Measuring Temperature with RTDs – A Tutorial

1 _____Product and company names are trademarks or trade names of their respective Copyright 1996 National Instruments Corporation. All rights 1996 Application Note 046 Measuring Temperature withRTDs A TutorialIntroductionA resistance- Temperature detector (RTD) is atemperature sensing device whose resistance increaseswith Temperature . An RTD consists of a wire coil ordeposited film of pure metal. RTDs can be made ofdifferent metals and have different resistances, but themost popular RTD is platinum and has a nominalresistance of 100 at 0 are known for their excellent accuracy over awide Temperature range. Some RTDs have accuraciesas high as ( C) at 0 C. RTDs are alsoextremely stable devices. Common industrial RTDsdrift less than C/year, and some models are stableto within can be difficult to measure because they haverelatively low resistance (100 ) that changes onlyslightly with Temperature (less than / C).

2 Toaccurately measure these small changes in resistance,you may need to use special configurations thatminimize errors from lead wire an RTD is a passive resistive device, youmust pass a current through the device to produce ameasurable voltage. This current causes the RTD tointernally heat, which appears as an error. Selfheating is typically specified as the amount of powerthat willraise the RTD Temperature by 1 C, or 1 mW/ C. Youcan minimize self heating by using the smallestpossible excitation current. The amount of selfheating also depends heavily on the medium in whichthe RTD is immersed. An RTD can self heat up to100 times higher in still air than in moving Relationship ofResistance andTemperature in RTDsCompared to other Temperature devices, the output ofan RTD is relatively linear with respect totemperature. The Temperature coefficient, called alpha( ), differs between RTD curves.

3 Although variousmanufacturers may specify alpha differently, alpha ismost commonly defined as the change in RTDresistance from 0 to 100 C, divided by the resistanceat 0 C, divided by 100 C: ( / / C) = (R100 - R0)/(R0 * 100 C)where R100 is the resistance of the RTD at 100 C,and R0 is the resistance of the RTD at 0 example, a 100 platinum RTD with = will measure at 100 1 displays a typical resistance-temperaturecurve for a 100 platinum ( C)Resistance ( )Figure 1. Resistance- Temperature Curve for a 100 Platinum RTD, = the resistance- Temperature curve isrelatively linear, accurately converting measuredresistance to Temperature requires curve fitting. TheCallendar-Van Dusen equation is commonly used toapproximate the RTD curve:Rt = R0[1 + At + Bt2 + C(t - 100)3]where Rt is the resistance of the RTD attemperature = t, R0 is the resistance of the RTD at0 C, A, B, and C are the Callendar-Van Dusencoefficients shown in Table 1, and t is thetemperature in C.

4 For temperatures above 0 C, theC coefficient equals 0. Therefore, for temperaturesabove 0 C, this equation reduces to a quadratic. Ifyou pass a known current, IEX, through the RTD andmeasure the output voltage developed across theRTD, V0, you can solve for t:t=2(V0 IEXR0)IEXR0[A+A2+4B(V0 IEXR0)/IEXR0]where V0 is the measured RTD voltage and IEX is theexcitation platinum RTD curves follow one of threestandardized curves the DIN 43760 standard( = ), the Industrial or Americanstandard ( = ), or the InternationalTemperature Scale (ITS-90) that is used with wire-wound RTDs ( = ). The Callendar-VanDusen coefficients for each of these three platinumRTD curves are listed in Table measurement CircuitsBecause the RTD is a resistive device, you mustdrive a current through the device and monitor theresulting voltage. However, any resistance in thelead wires that connect your measurement system tothe RTD will add error to your readings.

5 Forexample, consider a two-wire RTD elementconnected to a measurement system that also suppliesa constant current source, IEX, to drive the shown in Figure 2, the voltage drop across thelead resistance, RL, adds to the measured voltage. + -RLV0 RTIEXRLF igure 2. Two-Wire RTD MeasurementFor example, a lead resistance of in each wire,RL, adds a error to the resistance a platinum RTD with = , the resistanceequals a /( / C) = C you are using lead lengths greater than 10 ft, youwill probably need to compensate for this leadresistance. The preferred RTD measurement methodis to use a four-wire RTD. One pair of wires carriesthe current through the RTD; the other pair senses thevoltage across the RTD. Because only negligiblecurrent flows through the sensing wires, the leadresistance error of RL2 and RL3 is negligible. Thisconfiguration is illustrated in Figure 3.

