Pressure measurement electric transducers for

Electrical output transducers exist in many forms and consist simply of elastic elements coupled with suitable displacement transducers such as capacitance, inductance and reluctance pick-ups, certain types of strain gauge, piezoelectric elements, potentiometers and eddy current probes. A selection of these is described in the following paragraphs. 

A typical capacitive sensor is illustrated diagrammatically in Fig. 6.11. The element consists of a gas filled parallel plate capacitor in which one plate is a flat circular diaphragm whilst the other is a fixed metal disc. When the pressure on the outside surface of the flexible diaphragm is increased by an amount SP compared with the pressure within the unit, the diaphragm is deflected by an amount w at a radius r, (assuming the distorted section to be smoothly curved) given by (Volume 6, Section 13.3.5)  

Capacitive pressure transducers are traditionally used for relatively low pressure ranges (up to about 800 kPa) and for vacuum measurement (Section 6.3.5). 

These employ a magnetically permeable member to increase and decrease respectively the inductances in the coils of a two coil inductance bridge. Changes in inductances are additive due to the way in which the coils are connected within the bridge circuit (Fig. 6.12). 

It can be shown that, for such a transducer, the inductance of each coil is given by(,)  

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