How it works – The Pirani Guage
The Pirani gauge is used to measure vacuum at low or roughing levels. This method of measuring vacuum was invented in 1906 by Marcello Pirani.
In vacuum terms a basic Pirani gauge offers a measurement range from an upper figure of around 0.5 Torr down to approximately 10−4 Torr.
The basic principle of vacuum measurement used in the Pirani gauge is that of measuring the resistance of a heated filament set inside a tube placed in the vacuum to be measured. The resistance of the heated filament is proportional to its temperature.
When the filament is heated at atmospheric pressure, molecules of gas in the air collide with the filament, thus cooling it by removing the heat from the filament surface. When the pressure surrounding the filament is reduced (ie when a vacuum pump is used to rough the chamber where the gauge is located) there will be increasingly fewer molecules colliding with the filament.
The result is that the filament temperature rises, since the cooling effect of the gas molecules colliding with the filament is reduced. An electrical circuit connected to the filament is able to detect the changes in its resistance.
There are two main types of measurement used in Pirani Gauges; constant current and constant resistance.
The constant current measuring method ensures that the current flowing through the filament is maintained at a fixed level. The resulting changes in the filament resistance are thus related to the changes in pressure of the gases surrounding the filament.
Conversely, the constant temperature gauge operates by maintaining a fixed temperature of the filament by altering the current flowing through it. This has the advantage of increasing the pressure range over which the gauge maintains its accuracy but it also requires more complex control electronics.
Unlike gauges such as capacitance manometers, which react to different gases equally, Pirani gauges (and thermocouple gauges) will indicate different readings depending on the gas, or mixture of gases for which the pressure is being measured. This results from the widely varying thermal conductivities of different gases; i.e. the extent to which one gas will reduce the gauge filament temperature will differ from that of others. Calibration curves, tables or factors are often supplied that enable a gauge reading to be adjusted to the gas under measurement.
An active Pirani gauge combines the control electronics and gauge head in one unit and most modern Pirani gauges take this form.
A good example of a state-of-the-art active Pirani Gauge is the APGX100 series manufactured by Edwards High Vacuum.
APGX100 gauges have the following features:
Cable connections and gauge adjustment conveniently located, thereby minimizing the space envelope required for access
Sensor tube can be baked to 150 ºC
Adjustable set-point for simple process control and interlocking
Remote calibration possible
CSA, C/US approved
Linear output - 1 volt per decade for easy interface with vacuum control systems
LED status indicator shows normal and fault conditions
Non standard flange fittings available on request.
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