Introduction. Vacuum pressure measurement is broadly divided into two categories: high and low vacuum. Many of the different techniques used to measure these categories have an overlap at some point in the pressure range. By combining several different types of gauge it is possible measure system pressure from 10mbar down to 10e-11mbar. VG Scienta offers both types of gauges in various options along with a comprehensive gauge controller. However for the user who requires a more basic configuration, a low cost alternative guage and controller is available. This offers vacuum pressure measurement in the range 1000mbar to 10-10mbar. For measuring in the low vacuum range (30mbar to 10-3mbar) we recommend the Pirani Gauge; for pressures below this (10-3mbar to 10-11mbar) we recommend the Ion Gauge. Pirani Gauge Heads The Pirani gauge is a roughing pressure vacuum gauge. It uses the thermal conductivity of gases to measure pressure. VG has 2 different types of Pirani gauge measuring slightly different pressure ranges; they are both available in a variety of styles with a range of mounting flanges.

Principle of Operation.

The Pirani gauge head is based around a heated wire placed in a vacuum system, the electrical resistance of the wire being proportional to its temperature. At atmospheric pressure, gas molecules collide with the wire and remove heat energy from it (effectively cooling the wire). As gas molecules are removed (i.e. the system is pumped down) there are less molecules and therefore less collisions. Fewer collisions mean that less heat is removed from the wire and so it heats up. As it heats up its electrical resistance increases. A simple circuit utilising the wire detects the change in resistance and once calibrated can directly correlate the relationship between pressure and resistance. In this way you can use a calibrated meter to indicate pressure.

This effect only works in the pressure region from atmosphere to around 10e^-3 mbar. Therefore other forms of gauge have to be used to measure pressures lower than this.

The VG Scienta range of Pirani Gauges is divided into two types: constant current, and constant resistance. The name refers to how the electrical measurement of the wire is controlled.

The constant current type has a power supply that gives a constant current all the time to the filament. Therefore the filament resistance changes are measured.

The constant resistance type has a power supply that changes the current supplied to keep the resistance of the filament the same. The constant resistance type has a slightly larger pressure range but requires more complicated electronics to control it.