520

3.6 Transmitters—Electronic

A. F. MARKS (1970) P. B. BINDER, L. D. DINAPOLI (1985)

C. G. WALTERS (1995) D. S. NYCE (2005)

Range: Temperature: Thermocouples can measure as low as –518°F (–270°C) with a K, E,or T-type thermocouple, and as high as 4200°F (2300°C) with a Nanmac model HTPtungsten/rhenium (W/Re) thermocouple. Platinum RTDs can measure from –328 to1112°F (–200 to 600°C) with Tegam model 8693 probe.

Pressure: Absolute, gauge, and differential pressures. Fullscale ranges from 0.1 in.H2O (25 Pa) (with Druck model LPM 1000), to 200,000 PSI (1,380 MPa) withOmegadyne model PX91.

Flow: Virtually unlimited. Orifice size can be varied for use with DP cell, electro-magnetic induction, paddle, thermal conductivity, etc.; can vary pipe size.

Level: Maximum level is unlimited with bubbler or differential pressure. Radar up to114 ft (35 m) with Solartron/Mobrey model MRL 700, magnetostrictive up to 100 ft(30.6 m) with MTS Temposonics flexible Level Plus.

Motion: Full scale ranges from 0.020 in. (0.5 mm) with Schaevitz LVDT model LBB,to over 20 ft (6 m) with MTS Temposonics magnetostrictive models.

Inaccuracy: Fully compensated transmitters are available with error as little as 0.1%of full range over their operating temperature range. Lower cost devices are normally0.25 to 1%. Transmitters with especially high or low full scale ranges may have errorsof 2% or more.

Costs: Process control pressure, flow, and level transmitters in common full-scaleranges cost $500 to $2000. Temperature and motion sensors are typically less than$500. Replacement RTDs and thermocouples are $25 to $100.

Partial list of Suppliers: ABB (abb.com/instrumentation)Acromag (www.acromag.com)Action Instruments (www.actionio.com)AGM Electronics (www.agmelectronics.com)Air Monitor Corporation (www.airmonitor.com)Ametek (www.ametekusg.com)AVL (www.avl.com)Barksdale (www.barksdale.com)Brandt Instruments (www.thermo.com)Bristol Babcock (www.bristolbabcock.com)Burkert (www.burkert-usa.com)CR Magnetics (www.crmagnetics.com)Danfoss (www.danfoss.com)Dresser Instruments (www.dresserinstruments.com)Drexelbrook Engineering (www.drexelbrook.com)

LT

TT

PT

FT

MT

Level transmitter

Temperature transmitter

Pressure transmitter

Flow transmitter

Motion transmitter

Flow sheet symbols

© 2006 by Béla Lipták

3.6 Transmitters—Electronic 521

Dwyer Instruments (www.dwyer-inst.com)Dynisco (www.dynisco.com)Elan Technical (www.elantechnical.com)Endevco (www.endevco.com)Endress + Hauser (www.systems.endress.com)Fischer & Porter (www.fluidprocess.com)Fisher Controls (www.emersonprocess.com)Foxboro (www.Foxboro.com)GP:50 (www.gp50.com)Great Lakes Instruments (www.glint.com)Greyline Instruments (www.greyline.com)HiTech Technologies (www.hitechtech.com)Honeywell (www.Honeywell.com/sensing)Inor Transmitter (www.inor.com)ITT Industries (www.ittcannon.com)Johnson Controls (www.jci.com)Jordan Controls (www.jordancontrols.com)Jumo Process Control (www.jumousa.com)Kavlico (www.kavlico.com)Kulite Semiconductor Products (www.kulite.com)Love Controls (www.love-controls.com)Magnetrol (www.magnetrol.com)Measurement Specialties/Schaevitz Sensors (www.msiusa.com/schaevitz)Mensor (www.mensor.com)Minco Products (www.minco.com)MKS Instruments (www.mksinst.com)Moore Industries (www.miinet.com)M-System Technology (www.m-system.com)MTS Systems (www.mtssensors.com)Neutronics (www.neutronicsinc.com)Powers (www.mmcontrol.com)Pyrometer Instrument (www.pyrometer.com)RDP Electrosense (www.rdpelectrosense.com)Revolution Sensor (www.rev.bz)Ronan Engineering (www.ronan.com)Rosemount (www.rosemount.com)Sensidyne (www.sensidyne.com)Sensitech (www.sensitech.com)Sensotech (www.sensotech.com)Setra Systems (www.setra.com)S-Products (www.s-products.com)Teledyne Analytical Instruments (www.Teledyne-ai.com)Teledyne Hastings (www.hastings-inst.com)Thermo Electric (www.thermo-electric-direct.com)Transicoil (www.flwse.com)Vaisala (www.vaisala.com)Validyne Engineering (www.validyne.com)Wilkerson Instrument (www.wici.com)Yokogawa (www.Yokogawa.com)

TRANSMITTER CONFIGURATIONS

Industrial products and processes, commercial products, andautomotive products all have requirements to access data thatindicate physical parameters either continuously or on demand.Since industrial process control is the target of this work, atten-tion in this chapter is concentrated on the measurement of tem-perature, pressure, flow, level, and motion. Although manyindustrial processes require additional measurements, those

listed are the most common. To facilitate a closed-loop controlsystem, information from the process must be obtained beforea controller can determine what action may be required by acontrol element. Some popular names for the sensing devicesthat provide the information are sensors, transducers, and/ortransmitters. Other names are also used, but we will define thesethree for the purposes of this chapter on transmitters.

A sensor is an input device that provides a usable outputin response to the input measurand.1 A sensor is also

© 2006 by Béla Lipták

522 Transmitters and Local Controllers

commonly called a sensing element, primary sensor, orprimary detector. The measurand is the physical param-eter to be measured.

An input transducer produces an electrical output thatis representative of the input measurand. Its output isconditioned and ready for use by the receiving electron-ics.1 (The terms “input transducer” and “transducer” canbe used interchangeably.)

The receiving electronics can be an indicator, controller,computer, PLC, etc. The term “transmitter,” as commonlyused with industrial process control instrumentation, has amore narrow definition than those of a sensor or transducer:

A transmitter is a transducer that responds to a measuredvariable by means of a sensing element and converts it toa standardized transmission signal that is a function onlyof the measured variable.2

Transmitters can have any of several electrical connectionschemes. The most common and easiest to use is the two-wire,loop-powered configuration. This is generally the basic con-figuration assumed by engineers for industrial process controlsystems when digital communication is not required. As shownin Figure 3.6a, only two wires are used to accommodate bothpower to the transmitter and output signal from the transmitter.

