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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Pressure Measurements
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