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Vers. No. 1.0102 Released: 2/12/20116 Functional Procedures → Measuring with thermocouples6.5 Measuring with thermocouplesThermocouples consist of two “thermoelectric wires” which areformed from different materials, e.g. platinum and platinum/rho-dium, and are joined together at one end, usually by welding. Ifthis contact point and the other ends of the thermoelectric cableshave different temperatures, a “thermoelectric voltage” is pro-duced at the contact point. This voltage is essentially propor-tional to the temperature difference between the contact pointand the ends of the cables.Since thermocouples only measure a temperature difference (dif-ference between the temperature at the contact point and themeasured temperature at the terminal strip on the Q.brixx mod-ule), the terminal temperature must be known or the “transition”from the thermocouple cable or compensating cable to the cop-per cable must occur at a known temperature level. The first caseis known as internal cold junction compensation (TCint) and thesecond case as external cold junction compensation, TCext.To acquire the temperature with internal cold junction compen-sation an additional temperature probe is used which measuresthe reference temperature. For the Q.brixx modules a cold junc-tion compensation terminal with an integrated Pt1000 tempera-ture probe is used. In this way, the temperature at the “transitionpoint” is determined and the voltage produced by the thermocou-ple is corrected depending on the type of thermocouple.To measure the temperature using external cold point compensa-tion, a second thermocouple of the same type is needed which isconnected in series with the first one. The polarity is chosen suchthat the thermoelectric voltages subtract. The second thermocou-ple is located at a fixed reference temperature (usually 0 °C).Then, the Q.brixx module calculates the temperature at the mea-suring point based on the linearization curve. However, theQ.brixx module requires the information which reference temper-ature (cold junction temperature) is being used. |
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