The loop current is usually 4 to 20 mA, but 1 to 5 and10 to 50 mA have been used. Important calibration parame-ters with a current loop are zero, full scale, and span. Withthe 4- to 20-mA range, the loop current is normally 4 mAwhen the measurand is at zero, and 20 mA when the mea-surand is at full scale. The difference between zero and full

scale, 16 mA, is called the span. Thus, the span correspondsto the indicated range of the measurand.

When considering a motion transmitter, for example, therange of the measurand could be 0.0 to 100.0 mm, correspondingto a 4- to 20-mA loop current (output span is then 16 mA); theoutput scaling factor is 0.16 mA/mm (which is 100 mm/16 mA).

Two-Wire Loops

The main advantage of a two-wire loop is that it minimizesthe number of wires needed to run both power and signal. Theuse of a current loop to send the signal also has the advantagesof reduced sensitivity to electrical noise and to loading effects.The electrical noise is reduced because the two wires are runas a twisted pair, ensuring that each of the two wires receivesthe same vector of energy from noise sources, such as electro-magnetic fields due to a changing current in a nearby conductoror electric motor. Since the receiving electronics connected tothe transmitter is designed to ignore common-mode signals,the resulting common-mode electrical noise is ignored.

The sensitivity to loading effects is reduced because thecurrent in the twisted pair is not affected by the added resistanceof long cable runs. A long cable or other series resistance willcause a greater voltage drop but does not affect the current levelas long as enough voltage compliance is available in the circuitto supply the signal current. The circuit compliance to handlea given voltage drop from additional loop devices depends onthe transmitter output circuit and on the power supply voltage.

The typical power supply for industrial transmitters is 24VDC. If 6 volts, for example, are needed to power the trans-mitter and its output circuit, then 18 volts of compliance remainto allow for wire resistance, load resistance, voltage dropsacross intrinsic safety (IS) barriers and remote displays, etc.Where the current loop signal is connected to the main receiv-ing equipment or data acquisition system, a precision loadresistor of 250 ohms is normally connected. This converts the4- to 20-mA current signal into a 1- to 5-volt signal, since itis standard practice to configure the analog-to-digital converterof the receiving equipment as a voltage-sensing input.

Three- and Four-Wire Loops

In contrast to the two-wire current-loop configuration, somecurrent-loop devices require a three- or four-wire connection,as shown in Figure 3.6b. These are not loop powered and

FIG. 3.6aWiring configuration of a two-wire, loop-powered 4- to 20-mAtransmitter.

Two-wireloop powered

transmitter

Twistedpair

+24 VDC

Common

Receivingequipment

To A/Dconverter

250 Ω

FIG. 3.6bThree-wire (left) and four-wire (right) current-loop configurations.

ree-wiretransmitter

+24 VDC

Common

250 Ω

1 to 5 VDC

1 to 5 VDC

250 Ω

Four-wiretransmitter 24 VDC

+

−

© 2006 by Béla Lipták

3.6 Transmitters—Electronic 523

therefore have a separate means for providing power by add-ing one or two more wires.

In a four-wire configuration, the current-loop wires canbe a twisted pair, and the power supply wires a separatetwisted pair. This preserves the ability to reject electrical andmagnetic common-mode interference.

This is not so effective in a three-wire configuration dueto the common connection for the return current path. Typi-cally, though, when an instrumentation engineer specifies acurrent-loop transmitter for industrial process control, it isassumed that a two-wire, loop-powered 4- to 20-mA deviceis intended. Other data signals may also be impressed uponthe same wire pair, or alternatively, various digital commu-nication techniques can be used instead of a current loop.These are described in Section 4.16, which covers field bosesand network protocols.

MEASURED VARIABLES

Although almost any type of transducer can be configured asa transmitter, the most common types for industrial processcontrol comprise measurands of temperature, pressure, flow,level, and motion. These are presented here with an expla-nation of principle of operation. Transmitters for measuringother parameters will have the same possibilities for connec-tion and communication methods, with the main differencesbeing in the sensing element design.

Temperature

A temperature transmitter can utilize a sensing element basedon a solid state device (voltage or current output), or ther-mistor (resistance change) sensing element, but most useeither a thermocouple (TC) or resistance temperature device(RTD). Since they are the most popular, thermocouple andRTD-based temperature transmitters are presented here.Thermocouples and RTDs for use with temperature transmit-ters are available from many sources and will have similarcharacteristics if ordering the same type.

Thermocouples Thermocouples are one of the most widelyused devices for measuring temperature where the tempera-ture probe can contact the body to be measured. They aresimple and relatively easy to fabricate. A thermocouple half-circuit is formed by the joining of two dissimilar metalstogether, usually by welding the tips of the two wires. Thepoint of contact between the two wires is called a thermo-couple junction. The metal used for each wire can be a pureelement or an alloy.

A voltage potential is developed that can be measured inmillivolts across the wire ends opposite the junction. Theamplitude of the voltage depends on the difference in elec-tronegativity between the two metals and the difference intemperature between the junction and the other ends of thethermocouple wires. A complete thermocouple measuringcircuit includes two junctions. The pair of wires leading back

to a voltmeter can be the same metal or alloy as each other,shown in Figure 3.6c.

One junction is the reference, theoretically held at 0°Cand called the cold junction. The other junction is incorpo-rated into the measuring probe. Letters such as J, K, and Tare used to indicate the metals used. Standard tables list theoutput voltage generated vs. temperature and thermocoupletype, assuming that the cold junction is held at 0°C. (For suchtables and much more, refer to Section 4.13 in Volume 1 ofthis handbook.)

Temperature transmitters do not actually incorporate a“cold” junction but simulate it through the use of an elec-trical circuit and a special terminal block to accept thethermocouple (TC) extension wires. The terminal block isarranged so that the two terminals will remain at a temper-ature equal to each other. This provision is needed in orderto prevent the introduction of additional thermocouples at theextension wire connection points from affecting accuracy. Inthe electronic circuit, the cold junction simulator adjusts thezero point for the given thermocouple type, while the gainof an amplifier adjusts the sensitivity or span. In addition, alinearizing circuit is incorporated to improve the accuracyover the full operating range. The most popular thermocoupletypes are J, K, E, and T, with ranges and conductors as listedin Table 3.6d.

For a tabulation of inaccuracy, repeatability, ambient tem-perature, and supply voltage effects on a variety of pressureand temperature transmitters, refer to Table 3.6e.

