(4) On alternating current, even at low frequencies, the reversal of
current occurs so rapidly that the thermal inertia of the wire prevents any such
dissymmetry, and the temperature distribution is unchanged by these thermoelectric
effects.
This is why AC-DC differences occur.
The AC change is fast enough to
eliminate the Peltier and Thompson effects while two different polarity DC currents
will respond to the Peltier and Thompson effects.
(5) The causes of the AC-DC differences have briefly been explained here
and basically they are caused by the following:
(a) Heating of an electrical conductor varies throughout its length.
(b) Heating is not uniform through the heating element.
(c) It is impossible to locate the thermocouple junction in the exact
center of the heater.
(d) Heating and cooling takes place at all electrical junctions.
4. Circuit loading is responsible for more errors in the field of calibration than
any other source. This is normally caused by placing a low impedance instrument in
a measurement setup. The main disadvantage in the use of thermal converter meters
is the relatively low impedance.
This must always be taken into consideration.
First you must determine if the circuit under test can tolerate the low impedance.
Then you must determine whether or not that circuit will be altered by the low
input impedance. Input impedances for some of the better instruments of this type
range from 100-200 ohms per volt. You can see that such low input impedance will
limit their use.
a. In addition to the low input impedance, a second disadvantage is skin
effect. As the frequency increases, the thermal converter heater tends to increase
in resistance. This causes the heater temperature produced by a given current to
be greater than at lower frequencies which results in a higher output from the
thermocouple element and consequently an erroneous reading.
Essentially what
happens is, the current applied to a given conductor will, at higher frequencies,
tend to flow closer to the surface of the conductor. The resulting effect is that
the resistance tends to increase, giving off more heat for a given current and
causing considerably more radiation heat loss.
b. The primary methods of compensating for skin effect involves the physical
construction of the thermocouple element and its heater circuit.
Where frequency
range is critical the method of thermal conversion used is selected for the
increased sensitivity of measurement at the high frequencies.
Additionally, the
physical size of the heater element and its leads becomes critical as frequency is
increased.
Therefore, the size, length and type of heater wire are varied to
minimize the loss of frequency response due to skin effect.
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