Capacitance Diaphragm Vacuum Gauges

Pressure - force acting on a membrane - is determined from its deformation

Principle of a capacitance diaphragm vacuum gaugeDiaphragm vacuum gauges are particularly suitable for gas type independent pressure measurements up to high vacuum. They measure the elastic deformation (Δs) of a membrane, which is a result of different forces acting on different sides of the membrane (see figure).

The capacitance diaphragm vacuum gauge measures a relative pressure. A reference volume in the gauge is evacuated to a defined value p2, p2 being much smaller than the smallest pressure that has to be measured in the recipient p1. Thus, a total pressure measurement takes place with a negligible error, given by the residual pressure p2.

In the capacitance diaphragm vacuum gauge the pressure sensible membrane acts as one of the electrodes of a capacitor. The deformation (Δs) as a function of the pressure difference causes a change in its capacity, which can be directly and accurately measured. The membranes are manufactured either out of stainless steel with a small thermal expansion coefficient or out of metal coated ceramics. In practice, ceramic based membranes are less sensitive to temperature variations. Furthermore, they have a higher bias stability based on the better relaxation, an improved resistance to corrosion and a better usability under rough conditions. The membrane should be insensitive to length changes triggered by temperature variations, since this could affect the measurement. The thickness of the membrane is a crucial factor determining the smallest measurable pressure. Typically, a membrane with a given thickness can cover a pressure range of around 4 decades.

The accuracy can be improved by regulating the sensor temperature to constant 45°C. Thus, the influence of the temperature on the data is minimized. Advantage of the capacitance diaphragm vacuum gauge is the independence on the gas type, a high accuracy (typically 0.2 % of the measured value) as well as the resistance against corrosive gases.

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