3.1.1 Direct, gas-independent pressure measurement
Diaphragm vacuum gauges
In the case of a diaphragm vacuum gauge, pressure is measured in accordance with the definition. A pressure p is exerted on a diaphragm having a defined area A and deflects the diaphragm proportionally to the pressure. A sensor measures the deflection. Piezo-resistive or capacitive sensors receive the pressure signal and convert it into an electrical signal.
Piezo-diaphragm vacuum gauges
A simple and extremely robust method involves the use of a piezo-resistive pick-up. The design is shown in Figure 3.1. A diaphragm is arranged over a well-evacuated volume having a reference pressure p0, into which the expansion measurement resistances are diffused. The measured change in resistance as a result of diaphragm deflection serves as a parameter for the pressure. This pick-up is characterized by its insensitivity to gas inrush and its high accuracy.
Capacitive diaphragm vacuum gauges
In a capacitive vacuum gauge (Figure 3.2), deflection of the diaphragm is measured as the change in capacity of a plate capacitor that is formed by the diaphragm and a fixed counterplate in a well-evacuated space having a pressure p0. The diaphragm is comprised either of ceramic with a vacuum-metalized coating or of stainless steel. This method and diaphragms of varying sensitivity can be used to perform measurements of four decades each. The lower measurement limit is 10-5 mbar.
The limiting effects are:
- Change in clearance within the pressure transducer due to the influence of temperature
- Decreasing forces acting on the diaphragm at low pressures
The influence of temperature can be eliminated through electronic compensation of a known temperature drift or by means of an installed heater that maintains the sensor at a constant temperature. The influence of temperature can be further minimized through the use of ceramic diaphragm material.
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