PROCESS Temperature Sensor RTD or Thermocouple Weak LowLevel Signal ohm or mV Readout - PDF document

PROCESSTemperature(RTD orThermocouple)"Weak" Low-Level SignalControl SystemTerminations PROCESSTemperature(RTD orThermocouple)"Strong" High-Level SignalControl SystemTemperature Transmitter Transmitters vs. Direct WiringWhen making temperature measurements, two wayshave traditionally been employed to get processreadings back to a monitoring and control system.One method is to utilize sensor extension wiresto carry the low-level signals (ohm or mV) generatedby field-mounted RTD or thermocouple sensors. Another is to install temperature trans-mitters at or near the measurement point. Thetransmitter amplifies and conditions the sensorsignal, and transmits it over a twisted wire pair backDirect wiring strategies have generally been consid-ered less expensive and sometimes easier. Transmitteruse, because of cost considerations, was often reservedfor important loops and applications where signal andloop integrity was a must.Today, our highly functional, yet very affordable,microprocessor-based field-mount temperature transmit-ters are comparable in price to direct wiring strategies.When the additional advantages of using intelligenttransmitters are factored in you will, in most applications,also save considerable time and maintenance head-aches. This is especially true when the measurementWhy UseTemperature TransmittersInstead of Direct Wiring?Figure 2. A temperature transmitter amplifies and conditions theprimary sensor signal, then carries it over a twisted pair wire to the Figure 1. Sensor extension wires carry low-level (ohm or mV)signals generated by a field-mounted RTD or thermocouple. Here are just some of the reasons why you shoulduse Moore Industries’ intelligent temperaturetransmitters in place of direct wiring strategies:Cut Wiring CostsDirect wiring sensors to a control system requires the useof sensor extension wires. Not only are extension wiresfragile, they also cost three times more than the commonshielded copper wire used for a temperature transmitter’s4-20mA signal. Using the less expensive wires, transmit-ters can pay for themselves in wire and conduit costsalone. The longer the wire run, the greater the potentialsavings.In retrofit situations, you may wish to switch to trans-mitters, but are reluctant to do so because some mistak-enly believe that new copper wires must be run to accom-modate the 4-20mA. This is not the case. Temperaturetransmitters can be installed at the sensor, and the in-place RTD or thermocouple extension wires can be usedto transmit the 4-20mA back to the control system. Thismeans no additional installation time or material costs(including conduit) will be needed. And you still get all ofthe advantages of using temperature transmitters.Protect Signals from Plant NoiseCommon in nearly every industrial environment, RFI(radio frequency interference) and EMI (electromagneticinterference) can negatively affect process signals.Before you eliminate RFI and EMI as a possible culprit oferratic signals, consider just some of the commonsources: Mobile and stationary radio, television, andhand-held walkie-talkies; radio-controlled overhead cranes;radar; induction heating systems; static discharge; highspeed power switching elements; high ac current conduc-tors; large solenoids and relays; transformers; ac and dcmotors; welders; and even fluorescent lighting.If you have one or more of these in your plant, youmay have a RFI/EMI problem. The result is sometimesjust a minor inconvenience. Other times it can be asThe Interface Solution Experts Page 1 Intelligent, Universal Temperature TransmitterProgramming isusing a PC, hand-held configurator, or Sensor TypesThermocouple (J, K, E, T, R, S, B, N, and C) and RTD Scaleable, IsolatedReady for directinterface with DCS,PLC, SCADA, and devices andserious as a costly plant shutdown.In a direct wiring scheme, the low-level signalsgenerated by an RTD (ohm) or thermocouple (mV) areparticularly susceptible to the signal degrading effects ofRFI/EMI. Compounding theproblem, sensor extensionwires can behave muchlike an RFI/EMI antennaby actually drawingplant “noise” to thewires, and affectingweak, low-levelsignals. Con-versely, a properlydesigned tempera-ture transmittereffectively negates theeffects of incoming RFInoise