Pirani gage

Pirani gage is similar here the change in temperature of the wire is measured by the variation of the wire electrical resistance. The lowest measurable pressure is around 10-3 torr. The calibration (usually for dry air) is a function of the heat conductivity of the gas. Figure 1.30 shows a typical correction to be introduced for different gases. 

Fig. 1.30. Typical pressure correction for a Pirani gage for different gas species (courtesy of Alcatel).

Pirani gage for measuring small pressures (1-2000 microns)... 

The apparatus comprised a tube, containing a fine tungsten wire, to which was attached a pirani gage calibrated for hydrogen, and a gas pipette. As before, the apparatus was pumped to a hard vacuum and the wire flashed... 

On admission of a charge of hydrogen to the wire from the pipette the wire temperature rose abruptly. Successive charges were admitted until the pirani gage indicated the appearance of hydrogen in the gas phase, and thus the saturation of the adsorbed film on the tungsten. A wire of superficial area 0.55 cm. was found to chemisorb 4.4XIO molecules. 

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