Physical mechanism of helium detection in the Pirani vacuum sensor

A physical mechanism is proposed that describes the operation of a Pirani-type vacuum sensor in a helium atmosphere and in helium-air mixtures. It is shown that the cooling rate of the heater in the Pirani sensor is determined by the heat removal by helium atoms desorbed from the metal surface, and helium atoms carry away significantly more heat from the surface than chemisorbed molecules of atmospheric gases. In the case of helium- air mixtures, there is competition between helium atoms and molecules of atmospheric gases for adsorption sites, leading to a significant change in the temperature of the heater.