by converting a sensor’slow-level signal to a high-levelto RFI/EMI, and can accuratelywithstand long distance transmis-noisy plant, back to the controlroom. When specifying yourtransmitter, always check forRFI/EMI protection. If there’s nospecification given, it’s usuallybecause the instrument is notdesigned to resist noise. It willprobably not perform very well ina noisy plant environment.Stop Ground LoopsMake sure to choose an isolated transmitter (even today,not all are!). Our transmitter’s input/output/power signalisolation protects against signal inaccuracies caused byground loops. This is important even when using un-grounded thermocouples because their insulation willeventually break down.Reduce Hardware and Stocking CostsWith direct wiring, it is necessary to match the sensortype to input-specific DCS and PLC input cards. Sensora 4-20mA input card. And since numerous sensor typesare routinely used in a plant, a large number of differentcards must be ordered and kept on hand as spares. Thisis not only expensive, but can result in a lot of confusionOur temperature transmitters incorporate powerfulmicroprocessors that allow them to be easily configuredto accommodate nearly any sensor input type. Their4-20mA output signal is control-system ready. Thisallows you to standardize on (and stock) less expensive4-20mA DCS and PLC input cards.In an intelligent temperature transmitter strategy, youtransmitter to accommodate the different sensor type.The loop’s twisted pair wiring and existing 4-20mA inputboards don’t even have to be touched. Because younever know what sensor you’ll end up with, make sure togo with a universal transmitter that configures to acceptall common temperature sensor types and temperatureranges Enhance Accuracy and StabilityUsing temperature transmitters can substantially en-hance measurement accuracy. DCS and PLC systemsmeasure readings over the entire (very wide) range of asensor. It is well known that measuring a narrower rangeproduces far more accurate measurements. Transmitterscan be calibrated to any range within a sensor’s overallcapabilities. Their measurements are moreThe Interface Solution Experts Page 2 Our universal temperature transmitters configure to handlenearly every temperature sensor type and temperature range. Why UseTemperature TransmittersInstead of Direct Wiring? Why UseTemperature TransmittersInstead of Direct Wiring? The Interface Solution Experts Page 3 Sensor-to-Transmitter Trimming compensates for deviationsin a temperature sensor’s established curve.precise than is possible with most direct wiring strategies.Our transmitters deliver accuracy ratings of ±0.13°C(±0.23°F) when paired with a common Pt100 RTDsensor over a 200° span.If you need even better accuracy, you can trim ouruniversal transmitters to precisely match a particularsensor. Even though sensors are designed to have ahigh degree of conformance to an established curve,each one (even precision sensors) will vary slightly fromtheir stated specification. In the past, transmittersassumed an accurate sensor measurement and pro-cessed it accordingly. Our transmitters can be trimmedto match the measurement actually being taken by eachindividual sensor.Called Sensor-to-Transmitter Trimming, the transmit-calibration baths maintained at stabilized temperatures). The transmitter then “captures” two readingsfrom the sensor representing the upper and lower rangevalues, and stores them in non-volatile memory. Thetransmitter uses these values to compensate for devia-tions between the sensor’s stated linearization curve andits actual measurements. When our transmitters arepaired with a 1000 ohm RTD, this technique results inamazing measurement accuracy of up to ±0.014°C(±0.025°F) over a 100° span.To further enhance measurement accuracy, ourtransmitters can be trimmed to respond to two datapoints within the selected zero and span measurementrange. This advantage allows a complete range to bespecific segment of the range most critical to the pro-cess. For example, in , the actual sensor curveis used in place of the ideal RTD curve between 20°Cand 27°C. This provides incredible precision over aportion of span, while measuring the remainder of thespan with our transmitter’s usual outstanding accuracy.Prevent Mis-WiringIn place of numerous sensor lead-wire and DCS/PLCinput board