FIG. 3.6cThe measuring and the cold junctions of a thermocouple-type tem-perature detection circuit.

TABLE 3.6dThe Most Common Thermocouple Types, Their Ranges, andConductors Used

Thermocouple Type Conductors Used Temperature Range

J Iron-Constantan –210 to 1200°C

K Chromel-Alumel –270 to 1372°C

E Chromel-Constantan –270 to 1000°C

T Copper-Constantan –270 to 400°C

Measuringjunction

Coldjunction

Iron

Constantan

Iron

Terminalblock

© 2006 by Béla Lipták

524 Transmitters and Local Controllers

RTDs Most RTDs are made with a platinum resistance ele-ment and calibrated to have a nominal resistance at 0°C ofeither 100 or 1000 ohms, although other resistances are avail-able. (For a detailed description of RTDs refer to Section 4.10in Volume 1 of this handbook).

The resistive element can be made from wire coiled ontoa nonconductive form or can be a film deposited onto anonconductive substrate. The form or substrate is usuallymade of a ceramic material. The element resistance increasesas temperature increases. A constant current is applied, andthe voltage drop across the element is measured. The currentmust be kept low enough so that the power dissipation is notsufficient to appreciably heat the RTD.

If the copper extension wires are long, the resistancereading can be corrupted by the resistance change of the wiresdue to ambient temperature changes. To prevent this, a four-wire circuit called a Kelvin connection can be used, as shownin Figure 3.6f.

In the four-wire configuration, the current is appliedthrough one pair of wires, and the voltage is measured by

the other pair of wires. There is minimal current in the volt-age-sensing pair, thus avoiding a voltage drop and preservingthe accuracy of the RTD resistance reading. Table 3.6g showsthe performance data for standard “smart” RTD transmitters.

When using any type of contact probe for the measure-ment of temperature, it is important to note that this techniquedoes not work well in vacuum. The sensing area (usually thetip) of the temperature probe must touch a thermally conduc-tive medium. Air is acceptable as a thermally conductive

TABLE 3.6eTypical Inaccuracies and Other Characteristics of Electronic Pressure and Temperature Transmitters

All Errors Are in Units of ±% Full Span

Type of Instrument Inaccuracy Repeatability50°F (28°C)

Ambient Effect

Supply Voltage Change by 1.0 Volt

or by 10%

Absolute PT—mmHg rangePSIA range

1%0.5%

0.5%0.1%

1%1%

0.1%0.1%

Gauge PT—below 2000 PSIG(14 MPa)

above 2000 PSIG(14 MPa)

0.25% or 0.5%

0.5%

0.1%

0.1%

1%

1%

0.1%

0.1%

D/P Cell—below 500″ H2O (125 kPa)below 850″ H2O (212.5 kPa)PSID range

0.5%0.75%0.5%

0.1%0.1%0.1%

1%1%1%

0.1%0.1%0.1%

Repeaters—up to 35 PSIA (242 kPa)up to 100 PSIG (690 kPa)

2″ H2O max (0.5 kPa)1″ H2O max (0.25 kPa)

0.2″ H2O (50 Pa)0.3″ H2O (75 Pa)

1%1%

N.A.N.A.

Filled TT—force balancemotion balance

0.5%0.5%

0.2%0.1%

0.75%1.0%

N.A.0.1%

RTD-Based TT(platinum) 0.15% 0.05% 0.75%

0.02%

TC-Based TT 0.1% or 5 microV 0.05% See below 0.02%

Ambient error sources include 1) reference junction error of 40 microvolt maximum, 2) span error of 0.5%, and 3) zero errorof 0.5%.

PT = Pressure Transmitter; TT = Temperature Transmitter.

FIG. 3.6fThe four-wire circuit, which is also called the Kelvin sensing cir-cuit, that is used in RTD thermometry.

Currentsource

Voltagemeasurement

RTD

© 2006 by Béla Lipták

3.6 Transmitters—Electronic 525

medium for measuring relatively slow temperature changes,but heat is not conducted in a vacuum. A vacuum chamberis heated by radiation (or conduction through some othermedium, such as the chamber walls), so another sensingmethod should be used in vacuum. One example of a pre-ferred method would be to aim an infrared detector at aninterior surface.

Pressure

Industrial pressure transmitters measure fluid pressure, eithergas or liquid, while using any of several types of sensingelements. (For a detailed discussion of all the pressure sen-sors used in the processing industry, refer to Chapter 5 inVolume 1 of this handbook.)

The most popular pressure sensors are the strain gauge,LVDT, capacitive, resonant wire, and inductive or variablereluctance types.

Strain Gauges Strain gauges can be wire, bonded or unbondedfoil, thin film, or semiconductor. In any of these types, theresistance varies with strain, although capacitance- andinductance-based gauges have also been built. Figure 3.6hprovides a pictorial representation of a bonded foil-type straingauge and a diffused semiconductor type.

In the bonded-foil gauge, the adhesive also provides elec-trical insulation from the strain member. Due to its small sizeand relatively low cost, a diffused semiconductor sensing ele-ment is the most common strain gauge implementation. Sincean adhesive is not required, it is easier to manufacture thanthe bonded type, but has an intrinsic temperature sensitivity

that results in both strong zero and span shifts. Accordingly,effective means for temperature compensation must beemployed.

The compensation scheme usually corrects for nonlin-earity of the sensing element in addition to correctingtemperature errors. A diffused semiconductor strain gaugepressure transmitter can be designed so that the fluiddirectly wets the semiconductor diaphragm if the measuredmedia are noncorrosive, but an isolation diaphragm ofstainless steel or other metal is usually used to ensurecompatibility with various measured media. The spacebetween the metal diaphragm and the semiconductor sens-ing element is filled with a liquid (usually a degassedsilicone oil) to transmit the pressure from the diaphragmto the sensing element.

