level detection, and the detected point can be the interface between a conductive and nonconductive liquid.
Figure 3 shows an arrangement with two dual-tip probes that detect maximum and minimum levels. When the level reaches the upper probe, a switch closes to start the discharge pump; when the level reaches the lower probe, the switch opens to stop the pump.
One of the oldest and most common methods of measuring liquid level is to measure the pressure exerted by a column (or head) of liquid in the vessel. The basic relationships are:
P = mHd
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, so long 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 be compensated for or expressed with relation to the actual temperature of the measured liquid. This is the case with hydrostatic tank gauging (HTG) described below.
For decades, DP-type instrumentslong before the DP cellwere used to measure liquid level. Orifice meters, originally designed to measure differential pressure across an orifice in a pipeline, readily adapted to level measurement. Todays smart DP transmitters adapt equally well to level measurements and use the same basic principles as 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 the high-pressure side of the meter body and the low-pressure side is open to the atmosphere. If the vessel is pressurized or under vacuum, the low 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. In fact, newer smart transmitters and conventional 4 20 mA signals for communications to remote DCSs, PLCs, or other systems have actually resulted in a revival of this technology. Problems with dirty liquids and the expense of piping on new installations, however, have opened the door for yet newer, alternative methods.
Hydrostatic Tank Gauging. One growing, specialized application for systems that involve hydrostatic measurements is hydrostatic tank gauging (HTG). It is an emerging standard way to accurately gauge liquid inventory and to monitor transfers in tank farms and similar multiple-tank storage facilities. HTG systems can provide accurate information on tank level, mass, density, and volume of the contents in every tank. These values can also be networked digitally for multiple remote access by computer from a safe area.
Figure 4 shows a simplified system that incorporates only one pressure transmitter (PT) with a temperature transmitter (TT) and makes novel use of a level transmitter (LT) to detect accumulation of water at the bottom of a tank. Mass (weight) of the tanks contents can be calculated from the hydrostatic head (measured by PT) multiplied by the tank area (obtained from a lookup table). The liquids temperature-density relationship can be used to calculate the volume and level, provided the tank is not under pressure. Data fed into a computer system make it possible for all calculations to be automatic, with results continuously available for monitoring and accounting purposes.
The level transmitter, with its probe installed at an angle into the bottom portion of the tank, is an innovative way to detect accumulation of water, separated from oil, and to control withdrawal of product only. Moreover, by measuring the water-oil interface level, the LT provides a means of correcting precisely for the water level, which would incorrectly be measured as product.
Though the DP transmitter is most commonly used to measure hydrostatic pressure for level measurement, other methods should be mentioned. One newer system uses a pressure transmitter in the form of a stainless steel probe that looks much like a thermometer bulb. The probe is simply lowered into the tank toward the bottom, supported by plastic tubing or cable that carries wiring to a meter mounted externally on or near the tank. The meter displays the level data and can transmit the information to another receiver for remote monitoring, recording, and control.
Another newer hydrostatic measuring device is a dry-cell transducer that is said to prevent the pressure cell oils from contaminating the process fluid. It incorporates special ceramic and stainless steel diaphragms and is apparently used in much the same way as a DP transmitter.
Radar or Microwave
Radar methods of level measurement are sometimes referred to as microwave types. Both use electromagnetic waves, typically in the microwave X-band (10 GHz) range. This technology is being adapted and refined for level measurement, so you should check out the latest offerings. Most applications have been designed for continuous level measurement.
Basically, all types operate on the principle of beaming microwaves downward from a sensor located on top of the vessel. The sensor receives back a portion of the energy that is reflected off the surface of the measured medium. Travel time for the signal (called the time of flight) is used to determine level. For continuous level meas urement, there are two main types of noninvasive systems, as well as one invasive type that uses a cable or rod as a wave guide and extends down 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 on top of the tank, a sensor oscillator sends down a linear frequency sweep, at a fixed bandwidth and sweep time. The reflected radar signal is delayed in proportion to the distance to the level surface. Its frequency is different from that of the transmitted signal, and the two signals blend into a new frequency proportional to distance. That new frequency is converted into a very accurate measure of liquid level.
The sensor outputs a frequency-modulated (FM) signal that varies from 0 to ~200 Hz as the distance ranges from 0 to 200 ft (60 m). An advantage of this technique is that the level-measurement signals are FM rather than AM, affording the same advantages that radio waves offer. Most tank noise is in the AM range and does not affect the FM signals.
The second noninvasive technology, pulsed radar or pulsed time-of-flight, operates on a principle very similar to that of the ultrasonic pulse meth od. The radar pulse is aimed at the liquids surface and the transit time of the pulses re turn is used to calculate level. Because pulse radar is lower power than FMCW, its performance can be affected by obstructions in the tank as well as foam and low-dielectric materials (K < 2).
Antennas for the noninvasive methods come in two designs: parabolic dish and cone. Sche matically, Figure 5 shows that the parabolic dish antenna tends to direct the signals over a wider area while the cone tends to confine the signals in a narrower downward path. The choice of one or the other, and its diameter, depends on application factors such as tank obstructions that may serve as reflectors, the presence of foam, and turbulence of the measured fluid.
