National Instruments NI 4350 User Manual

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Chapter 3 NI 4350 Operation©National Instruments Corporation3-7NI 4350 User ManualFigure 3-2. Effect of the Cold-JunctionWith the NI 4350 instruments, you can perform cold-junctioncompensation in software. To do this, you can use the thermistortemperature sensor on the NI 4350 accessory to measure the ambienttemperature at the cold-junction and compute the appropriatecompensation for the unwanted thermoelectric voltages using software.You have several options for performing cold-junction compensation,as shown below.• If you are using the NI435X instrument driver, LabVIEW,LabWindows/CVI, VirtualBench, or the DAQ Channel Wizard,your software will automatically perform cold-junctioncompensation on all channels configured as thermocouplechannels.• If you are using LabVIEW and are not using the instrument driveror the DAQ Channel Wizard, your software includes examples thatperform these temperature-to-voltage and voltage-to-temperatureconversions for the cold-junction thermistor and various types ofthermocouples based on the National Institute of Standards andTechnology (NIST) standard reference tables. These examples arelocated in the DAQ analog input example library and have 4350 intheir title.• If you are not using either of the previous software options, followthe steps below to perform cold-junction compensation:1. Measure the resistance of the thermistor cold-junction sensor,R thermistor cold-junction , and compute the cold-junctiontemperature, T cold-junction , using the thermistorresistance-temperature conversion formula.2. From this temperature of the cold-junction, T cold-junction ,compute the equivalent thermocouple voltage, V cold-junction , forthis junction using a standard thermocouple conversionformula.+ +++– –––V 2Vthermocouple VmeasuredV1Vmeasured = Vthermocouple + V1 – V2 where V 1 – V2 = Vcold-junction