Agilent Technologies 34420A, User Manual

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Thermocouples
A thermocouple converts temperature to voltage. When two wires,
composed of dissimilar metals, are joined, an open circuit voltage is
generated. The voltage is a function of the the junction temperature and
the types of metals in the wire. Since the temperature characteristics of
special dissimilar metals is well known, a conversion from the voltage
generated to the temperature of the junction can be made. For example,
a voltage measurement of a type T thermocouple (made up of copper and
constantan wire) might look like this:
Notice, however, that the connections made between the thermocouple
wire and the meter make another thermocouple where the thermocouple’s
constantan (C) lead connects to the meter’s copper (Cu) input terminal.
The voltage generated by this second thermocouple affects the voltage
measurement of the type T thermocouple.
If the temperature of the thermocouple created at J2 (the LO input
terminal) is known, the temperature of the type T thermocouple can be
calculated. One way to do this is to connect two type T thermocouples
together to create only copper to copper connections at the meter’s input
terminals, and to hold the second thermocouple at a known temperature.
Meter
7
Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
265