Diaphragm

About diaphragm

The diaphragm is one of three types of elastic-element pressure transducer. Diaphragm-type instruments predominate in the measurement of pressures up to 10 bar. Applied pressure causes displacement of the diaphragm and this movement is measured by a displacement transducer. Both gauge pressure and differential pressure can be measured by different versions of diaphragm-type instruments. In the case of differential pressure, the two pressures are applied to either side of the diaphragm and the displacement of the diaphragm corresponds to the pressure difference. The magnitude of displacement in either version is typically 0.1 mm, which is well suited to a strain gauge type of measuring transducer.


Four strain gauges are normally used in a bridge configuration, in which an excitation voltage is applied across two opposite points of the bridge. The output voltage measured across the other two points of the bridge is then a function of the resistance change due to the strain in the diaphragm. This arrangement automatically provides compensation for environmental temperature changes. Older pressure transducers of this type used metallic strain gauges bonded to a diaphragm typically made of stainless steel. Apart from manufacturing difficulties arising from the problem of bonding the gauges, metallic strain gauges have a low gauge factor, which means that the low output from the strain gauge bridge has to be amplified by an expensive d. c. amplifier. The development of semiconductor (piezoresistive) strain gauges provided a solution to the low-output problem, as they have gauge factors up to 100 times greater than metallic gauges. However, the difficulty of bonding gauges to the diaphragm remained and a new problem emerged regarding the highly non-linear characteristic of the strain–output relationship.


The problem of strain gauge bonding was solved with the emergence of monolithic piezoresistive pressure transducers, and these are now the most commonly used type of diaphragm pressure transducer. The monolithic cell consists of a diaphragm made of a silicon sheet into which resistors are diffused during the manufacturing process. Such pressure transducers can be made to be very small and are often known as microsensors. Also, besides avoiding the difficulty with bonding, such monolithic silicon measuring cells have the advantage of being very cheap to manufacture in large quantities. Although the inconvenience of a non-linear characteristic remains, this is normally overcome by processing the output signal with an active linearization circuit or incorporating the cell into a microprocessor-based intelligent measuring transducer. The latter usually provides analog-to-digital conversion and interrupt facilities within a single chip and gives a digital output which is readily integrated into computer control schemes. Such instruments can also offer automatic temperature compensation, built-in diagnostics and simple calibration procedures. These features allow measurement accuracies of up to ±0.1% of full-scale reading.


As an alternative to strain-gauge-type displacement measurement, capacitive transducers are sometimes used. These can also be diffused into a silicon chip, and thus enable the fabrication of very small microsensors. As a further option, developments in optical fibers have been exploited in the Fotonic sensor, which is a diaphragm-type device in which the displacement is measured by optoelectronic means. In this device, light travels from a light source down an optical fiber, is reflected back from the diaphragm and travels back along a second fiber to a photodetector. There is a characteristic relationship between the light reflected and the distance from the fiber ends to the diaphragm, thus making the amount of reflected light dependent upon the measured pressure.

 

Whilst the discussion so far has been about measuring static pressures, diaphragm-type instruments can also be used to measure dynamic pressures. The principal displacement measuring device used in such circumstances is the piezoelectric crystal. However, the strain gauge in its various forms can be used as an alternative, and there are particular advantages in its use in respect of its greater measurement sensitivity and also its ability to measure static pressures. A further alternative for measuring dynamic pressures is the diaphragm-based capacitive displacement transducer.


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