Dynamical behaviour of the Pirani sensor

The conventional Pirani gauge has poor performance at atmospheric pressures since here the stationary thermal conductivity of gases saturates. In contrast, the heat capacity of the gas does not saturate. It is accessible by a pulsed operation of the wire. The present paper gives an investigation of the Pirani gauge signal for the non-stationary heating and cooling processes. As follows from the experimental data and theoretical estimates, at atmospheric pressure a substantial amount of energy is stored in the gas, much more than in the wire. However, the energy of the gas has only a rather small effect on heating and cooling rates. The reasons for this behaviour are the strong heat losses due to the thermal conductivity of the gas, the rather weak thermal coupling between the wire and surrounding gas as well as the smallness of the average temperature elevation of the gas over ambient. Nevertheless, the effect of the heat capacity of the gas on the rates is sufficiently strong to provide a usable measuring signal. The pulsed operation of the wire and the recording of the time-dependent signal can be accomplished by a smart controller. Such an instrument would provide a substantially improved performance of the Pirani sensor at pressures above 100 mbar. (C) 2004 Published by Elsevier Ltd.