With pressure transmitters utilizing LVDT, capacitive,inductive, or variable reluctance sensing elements, a pressure-sensing metal diaphragm, capsule, or bourdon tube is exposedto the pressure. The diaphragm, capsule, or bourdon tube con-verts the pressure into motion. Then the motion is measured

TABLE 3.6g Performance Data for Standard and “Smart” RTD Transmitters

Standard* Smart*

PerformanceCriteria*

Platinum Element Nickel Element

DigitalOutput Analog Output (4- to 20-mA DC)

Inaccuracy ± 0.15% or±0.15°F (0.08°C)

± 0.25% ± 0.035% or± 0.18°F (0.1°C)

± 0.05% or± 0.18°F (0.1°C)

Repeatability ± 0.05% ± 0.05% ± 0.015%or ± 0.18°F (0.1°C)

± 0.025% or± 0.18°F (0.1°C)

Zero Shift/6 mo. ± 0.1% ± 0.2% ± 0.06% R or ± 0.1% R or

Span Shift/6 mo. ± 0.1% ± 0.4% 0.18°F (0.1°C) 0.18°F (0.1°C)

Supply Voltage Variation

± 0.2% or 0.02°F (0.01°C) — (0.005%)/Volt

Ambient Effect (100°F or 55°C)

± 0.75% Included above ± 0.1%

*When two values are given the error is the higher of the two. When % is given it refers to % of span or % of calibratedspan, except if %R is shown, which means % of actual reading.

FIG. 3.6hStrain gauge configurations: bonded foil (left) and diffused semi-conductor (right).

ElectricalcontactsGauge

Strain member

Diffusedsemiconductor

gage

Semiconductorstrain member

ElectricalcontactsAdhesive

© 2006 by Béla Lipták

526 Transmitters and Local Controllers

and converted into an electronic signal by the sensing elementand associated electronics module. Figure 3.6i shows somerepresentative configurations for pressure sensing.

LVDT The diffused semiconductor strain gauge was shownin Figure 3.6h. A pictorial representation of an LVDT (linearvariable differential transformer) is shown in Figure 3.6j. AnLVDT normally comprises three coils of wire, arranged in aline as shown. The center coil is the primary and is drivenby an AC waveform, usually a sine wave at a frequency of500 Hz to 10 kHz. The circuit generating the drive for theprimary coil and receiving the signal from the secondaries iscalled a signal conditioner. A ferromagnetic core is positionedwithin the bore of the bobbin on which the coils are wound.

The two secondaries are connected in series so theirvoltages subtract. When the core is centered, the voltage ofeach secondary has the same amplitude but opposite polarity,so the output voltage is approximately zero. As the coremoves one way or the other, the output becomes more pos-itive or negative respective to the direction. In order to pro-duce this bipolar output, a demodulating circuit must be usedto change the output voltage of the secondaries to DC.

There are many circuits to do this, and so they are notshown here, but they can be passive or synchronous. A pas-sive demodulator uses diodes, capacitors, and resistors anddoes not rely on the phase of the primary voltage. Synchro-nous demodulators use semiconductor switches that con-duct with a timing depending on the phase of the primary.

A synchronous demodulator has better performance than apassive one when the cable to the LVDT is longer than a fewinches from its signal conditioner.

Photoelectric Transducers Figure 3.6k shows a typicalschematic of a photoelectric transducer where the positionof the photocoder is proportional to the motion of a primarysensing element. Light from the source shines through per-forations in the shutter to energize photoelectric cells. Theoutput of these cells is scanned and the pulses are amplifiedto produce a digital signal or they are rectified to produce aDC analog signal.

Capacitive Transducers Figure 3.6l shows a capacitance-type pressure or differential pressure transducer, which is ofthe motion-balance type. Positioned between two fixedcapacitor plates is a highly prestressed thin metal diaphragm.This forms the separation between two gas-tight enclosuresthat are connected to the process. The difference in pressurebetween the two chambers produces a force that causes thediaphragm to move closer to the fixed capacitor plate of thelow-pressure chamber. The transducer is excited by a 10-kHzvoltage with an amplitude proportional to the difference inpressure.

FIG. 3.6iMechanical pressure sensor configurations: diffused semiconduc-tor strain gauge (left) and LVDT with pressure capsule (right).

Metaldiaphragm

Oil-filledchamber

Pressureport Housing Pressure

port

Core LVDT

Referenceport

Pressurehousing

Capsule

Referenceport

Diffusedsemiconductor

strain gage

FIG. 3.6kPhotoelectric encoder transducer.

Scanner disc andphotocells

DiscPhotosensor

Light, disc andphotocell assembly

Opaquehousing

Opaquehousing

LightTransparent

T bar

FIG. 3.6j Cutaway view of an LVDT-type pressure sensor (left), and the outside view of the same sensor showing the core as it is about to enter thebore (right).

Bobbin

Primarycoil

Secondarycoils (2)

Cable

End plate (2)

Core

© 2006 by Béla Lipták

3.6 Transmitters—Electronic 527

In some designs, the transducer is filled with an inertfluid (usually silicone oil) to prevent contamination of thecapacitor plates or transducer interior by the process fluid orgas. A metal barrier diaphragm keeps the fill fluid in thetransducer and the process out of the transducer.

Capacitive Semiconductor Transducers The capacitive semi-conductor transducer is very similar to the metallic capacitancetransducer except that the sensing diaphragm is micro-machined from silicon instead of metal. Figure 3.6m showsa silicon capacitive cell. The semiconductor transducer hasalmost no hysteresis and has excellent elasticity. The sensingdiaphragm is sealed between two rigid plates to complete thecapacitor cell. Silicon is normally sealed with low-melting-point glass to create a hermetic seal, instead of being weldedlike the metallic cell.

Potentiometric Transmitters Figure 3.6n shows a potenti-ometer (resistance) driven by a bourdon tube in a mannersimilar to the movement of a pressure gauge. Rotation dueto a pressure change turns the shaft of a precision potenti-ometer, and the change in resistance is proportional to theprocess pressure.

Pressure Measurement Types Pressure measurements canbe gauge, absolute, or differential. A pressure-sensing dia-phragm, for example, has two sides. To measure gauge pres-sure (PSIG), one side (the reference port) is exposed to theambient atmospheric pressure, and the other side (the mea-suring port) is exposed to the measurand. Thus, the measure-ment is made relative to the ambient pressure.

The reference port may or may not be accessible to theuser. To measure absolute pressure (PSIA), the reference portis sealed off while containing a vacuum. To measure differ-ential pressure (PSID), both ports are connected by the userand the pressure difference is indicated. A pressure transmit-ter specially designed to accept a wide common mode pres-sure range and high overpressure is called a DP cell.

Flow

Flow transmitters are widely used in all types of processcontrol applications. (For a detailed discussion of the vari-ous flow detectors, refer to Chapter 5 in Volume 1 of thishandbook.)

Airflow is measured to monitor and control the perfor-mance of fans and dampers. Liquid flow is measured to controlboilers, chillers, heat exchangers, and the supply of liquidcomponents and final product. Common methods employmeasuring the pressure drop across a section of pipe, duct,orifice plate, venturi, or pitot, as well as direct methods usinga paddle, thermal conductivity, vortex shedding, or turbine.

