Pressure is defined as force per unit of area: p = F / A (Formula 1-3), where F is force and A the area to which the force is applied. The SI unit of pressure is 1 N / m2 = 1 Pa. Other frequently-used units of pressure are: 1 mbar = 100 Pa and 1 torr = 133,322 Pa. If pressure is measured via the force that is exerted on an area, it is termed a pressure measurement that is independent of the type of gas in question.
Pressure measurement on the basis of force reaches its limits at pressures of less than 1 mbar, because the exerted forces become too small. Consequently other processes must be used. The thermal conductivity of the enclosed gas can be used, for example, or the gas molecules can be ionized, the ion streams flowing between electrodes measured.
A spinning rotor gauge (SRG), a so-called gas friction gauge, is used for calibration purposes . A sphere is magnetically suspended in the vacuum and caused to rotate rapidly, at which point the drive is then de-energized. The pressure of the type of gas that is present can be calculated from the decrease in rotational frequency due to gas friction. These devices measure pressures p of more than 10-7 mbar. Calibrated spheres can be used as a transfer standard.