6 +RL2V0 RTRL4RL1RL3-IEXF igure 3. Four-Wire RTD MeasurementTable 1. Callendar-Van Dusen Coefficients Corresponding to Common RTDsStandardTemperatureCoefficient ( )AB C*DIN x x x x x x x x x 10-12* For temperatures below 0 C only; C = for temperatures above 0 reduce costs, you may instead want to use a three-wire RTD. By using the three-wire RTD in aWheatstone configuration with a current source, asshown in Figure 4a, you can compensate for the leadresistances. Notice that, in this bridge configuration,the effects of RL1 and RL3 cancel each other outbecause they are located in opposite arms of thebridge. Lead resistance RL2 does not add significanterror because little current flows through , you can use a current excitation sourceand connect the three-wire RTD as shown inFigure 4b. In this configuration, the resistance ofonly one lead, RL1, adds error to the measurement .

7 RL2 RTRL1RL3R1R2R3+V0a. Three-Wire RTD in a Wheatstone Configuration-IEX +RL2V0 RTRL1RL3b. Three-Wire RTD with a Current Excitation Source-IEXF igure 4. Three-Wire RTD measurement with aWheatstone Bridge and a Current SourceRTD Measurements withSCXIS ignal conditioning is generally required to interfacean RTD to a measurement device such as a plug-indata acquisition (DAQ) board. Signal conditioningproduces current excitation for the RTD, amplifiesthe output signal, and filters the signal to removeunwanted noise. You can also use signalconditioning to electrically isolate the RTD and themonitored sytem from the DAQ system and the Conditioning eXtensions for Instrumentation(SCXI) is a signal conditioning front end you can usewith plug-in DAQ boards, or as a complete, externalDAQ system. SCXI modules condition signals from avariety of signal sources, such as RTDs, thermistors,and thermocouples, and pass the conditioned signalto the plug-in DAQ board.

8 The plug-in board canthen acquire the signals directly into PC , you can connect the SCXI system to anRS-232 or RS-485 serial network to a PC for remotedata SCXI product line has a variety of analog anddigital signal conditioning modules for various typesof signals. The SCXI -1121 and SCXI-1122 are wellsuited for SCXI -1121 is an isolated amplifier andmultiplexer module with four isolated input of the four channels has a jumper-selectablegain amplifier (gains from 1 to 2,000) and a lowpassfilter (4 Hz or 10 kHz). The SCXI -1121 also has fourchannels of isolated voltage or current can configure each channel independently toproduce a constant mA, mA, V, V SCXI-1122 is a 16-channel isolated multiplexermodule with a programmable amplifier (gains to 2,000) and a single mA currentexcitation Monitoring 16 RTDs withthe SCXI-1121 For this example, assume that you want to use 16100 four-wire RTDs to monitor the efficiency of aproduction process.

9 You can monitor all 16 RTDswith four SCXI-1121 modules installed in a four-slotSCXI -1000 chassis. The modules and chassis areconnected to a plug-in PC DAQ board that acquiresthe analog signals from all four modules and storesthe digitized readings into PC plug -in board in this example is theAT-MIO-16F-5, which is a 12 -bit, 200 kHzmultifunction I/O board for IBM PC AT andcompatible computers. The AT-MIO-16F -5 board isused in the example because of its accuracy, highscan rate, and self-calibration capability. You canalso use similar multifunction I/O boards for theMacintosh and PS/2 platforms with the SCXI SCXI chassis is connected to the AT-MIO-16F -5with the SCXI -1345 shielded cable assembly, whichis available in lengths up to 10 m. The RTDs arewired into SCXI-1320 terminal blocks, which haveconvenient screw terminals with strain 5 is a diagram of the Shielded Cable AssemblySCXI1100 4 SCXI-1320 Terminal Blocks4 SCXI-1121 ModulesSCXI1140 SCXI-1000 ChassisSCXI-1001 MAINFRAME SCXIAT-MIO-16F-5 DAQ BoardSCXI1140 SCXI1100 16 RTDsSCXI1140 SCXI1140 SCXI1100 SCXI1100 Figure 5.

10 SCXI System for Monitoring 16 RTDsAn IBM PC/XT/AT or compatible computer controlsthe Temperature monitoring system. Therefore,application software choices for controlling thesystem include National Instruments' LabWindows for DOS or LabVIEW for Windows. Alternatively,you can use a general-purpose programminglanguage such as C, BASIC, or Pascal in DOS orWindows and control the DAQ hardware with theNI-DAQ driver software that is included with allNational Instruments plug-in DAQ an SCXI System forRTD MeasurementFirst, configure the SCXI-1121 modules for the RTDmeasurements. Configuring includes setting theexcitation mode, gain, and bandwidth for each inputchannel of the SCXI-1121. Follow these steps toconfigure your SCXI-1121 the Excitation mode. Because you are usingfour-wire RTDs, configure each channel forconstant current excitation. You can configurethe current source for mA or mA.