A paddle-type flow transmitter is shown in Figure 3.6o.The paddle, which is positioned in the flow stream, receivesincreasing force as the flow increases. This force can bemeasured by a strain gauge, or the paddle can be allowed tomove with the force and then measured by a motion sensor.

Process flow transmitters utilizing a pressure drop com-bine a standard pressure transmitter with a pressuredrop–generating element that is placed in line with the flow.Some of these are depicted in Figure 3.6o. When a fluid flowsthrough an orifice, a pressure differential develops across thetwo sides of the orifice. The amount of pressure drop variesas the square of the flow rate, so a square root function isadded within the transmitter or within the data acquisitionsystem to return a linear output reading vs. flow rate.

In a pitot arrangement, a port perpendicular to the flowmeasures a reference pressure, and a port angled into the flowmeasures a pressure that increases with increased flow dueto the impact pressure of the fluid. A venturi arrangementprovides a more linear change in pressure vs. flow rate andprovides less obstruction of flow than would an orifice.

Pitot turbine meters are inserted into the flow to detectits velocity, while full flow turbine meters are more accuratebecause the full flow passes through the turbine blades. Tur-bine meters can measure the flow rate by detecting the RPMdeveloped. In this case a small magnet in the impeller caninduce voltage pulses into a nearby pickup coil. The pulserate then represents the flow rate.

FIG. 3.6l Capacitance pressure transmitter.

FIG. 3.6mSilicon capacitive cell.

Jumperwire P1 Screen Sealed housing

Substratesupport

Bufferamplifier

Output

Ceramicsubstrate

GoldfilmP2Pressure

portFilm

termination

Diaphragm

10 K Hzcarrier

PressureSilicon

Diaphragm

GlassSeal

C +

C −

© 2006 by Béla Lipták

528 Transmitters and Local Controllers

When using thermal conductivity to measure flow, a ref-erence temperature sensor (nonheated) on the upstream sideprovides a reading that is subtracted from the heated temper-ature sensor. The temperature difference decreases withincreased flow.

Flow transmitters can also detect the speed of sound inthe measured fluid, electromagnetic induction, and the floatposition of rotameters. To measure fluid flow depending onthe speed of sound in a medium (the fluid), a sonic wave iscaused to propagate in the fluid along the path of the flow tobe measured. With zero flow, a known amount of time willelapse as the wave travels from the sender to the receiver.This time will increase as the flow velocity in the oppositedirection to that of the waves rises or will decrease if theytravel in the same direction. The amount of time measuredindicates the flow rate. This varying rate of wave propagationdue to the relative difference in velocity between the senderand the medium is called the Doppler effect.

When using electromagnetic induction (called the Fara-day effect) to measure the flow of electrically conductiveliquids, the fluid flows in a magnetic field. Electrodes thatare electrically insulated from the pipe pick up a voltagepotential with a magnitude depending on the flow rate.

Rotameters can be instrumented with magnetic, inductive,optical, or other sensors so that the position of the float withinthe measuring tube is measured. If a rotameter with a glassmeasuring tube is used, local visual indication is also available.

Level

Transmitters for measuring liquid level are commonly usedfor process control, but also for inventory control and custodytransfer. (For a detailed discussion of this topic, refer toChapter 3 of Volume 1 of this handbook.) Level measurementfor solids is not as accurate or reliable, due to the nonuniformsettling that often occurs.

A liquid level measuring range can be less than 1 meter fora small reaction vessel, or may be more than 60 ft (20 meters)for a petroleum storage vessel. The highest accuracy can beobtained by using a contact method, such as a liquid levelfloat measured by a magnetostrictive transmitter. Sometimesa contact method is not suitable due to the sticky nature ofthe product being measured, such as tar. Noncontact mea-surements can be made with transmitters employing radar orultrasonic waves.

A simple but always popular way to indicate liquid levelis through the use of a sight glass, but this typically providesno electronic output for use by automated equipment.

A variation of a sight gauge uses a closed metal pipe thathouses a float containing a permanent magnet. As the float fol-lows the liquid level in the vessel, the float position is detectedby an externally mounted magnetostrictive transmitter. Thetransmitter then provides an accurate electronic reading of thelevel. Several level transmitter designs are shown in Figure 3.6p.

A simple and popular system for measuring liquid levelin vessels open to atmospheric pressure is the bubblershown in Figure 3.6p. Here, a small amount of airflow isintroduced into a tube that extends down to the tank levelfrom which the head measurement is to be made. Bubblesslowly escaping from the tube end ensure that the entiretube is full of air at the same pressure as at the bottom endof the tube. A gauge pressure transmitter measures the airpressure and thus indicates the liquid head.

FIG. 3.6nPotentiometric transmitter.

Frame

Piniongear

Hairspring

Gearsector

LeverB

Frame

Drivengear

Drivegear

Potentiometer

Pivotfor

lever B

Pressureconnection

LeverB

LinkageLever A

Motionvector

Bourdon tube

FIG. 3.6oSix flow transmitter designs, each operating on a different workingprinciple.

FTPT PT

PitotOrifice

Paddle

PT

Venturi Turbine

FT FT

Ref Heatedermal

conductivity

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3.6 Transmitters—Electronic 529

A differential pressure transmitter can also be connectedto pressure taps at different elevations that have a knowndistance between them. This configuration allows the detec-tion of level if the liquid density is known and that measure-ment is independent of vessel pressure. This installation hasan advantage over the bubbler in that the level can be deter-mined when the vessel is pressurized at various amounts ofpressure during normal operations.

A magnetostrictive level transmitter can have a flexibleprobe, which is easy to carry up a ladder to the top of a largevessel. The probe tip is attached to the vessel bottom by amagnet, weight, or clamp. A float rides on the liquid level.A second float can be weighted to sink through oil, for exam-ple, and float on the layer of water near the vessel bottom.This allows measurement of the amount of water, and theamount of oil is the top float position minus the water floatposition.

These sensors can measure with high resolution. Insideeach float is mounted a permanent magnet. The probe con-tains a magnetostrictive wire, called the waveguide, in whichcurrent pulses are periodically introduced. The interaction ofthe magnetic field from the current pulse with the magneticfields from the float magnets generates sonic waves in thewaveguide at the location of each float. The sonic wavestravel to the transmitter end of the waveguide at a velocitydepending on the waveguide material.

