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Pressure
We have already discussed some basics of pressure (e.g. measurement units).
Now I would like to familiarize you with ways we mesure pressure.
The pressure within a UHV system can vary from atmospheric (when it is vented) to 10-11 mbar and no one gauge can measure across this wide range of pressures.
Pirani gauge (~ 100 mbar to 10-3 mbar)
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Arguably the most common gauge used in surface science for measurement at the high pressure end of the scale is the Pirani gauge. (Measurement of pressures greater than ~ 100 mbar may be via a diaphragm gauge).
The physical principle underlying the operation of a Pirani gauge is the pressure dependence of the thermal conductivity of a gas below a pressure of ~ 100 mbar. (Above 100 mbar thermal conductivity of a gas does not vary with pressure because the increase in the number density of molecules is exactly compensated by a reduction in the mean free path. However the smaller number density of molecules at lower pressures means that the kinetic model picture of heat transfer as being due to colliding molecules breaks down.)
A metal filament, usually tungsten, is held within a metal tube connected to the vacuum system and is heated by an electric current. The variation of the filament temperature (and thus its resistance) with pressure is monitored using a simple Wheatstone bridge set-up.
Ion gauge (~ 10-3 mbar to 10-11 mbar)
This is the most important device in surface science for measurement of total pressure in the medium vacuum to UHV regimes. Its operation is based on the ionisation of molecules via a sufficiently high energy electron beam. The ionisation rate and thus the ion current (I+) is directly dependent on the gas pressure.
The Ion Gauge consists of three distinct parts, the filament, the grid, and the collector. The filament is used for the production of electrons by thermonic emission. A +ve charge on the grid attracts the electrons away from the filament; they circulate around the grid passing through the fine structure many times until eventually they collide with the grid. Gas molecules inside the grid may collide with circulating electrons. The collision can result in the gas molecule being ionised. The collector inside the grid is -ve charged and attracts these +ve charged ions. Likewise they are repelled away from the +ve grid at the same time. The number of ions collected by the collector is directly proportional to the number of molecules inside the vacuum system. By this method, measuring the collected ion current gives a direct reading of the pressure.
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