represent the frequency variations of the micro-
thermocouple gauge as a function of the static pressure (air) in
the vacuum measurement chamber for a K type microthermocouple
of diameter 25.4
mm. The two different cases present
the same behavior. Each experimental curve is composed by
three parts corresponding to the quality of energy transfer between
the sensor and the gas which is pressure-dependent [13].
First, from 0.1 Pa to about 10 Pa, the sensor response is independent
of the pressure and no significant variations are observed. For
this vacuum range, heat transfer occurs by radiation because of the
small amount of gas inside the volume. The mean free path of the
gas molecules is larger than the dimension (internal diameter of
the chamber) of the chamber. The wire cannot exchange heat with
surroundings and its temperature increases so that it cannot be
cooled: the oscillation frequency f (Hz) does not vary. The second
part corresponds to the range between 10 Pa and 400 Pa