(RMS) value of the applied AC input.
Figure 1-1 is a diagram of a basic
thermocouple.
The AC input is applied to the measuring junction and the heating
effect of this AC input causes the measuring junction to increase in temperature,
thereby producing a DC output to the meter. Remember that an equivalent value of
DC will produce the same temperature at the measuring junction as an equal value of
RMS AC.
Figure 1-1.
Basic thermocouple
3. Now we know that thermocouples produce a DC output when the measuring junction
is heated; let's discuss some of the laws that are associated with thermocouples.
We will not mention every law, but only those that will be of some concern to you
and your use of thermocouples.
4. At the junction of two dissimilar metals, there exists an EMF known as the
Seebeck effect, which is a function of temperature (fig 1-2).
If the circuit is
closed by another remote junction of the two conductors, another opposing EMF
exists at the other junction. If these junctions are at the same temperature, the
EMFs are equal and opposite, and no current flow will result.
However, if one
junction is at a higher temperature, the EMF at the hot junction will exceed that
at the cold junction, and a current will flow, which is dependent on the resistance
of the circuit. This current flow through a resistance involves a dissipation of
energy in heating the conductor, or the current may be used to perform work. The
electric energy derived from an absorption of heat at the hot junction and a
rejection of heat at the cold junction, so that the device is a thermodynamic
engine for the conversion of heat to electrical work.
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