User`s guide

269
Chapter 7 Tutorial
Measurement Fundamentals
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7
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:
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.
Internal DMM
34970A Refresh UG.book Page 269 Wednesday, February 17, 2010 12:34 PM