When a current pulse is introduced into the waveguide,a timer is started. When a sonic wave is detected at the endof the waveguide, the timer is stopped. The measured timeindicates the position of the float, and thus the liquid level.If more than one float is used, then multiple timers are used.

In a capacitance-level probe, the relative permittivity(also called the dielectric constant) of the measured mediummust be substantially different from the surrounding environ-ment. The probe responds with a variable capacitance,depending on how much of the probe length is submergedor adjacent to the measured medium.

Motion

Popular transducers used in motion transmitters include theLVDT, magnetostriction, and inductive types. The LVDT isgood for relatively short strokes but becomes less accurateand more expensive with measuring lengths over a fewinches. Magnetostrictive linear motion transmitters are highlyaccurate and available with lengths of less than a foot, to wellover 20 feet. Inductive types are practical for lengths of upto a few feet.

A simple LVDT is constructed with three coils of wirewound onto a bobbin. A ferromagnetic core passes withinthe bore through the center of the bobbin. The center coil isthe primary and is powered from a stable with an AC source,as shown in Figure 3.6q.

The two secondaries are typically connected in seriesbucking, so the voltages subtract. Synchronous demodulationis also used (with semiconductor switches instead of diodes)for better performance. As the core moves within the bore,the output voltage becomes more positive with movement inone direction and more negative with movement in the otherdirection. The output voltage vs. input position is relativelylinear, in the range of 0.1 to 0.5%.

A magnetostrictive linear position transducer operates aswas described earlier. Instead of mounting within floats, themagnet assemblies have means to mount to the member tobe measured. They are called position magnets.

At one time, it was not thought possible to operate amagnetostrictive transducer with the low power needed for a

FIG. 3.6pA number of level transmitter designs.

LT

PT

Bubbler

LT

PT

DP cell

LT

Magnetostrictive

CapacitiveRador orultrasonic

FIG. 3.6q Linear variable differential transformer.

ACexcitation

A

B

A

B

A

B

A

B

Primarywinding

Secondarywinding

Connected to themeasuring element

Detail (C)

Detail (A)

Detail (D)

Detail (B)

Rectifier

© 2006 by Béla Lipták

530 Transmitters and Local Controllers

two-wire transmitter. This problem was solved in 1990, andseveral companies now offer motion transmitters based onthis technology.3 Magnetostrictive linear position transmitterscan have a nonlinearity of less than 0.01%.

Inductive motion transducers are constructed similarly toLVDTs, but as few as one coil may be used. As the coremoves farther into the coil, the inductance increases. Then,the inductance can be converted into a signal voltage by usingan oscillator and measuring the inductive reactance. The non-linearity error is in the range of 0.2 to 0.5%.

The AC tachometer is an example of a device that gen-erates variable frequency signals for transmission. Here amagnetic slug, gear tooth, key in a shaft, or other device iscounted as pulses created in a coil (Figure 3.6r). When themagnetic slug comes across the face of the coil, a voltage isgenerated in one direction as it leaves.

These pulses are a direct measurement of the speed ofshaft rotation. When counting gear teeth where the spacebetween them is similar to the width of the teeth, a smoothAC signal without wide gaps between pulses is generated.The design of the sensor probe has to be made with knowl-edge of the tooth shape, size, etc., so that good waveformscan be generated for easy measurement.

IMPORTANT FEATURES

Inaccuracy

The basic performance criteria that define the error limita-tions of a transmitter are contained in a specification calledthe static error band. This includes the combined effect ofnonlinearity, hysteresis, and repeatability. The next group oferrors are contributed by calibration error, temperature-induced errors, and line/load regulation. There are many addi-tional performance criteria that can be specified, but theytypically are related to the specific type of transmitter, so theywill not be presented here.

Nonlinearity Nonlinearity is expressed as a percent of fullrange output (%FRO) and is determined from the maximumdifference between a datum on the output vs. measurand plotand a best straight line (BSL) drawn through the data. Non-linearity error can be specified in several different ways,including independent, zero based, and terminal based.

With independent nonlinearity, the BSL does not have togo through the origin or any particular datum. It can be foundusing a computer program with an iterative process, adjustingthe slope and intercept of a straight-line equation until theerrors are minimized. A more popular way to find an indepen-dent BSL, while avoiding an iterative process, is to use a least-squares approximation.

This function is available on most calculators and spread-sheet programs. It provides a BSL approximating that achiev-able with iteration but less perfectly if there are any strongirregularities in the data. The least-squares approximation isthe most commonly used method because it can be directlycalculated.

In zero-based nonlinearity error specifications, the straightline goes through the origin, but the full-scale point is adjusted(changing the slope of the line) to minimize the error. This isused when it is most important to have an accurate zeroindication.

A terminal-based BSL is drawn through both the originand the full-scale datum. This normally produces the highestnonlinearity error number for a given device but may bepreferred when no manual or automated adjustments are pos-sible after installation.

Hysteresis Hysteresis in a process transmitter is the maxi-mum difference between readings taken at the same measurandwith upscale and downscale approaches to the readings. It isdue to inelastic qualities of the sensing element, friction, andbacklash. When linkages are used to connect the measurand tothe sensing element, hysteresis can be increased. A related prop-erty is called deadband, which is the range through which themeasurand may be varied by reversing direction without pro-ducing an observable response in the output signal.

Repeatability Repeatability is the difference between con-secutive readings taken at the same measurand, approachingfrom the same direction, other conditions remaining constant.As long as the repeatability error is low, an intelligent systemcan reduce the errors from other sources by using the knowncharacteristics of those errors and compensating accordingly.

Zero Suppression and Elevation

There may be some measurement values that are of no inter-est in a transmitter installation for process control, eventhough that range falls within the operating range of thetransmitter. In some cases it will be desirable to ignore thisunimportant area in order to improve the readability withinthe range of interest.

FIG. 3.6r Magnetic tachometer sensor.

Outputsignal

Permanent magnet

Sensor winding

Sensor

Shaft

Magneticslug 0

+

−

Output

Voltagegenerated

Time

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3.6 Transmitters—Electronic 531

If, for example, a temperature transmitter is rated for arange of 0 to 1000°C but readings of less than 200°C are ofno interest, a zero suppression of 20% can be used. Then the4.0 to 20.0 mA output signal would correspond to a temper-ature range of 200 to 1000°C, yielding a higher sensitivity(in mA/°C).

