Chapter 8 Tutorial

Measurement Fundamentals

Thermocouple Measurements A thermocouple converts temperature to voltage. When two wires composed of dissimilar metals are joined,

a voltage is generated. The voltage is a function of the junction temperature and the types of metals in the thermocouple wire. Since the temperature characteristics of many dissimilar metals are well known, a conversion from the voltage generated to the temperature of the junction can be made. For example, a voltage measurement of a T-type thermocouple (made of copper and constantan wire) might look like this:

Internal DMM

Notice, however, that the connections made between the thermocouple wire and the internal DMM make a second, unwanted thermocouple where the constantan (C) lead connects to the internal DMM’s copper (Cu) input terminal. The voltage generated by this second thermocouple affects the voltage measurement of the T-type thermocouple.

If the temperature of the thermocouple created at J2 (the LO input terminal) is known, the temperature of the T-type thermocouple can be calculated. One way to do this is to connect two T-type thermocouples together to create only copper-to-copper connections at the internal DMM’s input terminals, and to hold the second thermocouple at a known temperature.

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