# 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.

Figure 3.1: Design of a diaphragm vacuum gauge
Source: Wurz, Handbuch der Vakuumtechnik, Wiesbaden 2006, p. 453

## 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.

Figure 3.2: Design of a capacitative diaphragm vacuum gauge
Source: Wurz, Handbuch der Vakuumtechnik, Wiesbaden 2006, p. 459

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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