Conversely, if the same transmitter were set instead fora 20% zero elevation, a 7.2- to 20.0-mA output signal wouldcorrespond to a temperature range of 0 to 1000°C (the lowerend of the current range would be increased by 20% of the16-mA span). Zero elevation may be used in order to matchthe output signal current to that expected by another pieceof equipment.

Turndown

When one part of a process operates in the 90- to 100-PSIGrange, while another part operates within a 15- to 20-PSIGrange, an engineer might specify the installation of two dif-ferent pressure transmitters, each having the appropriate full-scale range.

Ordering the transmitters and then stocking spare partscan become a logistical problem in a processing plant wherehundreds of pressure transmitters are installed. A preferredmethod is to specify a pressure transducer that can providethe required accuracy over several ranges.

This can be done in the cited case if the transducer isdesigned to operate over a full range of 0 to 100 PSIG but alsocan be readjusted (turned down) to read accurately at 20 PSIG.In order to do this effectively, the sensing element used in thetransmitter must be highly accurate, so that the resulting accu-racy after turndown will still be adequate to provide the neededperformance.

The ratio of the highest full scale to the least full scalethat can still deliver the rated accuracy is called the turndownratio. Turndown ratios of 10 are common, but some up tomore than 100 are also available. This feature can be usedwith any type of process transmitter and is commonly usedwith pressure transmitters.

External Calibration

In older equipment, it was often required that the instrumentcover be removed to facilitate calibration or other adjustment.For explosion-proof equipment, this required a safety tag andappropriate means to ensure safety during the operation, oftenmeaning that the process had to be shut down completely.

Many modern transmitters allow for calibration withoutremoving the instrument cover. Some have zero and spanadjustments under a nameplate, which is easily slid to theside to provide the access. This provides the convenience ofallowing adjustment while the only tool needed is a screw-driver.

Some transmitters have provision for the use of a hand-held calibrator that plugs into an electrical connector or usesmagnetic or optical coupling to enable the digital connection.

This method has more flexibility but requires the often-proprietary handheld calibrator.

Still others, such as those using the HART protocol,provide means for uploading or downloading informationdigitally over the same pair of wires operating the 4- to20-mA signal. Digital communications will be discussed laterin this section.

Intrinsic Safety

When the transmitter will be used in a hazardous location,where flammable or explosive gases, vapors, fibers, or pow-ders may be present, a protection method must be employed.Popular methods include mounting the transmitter within anexplosion-proof housing, in a purge cabinet, or as part of anintrinsically safe system.

In an explosion-proof housing, it is possible that theenclosed atmosphere may burn or explode, but the resultingpressure is withstood by the strength of the housing. A flamepath allows the high-pressure gas to escape while beingcooled. This prevents ignition of the external atmosphere andbleeds away the pressure so the worker is not injured whenthe housing cover is subsequently removed.

When the transmitter is mounted in a purge cabinet, freshair flows through the cabinet at a sufficient rate to preventthe buildup of a flammable mixture. Both explosion-proofand purged equipment relies on the installation method toprovide the safety features. Intrinsically safe installations,however, require the transmitter itself to be designed withinspecific safety criteria.

A transmitter can be installed into an intrinsically safesystem through the use of an approved safety barrier device.In the U.S., the approval agency is usually Factory Mutual(FM) or Underwriters Laboratories (UL) but can be one ofthe other smaller agencies if acceptable under local regula-tions where the equipment will be installed. The safety barrieris installed in the safe area (typically, the control room), inseries with the wiring between the safe area and the hazard-ous area (the process).

The safety barrier limits the electrical current and voltageto levels that are too low to provide energy sufficient to ignitethe type of hazardous materials expected to be present. Inaddition, the transmitter must be designed so that there is noenergy storage, arcing, or hotspot sufficient to cause ignition.The specific requirements are listed in the National ElectricalCode, and the publications of FM, UL, and NFPA (NationalFire Protection Agency).

DIGITAL COMMUNICATION

As described earlier, the 4- to 20 mA two-wire current loopis the assumed standard for analog communication for indus-trial process control. In addition, there are several popularmethods for digital communication from process transmit-ters. Some of these are HART, CAN, and PROFIBUS.

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532 Transmitters and Local Controllers

These and other digital networks and buses are described infull detail in Volume 3 of this handbook.

The HART (highway addressable remote transmitter)protocol was originally developed by Rosemount in 1986. Itis now owned by the HART Communication Foundation. Itutilizes a standard current loop installation, but allows digitalsignals to be impressed onto the same pair of wires. Thedigital signals do not affect other equipment that may beusing the 4- to 20-mA signal.

The digital words are placed on the lines using a fre-quency shift keying (FSK) method according to the Bell 202telephone standard. A logical zero is 2200 Hz, and a logicalone is 1200 Hz. This allows information to be sent from thetransmitter to the receiving equipment, as well as allowingthe receiving equipment to send signals to the transmitter forcalibration, configuration, etc. It is also possible to disablethe current loop signal, if desired, and use only the digitalcommunications. Since it uses Bell 202 FSK, it is a relativelyslow communication method.

CAN was introduced by Robert Bosch, GmbH in 1986and originally intended for automotive use. It stands for con-troller area network and is a much higher-speed system thanHART, but it uses a different hardware layer. Power is pro-vided by one pair of wires, and bidirectional signals are sentand received on a second pair.

CAN is a bus system and several devices can be con-nected to the same sets of wires. It can operate in a master–slave mode, or in a mode where any device can take the buswhen needed. Bus contention is solved by a priority levelassigned to each device. A popular implementation of CANis called DeviceNet.

PROFIBUS is an open digital communication system thatuses application profiles, thus receiving the PROFI part ofits name. The profiles are provided by manufacturers andusers to define performance and other features of the device.These profiles are downloaded by the connected equipmentto interpret how to interface with the device.

The hardware interface is by an RS 485 connection, usingtwo wires for bidirectional signals. Another pair of wires isused for power. There are several different implementationsof PROFIBUS that provide tradeoffs among baud rate, powerconsumption, transmission distance, fiber optic, and intrinsicsafety.

There are many other protocols for digital communica-tions with transmitters and industrial instrumentation. Themost popular ones change over time, and new ones are added.The reader can also investigate Foundation Fieldbus, MOD-BUS, IEEE 1451, and Ethernet, which are all described indetail in Volume 3 of this handbook.

TRANSMITTER SELECTION

In addition to selecting the proper range and accuracy ratingfor a transmitter installation, it is important to keep in minda few additional criteria for proper selection. The materials

of construction of the transducer elements that will contactthe process must be specified to avoid corrosion by or con-tamination of the measured media.

