Pressure measurement molecular vacuum gauges

Vacuum gauges may be broadly classified as either direct or indirect (10). Direct gauges measure pressure as force pet unit area. Indirect gauges measure a physical property, such as thermal conductivity or ionisation potential, known to change in a predictable manner with the molecular density of the gas. 

Hot-Cathode Ionization Gauges. For pressures below approximately lO " Pa, it is not possible, except under carehiUy controlled conditions, to detect the minute forces that result from the coUision of gas molecules with a soHd wall. The operation of the ion gauge is based on ionisa tion of gas molecules as a result of coUisions with electrons. These ions are then subsequendy collected by an ion collector. Ionisa tion gauges, used almost exclusively for pressure measurement in high, very high, ultrahigh, and extreme ultrahigh vacuums, measure molecular density or particle dux, not pressure itself. 

Lister and McDonald [135] have described in detail the construction and calibration of low temperature ethylene adsorption apparatus. In measurements of such low pressures, two obvious risks must be considered namely the desorption of water and thermal transpiration. By heating the entire system for a short while, or by permanently keeping the system under vacuum, most of the adsorbed vapors from the glass walls should be removed. Otherwise the slowly desorbing vapors will increase the pressure in the system during adsorption measurements leading to erroneous results. When low pressure measurements are made on a gauge held at a different temperature from that part of the apparatus where the adsorption takes place, correction for thermal molecular flow also needs to be considered. To obtain accurate results Lister and McDonald prepared and used correction data. 

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