The physical mechanism of operation of the Pirani vacuum sensor in the temperature stabilization mode

An experimental study of the processes on the surface of a metal heater (filament), which determine the physical mechanism of operation of a Pirani-type vacuum sensor in a wide pressure range of 1000 – 10$^{-3}$ mbar for the main components of air: nitrogen and oxygen has been made. It is shown that in the mode of stabilization of the constant temperature of the heater, the mechanism of power removal by the gas from the surface changes significantly with a change in pressure. At $p$ = 10$^{-3}$–10$^{-1}$ mbar, heat is carried away by molecules adsorbed on the surface over a layer of chemisorbed nitrogen or oxygen atoms. At high pressures, the molecular layer is almost completely filled, and additional heat removal is carried away by molecules adsorbing into the second (physisorbed) layer; in this case, the efficiency of heat removal sharply decreases, almost 1000 times per molecule.