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Gauges are used for a wide range of reasons within vacuum systems, including: pressure determination (is the UHV pressure low enough to optimize the experiment?), to act as system ‘inter-locks’ or ‘switches’ (when a secondary pump can be started) and to ‘control’ production processes.
Factors such as gauge reproducibility, accuracy, stability, resolution, linearity, response time, dynamic range and robustness as well as cost are all important considerations in choosing a gauge.
Vacuum gauges are either Total - the total pressure is the some of the partial pressure of each gas: Daltons law.
Partial which measure the (partial) pressure of individual gas species.
Additionally absolute gauges are termed ‘primary’ in that they can be calibrated from their own physical properties or ‘secondary’ for gauges which cannot be done so and require a reperene calibrator.
Gauges can also can be subdivided into two principle groups: those that directly measure pressure (the force exerted by impinging gas molecules) and those that measure pressure indirectly by measuring properties of gases which vary with pressure. Indirect gauges must be calibrated (or have their readings adjusted) for the specific gas present – their readings are thus “gas type dependent”. Commercially available direct gauges have a practical lower limit of 10-4 mbar so the use of indirect gauges is necessary at these and lower pressures.
Direct gauges further sub-divide into those where the difference between the measured pressure and a reference displaces a liquid or elastic/mechanical element.
Indirect gauges are often calibrated for nitrogen. For example, if the sensitivity for nitrogen is SN2 and the indicated (measured) pressure = PN2 then if the gas is known the true gas pressure (= Pgas) is:
Sgas = relative sensitivity for the gas.
In the case where SN2 = 1 then simply
Where there are multi gas species present then the situation becomes complicated. In the first instance the percentage composition of the gases needs to be known. Then from first-principles for a range of constituent gases where ri = Pi/PN2 is the relative constituent of the ith gas and the relative sensitivity of the ith gas is si = Pindicated(N2)/Ptrue, the relationship between the ‘true’ (or real) pressure and the indicated pressure is given by:
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