combinations, engineering designs anddrawings will only need to show one wire type (twistedwire pair) and one input board type (4-20mA). This onegreatly simplified, and the chances of loop mis-wiring arevirtually eliminated.Throughout the lifetime of a process, enhancements areroutinely made to accommodate the manufacture ofupgraded or even completely new products. Processchanges may require different measurement ranges orgreater temperature accuracy than was previouslyrequired. Either of these conditions may necessitate aIn a direct wired system, changing sensors generallymeans removing existing, and pulling new extension wire.This is because extension wire must be matched to thesensor type. Additional costs are incurred when thecontrol system’s costly input boards (if also sensor-typedependent) must be replaced to accommodate the newsensors. Intelligent temperature transmitters can be trimmed to placemeasurement emphasis on a specific segment of the range mostcritical to the process. LowerIDEAL RTD CURVE(USED BY DEFAULT)ACTUALCURVE ûC LOWER TRIM The Interface Solution Expertswww.miinet.comUnited States • info@miinet.comTel: (818) 894-7111  FAX: (818) 891-2816 Australia  sales@mooreind.com.auTel: (02) 8536-7200  FAX: (02) 9525-7296 Belgium  info@mooreind.beTel: 03/448.10.18  FAX: 03/440.17.97Tel: (0)344-617971  FAX: (0)344-615920Tel: 86-21-68406724  FAX: 86-21-50623585Tel: 01293 514488  FAX: 01293 536852 Diagnostic error message indicates that RTD Lead #3 is broken RTD Broken Lead WireWhy UseTemperature TransmittersInstead of Direct Wiring? © 2001 Moore Industries-International, Inc.Specifications and information subject to change without notice.Page 4Lower Maintenance Time and ExpenseTemperature transmitters have come a long way sincethe days of fixed-range, inflexible instruments. MooreIndustries transmitters are not only universal in regardsto input type and range; they also incorporate powerfulsensor diagnostics that save considerable time andmoney.Temperature transmitters with intelligent diagnosticcapabilities help you keep track of sensor operation andquickly find and diagnose sensor failures. Capable ofcontinually monitoring the sensor, if a wire breaks orotherwise stops sending a signal during operation, thetransmitter sends the output upscale or downscale towarn of sensor burnout and other unwanted conditions.Furthermore, our transmitters can tell you which wire hasbroken via an error message either on an integral digitaldisplay or using their PC configuration software. Specificfault messages eliminate the work of removing thesensor or checking all of the lead wires to diagnose aproblem . During startups, in the middle of thenight, or in the middle of winter, this can be a hugetimesaving advantage.Avoid Lead Wire ImbalancesWhere feasible, use 4-wire RTDs, and specify a tempera-ture transmitter that is able to accept a “true” 4-wire RTDinput. The advantage is that the fourth wire in a RTDcircuit effectively cancels out errors due to resistanceFigure 6. Our transmitters can indicate which wire has broken via anerror message on their integral digital display. Every ohm of imbalance in an RTD (100 ohm, Pt3850)sensor’s lead wires results in as much as 2.5°C measurement error. imbalances between the leads. Every ohm of imbalancein a RTD sensor’s lead wires can produce as much as a. Seriousimbalances may be present from the very first day ofcommissioning without you even being aware of them.Typical causes include manufacturing variances; leadlength differences; loose connections; terminal blockcorrosion; and work hardening from bending and otherstresses.Our intelligent temperature transmitters are capableof accepting “true” 4-wire RTD inputs and provide aconstant current source to the outer leads of the RTD. The voltage drop is measured across theinner leads, which is a high impedance loop. There isessentially no current flow in the voltage loop, so voltageis directly proportional to resistance. Lead resistance isignored. You will get a very accurate measurementproviding the resistance value of the RTD plus corrosion,plus wire resistance, is less than 2,000 ohms (typically).A 4-wire RTD costs about the same as a 3-wire and canbe used with less expensive, smaller gauge wire withoutconcern for added resistance. A 4-wire RTD compensates for all resistance imbalancesbetween lead wires.