Pressure measurement in a volume to be evacuated is an essential need. It allows us among others to check whether the applied pump system works properly during pump-down. In addition, it is often useful to have some kind of vacuum control that verifies the pressure in the vacuum system and, if necessary, switches off the vacuum equipment to avoid possible damage.
Currently, a broad variety of pressure gauge systems is in the market. All pressure measuring principles from u-tube manometer down to ionization gauge are extensively discussed in this chapter. Depending on the gauge type, pressures can be measured from 1 atmosphere down to below 10^-10 Pa. We note that no single pressure measurement principle covers this entire pressure range. However, in recent years, gauge heads consisting of a combination of two or more measuring principles, accommodated on a common connec­tion flange, have increasingly appeared on the market. Various combinations, also referred to as compound pressure gauges, cover in this way the pressure range from 1 atm to 10^-8 Pa. However, because of a number of reasons to be dealt with in this chapter, pressure gauges are not equally precise over this entire pressure range, but become increasingly inaccurate with decreasing pressure.
Pressure reading is either based on measure­ment of the pressure force or the particle density. Direct measurement of the pressure force is only possible at pressures in the range down to roughly 0.1 Pa. In the high or ultra-high vacuum ranges, pressure reading of most gauges is a measure of the particle density. As a result however, this pressure scale is only valid at the calibration temperature of the considered gauge. If the pressure gauge is located in an environment of different temperature, one needs correction of the reading.