3. A number of factors limit the AC to DC transfer accuracy of a thermocouple.
One of these factors is the AC-DC reversal error of the thermocouple.
a. In all AC to DC transfer measurements with a thermocouple, the average of
the DC response for two directions of current should be taken as the reference upon
which the AC measurements are based. The reversed DC average gives a better basis
than the response for one direction alone, as effects that lead to small
differences in the response for the two directions of direct current are reduced
when the average is taken.
b. Reversal differences may be expected in a contact thermal converter
because of the flow of the heater current through the hot junction of the
thermocouple. This can cause a voltage drop in the thermocouple circuit because of
the nonlinearities of the thermal and electrical circuits. This cause of reversal
difference is minimized by the special construction techniques used in most high
current thermal converters, and is eliminated by the use of a small insulating bead
between the heater and the thermocouple in the low current thermal converters.
c. Both Peltier and Thompson effects on a heater can also cause dissymmetry
in the temperature rise of the heater and can cause differences in the EMF of the
thermocouple unless the hot junction is exactly at the midpoint of the heater.
(1) If the input leads are of a different metal than the heater conductor,
Peltier heating and cooling may occur at the junctions of the heater conductor with
its input leads.
These will be equal and opposite in sign at the two terminals,
and, if we can assume complete thermal symmetry, their effect will be to raise the
temperature of one terminal by a small amount and to lower the other by an equal
amount. Of course, complete thermal symmetry is never attained so Peltier effect
may cause a change in the midpoint temperature of the heater.
(2) Thompson heating occurs in solid, single material conductors having a
temperature gradient in the direction of the current flow. This will cause a small
decrease in the temperature rise at the midpoint of the conductor and can lead to a
transfer error in a thermal converter.
(3) With only the normal resistance heating, and with the terminals of the
conductor at an equal and fixed temperature, the temperature distribution along the
conductor is parabolic. In addition, however, there is Peltier heating or cooling
at the junction of dissimilar metals and Thompson heating along each half of the
wire.
Unlike normal resistance heating, these are dependent on the direction of
the current flow and can cause a dissymmetry of the temperature distribution along
the conductor which reverses when the direct current through the conductor is
reversed.
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