Guided-wave radar (GWR) is an invasive method that uses a rod or cable to guide the micro wave as it passes down from the sensor into the material being measured and all the way to the bottom of the vessel. The basis for GWR is time-domain reflectometry (TDR), which has been used for years to locate breaks in long lengths of cable that are underground or in building walls. A TDR generator develops more than 200,000 pulses of electromagnetic energy that travel down the waveguide and back. The dielectric of the measured fluid causes a change in impedance that in turn develops a wave reflection. Transit time of pulses down and back is used as a measure of level.
The waveguide affords a highly efficient path for pulse travel so that degradation of the signal is minimized. Thus, extremely low dielectric materials (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 meas urement. GWR can handle varying specific gravity and media buildup or coatings. It is an invasive method, though, and the probe or guide may be damaged by the blade of an agitator or the corrosiveness of the material being measured.
Ultrasonic and Sonic
Both ultrasonic and sonic level instruments operate on the basic principle of using sound waves to determine fluid level. The frequency range for ultrasonic methods is ~20200 kHz, and sonic types use a frequency of 10 kHz. As shown in Figure 6, a top-of-tank mounted transducer directs waves downward in bursts onto the surface of the material whose level is to be measured. Echoes of these waves return to the transducer, which performs calculations to convert the distance of wave travel into a measure of level in the tank. A piezoelectric crystal inside the transducer converts electrical pulses into sound energy that travels in the form of a wave at the established frequency and at a constant speed in a given medium. The medium is normally air over the materials surface but it could be a blanket of nitrogen or some other vapor. The sound waves are emitted in bursts and received back at the transducer as echoes. The instrument measures the time for the bursts to travel down to the reflecting surface and return. This time will be proportional to the distance from the transducer to the surface and can be used to determine the level of fluid in the tank. For practical applications of this method, you must consider a number of factors. A few key points are:
The speed of sound through the medium (usually air) varies with the mediums temperature. The transducer may contain a temperature sensor to compensate for changes in operating temperature that would alter the speed of sound and hence the distance calculation that determines an accurate level measurement.
The presence of heavy foam on the surface of the material can act as a sound absorbent. In some cases, the absorption may be sufficient to preclude use of the ultrasonic technique.
Extreme turbulence of the liquid can cause fluctuating readings. Use of a damping adjustment in the instrument or a response delay may help overcome this problem.
To enhance performance where foam or other factors affect the wave travel to and from the liquid surface, some models can have a beam guide attached to the transducer.
Ultrasonic or sonic methods can also be used for point level measurement, although it is a relatively expensive solution. An ultrasonic gap technique is an alternative way to measure point level with low-viscosity liquids. A transmit crystal is activated on one side of a measurement gap and a receive crystal listens on the opposite side. The signal from the receive crystal is analyzed for the presence or absence of tank contents in the meas urement gap. These noncontact devices are available in models that can convert readings into 420 mA outputs to DCSs, PLCs, or other remote controls.
Selecting the Best Method
Figures 7 and 8 summarize some guidelines that will help you select the right level measurement method for your application. Remember, however, that initial cost is only one considerationa low initial cost may be far outweighed by high maintenance costs or loss of accuracy over time.
Suppliers often provide recommendations if you specify your needs, usually by filling out a form. Five types of information commonly define 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 the materials dielectric constant, K, conductivity in microsiemens per centimeter (mS/cm), viscosity in centipoise (cP), and density in pounds per cubit foot (lb./ft.3). Also describe consistency in such terms as watery, oily, like a batter, or like molasses. If this information is not available, send the supplier a sample for evaluation.
Process information. Give values of the normal temperature and pressure, as well as the minimum and maximum. If turbulence is present, indicate its degree as light, medium, or heavy. Describe vessel material: 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 as sump, reactor, storage, water separation at bottom, and so on. Provide a schematic diagram showing the vessel size and shape, the probe mounting and location, 0% and 100% of level, and the presence of an agitator or other internal obstruction.
Power requirements. Specify from the following: 115 VAC, 230 VAC, 24 VAC, or loop-
With a firm grasp of the principles underlying the methods, you should be able to intelligently choose among the options the supplier offers you.
For Further Reading
Bacon, J.M. June 1996. The changing world of level measurement, InTech.
Boyes, W. Feb. 1999. The Changing State of the Art of Level Measurement, Flow Control.
Carsella, B. Dec. 1998. Popular level-gauging methods, Chemical Processing.
Considine, D.M. 1993. Fluid Level Systems, Process/Industrial Instruments & Control Handbook. 4th Ed. New York, McGraw-Hill:4.130-4.136.
Gillum, D.R. 1995. Industrial Pressure, Level, and Density Measurement, 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 Measuring Process 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 and Level 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 Measurement Barriers, 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, Chemical Processing.
Ramirez, R.C. Oct. 1999. Microwaves calm down black liquor recovery, InTech:50-53.
RF Level Measurement Handbook. 1999. Princo Instruments Inc.
Gabor Vass is National Sales and Marketing Manager, Princo Instruments, Inc., Level Controls and Density Measurement Division, 1020 Industrial Blvd., Southampton, PA 18966; 800-221-9237, fax 215-355-7766, firstname.lastname@example.org, www.princoin struments.com.