Sometimes, one must use a transmitter design with arobust housing. For example, it is common for techniciansto use pressure transmitters mounted to vessel walls as stepswhile climbing up to perform routine maintenance andinspection duties.

When the transmitter will be exposed to wide changesin temperature during normal operation, the temperaturesensitivity specification of the transmitter may become themost important specification regarding total accuracy of thesystem.

Many communication protocols are now in common use.Consideration must be given to the cost of providing thehardware connection (wiring) as well as the suitability forproviding the desired information. A HART device may com-municate more slowly than a CAN device but can use existingcurrent loop wiring.

When using intrinsic safety barriers in a 4- to 20-mAcurrent loop, make sure that the sum of voltage drops for thetransmitter, readout devices, IS barriers, and load resistor addto less than the power supply voltage available for the loop.

Intelligent Transmitters

Although Section 3.10 in this chapter deals with smart trans-mitters in detail, they should also be mentioned in connectionwith digital communications. The main contribution of theaddition of a microprocessor to the transmitter has been theability to calibrate the unit over a much wider range than theactual span needed for the particular application. This resultedin much increased rangeability without sacrificing accuracybecause by memorizing the temperature and pressure effectson zero and span the smart transmitter can automatically cor-rect for these variations, and therefore the performance of theunit is only a function of repeatability, linearity, and hysteresis.

In addition to lower error and higher rangeability, thesmart transmitters are also more flexible. Because their cal-ibration curve is in the microprocessor’s memory, one canelectronically change the zero and the span of the transmitterthrough the keyboard of a portable terminal (Figure 3.6s),and the microprocessor will automatically match the mini-mum and maximum output signals to the newly set measure-ment inputs without affecting instrument calibration.

Desirable Features Checklist

The most desirable features are high resolution, accuracy,rangeability, reliability, and low cost. In order to meet theserequirements, a transmitter should be designed to have thefollowing features:

1. Small size and weight for easy installation and main-tenance.

2. Rugged design to withstand the industrial environ-ment.

© 2006 by Béla Lipták

3.6 Transmitters—Electronic 533

3. Minimum dependence on environmental conditionsfor accuracy; this requires good temperature stability,resistance to barometric change, weatherproofing,etc.

4. No need for adjustment due to load or line resistancevariations.

5. No potential hazard to personnel or equipment inexplosive atmospheres. Low-voltage operation withlimited current capacity assists in eliminating theseproblems. By definition, intrinsically safe equipmentprovides all three kinds of protection.

6. Convenient and accurate field calibration and main-tenance. In electrical transmitters this usually meansconveniently located test terminals. In explosion-proof designs, calibration should be possible withoutopening the housing.

7. Capacity to operate during voltage dips and poweroutages. Generally, this is accomplished elsewhere inthe control system.

8. Minimum number of transmission and power wires.In most systems installed today, the transmitter ispowered from the receiver over the same wires thatare used for transmission.

9. Output compatible with both measuring and control-ling instruments.

10. Optional local indication of output signal.11. Circuitry designed to facilitate troubleshooting and

maintenance. Most systems today are solid state (no

tubes) and are mounted on circuit boards or areencapsulated. Plug-in components make fast repairseasy, but encapsulated modules are considered to bethrowaway items.

Ability to be integrated into DCS systems and, in thecase of smart transmitters, to communicate over the datahighway provided with the particular system.

References

1. Nyce, D. S., “Linear Position Sensors, Theory and Application,”New York: John Wiley & Sons, 2003.

2. ANSI/ISA Standard S51.1-1979, “Process InstrumentationTerminology,” 1993.

3. Nyce, D. S., U.S. Patent 5,070,485, “Low Power MagnetostrictiveSensor,” 1991.

Bibliography

Blake, H., “Transmitters and Transducers with a Purpose,” paper presentedat AGA conference on transmission, 1968.

Brindley, K., Sensors and Transducers, Oxford, England: Heinemann Pro-fessional Publishing, 1988.

Carr, J. J., Sensors and Circuits, Englewood Cliffs, NJ: PTR Prentice Hall,1993.

Carstens, J. R., Electrical Sensors and Transducers, Englewood Cliffs, NJ:Regents/Prentice-Hall, 1992.

Demorest, W. J., “There’s More to Transmitter Accuracy Than the Spec,”Instruments and Control Systems, May 1983.

FIG. 3.6s Portable terminal can be used to re-range transmitters from a number of locations. (Courtesy of Yokogawa Electric Corp.)

Sensoroutput

Upperrangevalue

Lowerrangevalue

Process variable

0 0 100Single output (%)

Relay terminals

(Two-wire transmitter)

Meter compartment

Terminal board Power distributor

Storedcharacterization

data

© 2006 by Béla Lipták

534 Transmitters and Local Controllers

Fraden, J., Handbook of Modern Sensors, New York: Springer-Verlag,1996.

Herceg, E., Handbook of Measurement and Control, New Jersey: SchaevitzEngineering, 1976.

Magison, E. C., Intrinsic Safety, Research Triangle Park, NC: InstrumentSociety of America, 1984.

Minar, E. J. (Ed.), ISA Transducer Compendium, latest edition, ResearchTriangle Park, NC: Instrument Society of America.

Morris, H. M., “Microsensors Enhance Process Variable Transmitters’ Abil-ities,” Control Engineering, October 1991.

Norton, H., Handbook of Transducers, Englewood Cliffs, NJ: Prentice-Hall,1989.

Nyce, D. S., “Magnetostriction-Based Linear Position Sensors,” Sensors,11(4), 1994.

Nyce, D. S., Position Sensors for Hydraulic Cylinders, Hydraulics & Pneu-matics, November 2000.

Nyce, D. S., U.S. Patent 5,070,485, “Low Power Magnetostrictive Sensor,”1991.

Nyce, D. S., U.S. Patent 6,401,883, “Vehicle Suspension Strut Having aContinuous Position Sensor,” 2002.

Oliver, F. J., Practical Instrumentation Transducers, New York: HaydenBook Co., 1971.

Pallas-Areny, R., and Webster, J. G., Sensors and Signal Conditioning, 2nded., New York: John Wiley & Sons, 2001.

Webster, J. G., The Measurement, Instrumentation, and Sensors Handbook,Boca Raton, FL: CRC Press, 1999.

Winch, R. A., “Utility Puts Smart Transmitters to the Test,” InTech Magazine,August 1990.

© 2006 by Béla Lipták