Diaphragm pressure sensing element

The silicon piezoresistive sensor detects distortion. Therefore diffused resistor (gage) layout and diaphragm geometry design have a profound effect on the sensor characteristics. We utilize simulation technology for pressure-sensing element design. 

Pressure-sensing elements (a) flat diaphragm (b) corrugated diaphragm (c) capsule (d) bellows (e) straight tube (f) Cshaped Bourdon tube (g) twisted Bourdon tube (h) helical Bourdon tube (i) spiral Bourdon tube. (From Norton, H.N., Handbook of Transducers, Englewood Cliffe, NI Prentice-Hall, 1989, 294-330. Reprinted with permission.) Previously published in Chau, K.H.L., Pressure and sound measurement, in The Measurement, Instrumentation, and Sensors Handbook, Webster, I.G., Ed., CRC Press, Boca Raton, FL, 1999, p. 26-3.)... 

Elastic elements such as diaphragms and capsules are used as sensing elements in combination with different displacement transducers (e.g. inductive, potentiometric, etc.) for low/medium vacuum measurements down to about 1 torr. Elastic elements have been employed in conjunction with capacitive transducers for the measurement of pressures as low as 10 5 torr. 

A much more convenient method of direct pressure measurement involves the deflexion of a sensing element (e.g. a tensioned diaphragm made of a suitable metal or ceramic or a Bourdon tube) which separates... 

In most cases the pressure is detected using diaphragm sensors classified in terms of sensing element as metal strain gauge, piezoelectric semiconductor, electric capacitance, reluctance, and LVDT (linear variable-differential transformers) sensors. The structure of a silicon semiconductor sensor, where the pressure is detected by a silicon diaphragm, is shown in Fig. 5. In most cases these sensors can be used at temperatures lower than 70°C. 

A potentiometiic pressure sensor is another simple but effective pressure sensing device. In this case, the wiper arm of the potentiometer is linked to the diaphragm element. 

Components of the instrnment Primary sensing elements are prone to failures due to extreme conditions. E.g. in pressure transmitters overpressure stresses caused by pulsation or by hydraulic testing without isolation. Diaphragm seals with filled systems may slowly leak. Drifting occurs if excessive vacuum conditions arise. See manufacturer s limiting conditions for diaphragms. 

One important application of pneumatic transmission is in the operation of diaphragm actuators. These are the elements generally employed to drive the spindles of control valves (Section 7.22.3) and, if hard-wired transmission systems are employed, require devices which convert electric current into air pressure or air flowrate, i.e. electropneumatic (E/P) converters. The basic construction of a typical E/P converter is illustrated in Fig. 6.77. A coil is suspended in a magnetic field in such a way that when a current is passed through the coil it rotates. This rotation is sensed by a flapper/nozzle system (Section 7.22.1). The nozzle is supplied with air via a restrictor and its back pressure actuates a pneumatic relay. The output from the latter is applied to the feedback bellows and also acts as output from the E/P converter. Electropneumatic valve positioners employ the same principle of operation. 

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