Data Acquisition

Analog and digital signals 23T1Metal AT2VBVAVReference (cold)Junction at T2(Isothermal Block)Metal BMeasuring(hot)Junction at T 1Figure 2.10Thermocouple measurementIts operation is based on the principle that temperature gradients in electrical conductorsgenerate voltages in the region of the gradient.Different conductors will generate different voltages for the same temperature gradient.Therefore, a small voltage, equal to the difference between the voltages generated by thethermal gradient in each of the wires (V = V A – V B ), can then be measured at the referencejunction.Note that this voltage is produced by the temperature gradient along the wires and notby the junction itself. As long as the conductors are uniform along their lengths, then theoutput voltage is only affected by the temperature difference between the measurement(hot) junction and tile reference (cold) junction, and not the temperature distributionalong the conductor between them.2.6.1 Reference junction compensationCalculations determining the temperature corresponding to a given measured voltage of athermocouple assume that this voltage corresponds to a temperature gradient that is referencedto 0ºC. Clearly, where the reference junction is allowed to follow ambient temperature,this is not the case.Where ambient temperature variations of the reference junction would cause significanterrors in the temperature calculation from the voltage output of the thermocouple, twomethods of reference junction compensation exist:Maintain the reference junction at a constant known temperature such as an ice bath(0ºC). This is where the term ‘cold junction’ was originally derived.Measure the temperature of the reference junction and add the reference junctionvoltage. The reference junction voltage is equal to the voltage, which would be generatedby the same thermocouple if its measurement junction was at ambient temperature and itsreference junction was at 0ºC.Obviously the second option is far easier to implement and has led to the design ofmany cold junction compensation circuits. The necessary voltage correction can becarried out with software, hardware, or a combination of both.Hardware compensationHardware compensation requires dedicated circuitry to generate a compensation voltageaccording to the ambient temperature of the isothermal block, and add this voltage to thevoltage measured at the measuring junction. As the voltage vs temperature relationship