a. Direct.
A directly heated thermocouple circuit is shown in Figure 1-5.
It is called a direct or contact thermocouple because the junctions of the two
dissimilar metals are connected directly to the heater. This direct contact with
the heater results in a rapid response to any change in heater temperature caused
by any current through the heater.
(1) The complete assembly is mounted in an evacuated glass tube to increase
the sensitivity by minimizing heat loss to outside atmosphere.
In other words,
there are no drafts on the thermocouple which would cool the junction and give
erroneous readings.
The heater wire is a very fine, high resistance wire with
negligible frequency effects.
The length of the heater is kept short so that it
will pass high frequencies without much attenuation.
Since the heater is
physically small, the heat loss from radiation is also reduced to a minimum.
(2) Although the small size of the heater is beneficial, it imposes a limit
on the highest frequency signal that can be measured accurately due to skin effect.
Skin effect is the increase in resistance of the heater as the applied frequency is
increased.
At high frequencies, current tends to flow mostly on the outside
surface of a conductor. Skin effect may be reduced if a rod, or hollow wire of the
same diameter is used. By this expedient, we make the total skin surface available
larger in proportion to the total cross-sectional area of the conductor. Thus the
resistance of the conductor is made more constant from DC through the upper
frequency limit of the thermocouple.
Figure 1-5.
Directly heated thermocouple
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