Page 14 - Instrumentation and Measurement

Inclined manometers were developed to measure low pressures. The low-pressure arm is inclined, so
that the fluid has a longer distance to travel than in a vertical tube for the same pressure change. This
gives a magnified scale as shown in Fig. 2.2a.
Well manometers are alternatives to inclined manometers for measuring low pressures using low-
density liquids. In the well manometer, one leg has a much larger diameter than the other leg, as
shown in Fig. 2.2b. When there is no pressure difference the liquid levels will be at the same height
for a zero reading. An increase in the pressure in the larger leg will cause a larger change in the height
of the liquid in the smaller leg. The pressure across the larger area of the well must be balanced by
the same volume of liquid rising in the smaller leg. The effect is similar to the balance of pressure
and volume in hydraulic jacks.

Figure 2.2 Other types of manometers are the (a) inclined-tube manometer and (b) well-type
manometer.
2.2.2 Diaphragms, capsules, and bellows
Gauges are a major group of pressure sensors that measure pressure with respect to atmospheric
pressure. Gauge sensors are usually devices that change their shape when pressure is applied. These
devices include diaphragms, capsules, bellows, and Bourdon tubes.
A diaphragm consists of a thin layer or film of a material supported on a rigid frame and is shown in
Fig. 2.3. Pressure can be applied to one side of the film for gauge sensing or pressures can be applied
to both sides of the film for differential or absolute pressure sensing. A wide range of materials can
be used for the sensing film, from rubber to plastic for low-pressure devices, silicon for medium
pressures, to stainless steel for high pressures. When pressure is applied to the diaphragm, the film
distorts or becomes slightly spherical. This movement can be sensed using a strain gauge,
piezoelectric, or changes in capacitance techniques (older techniques included magnetic and carbon
pile devices). The deformation in the above sensing devices uses transducers to give electrical signals.
Of all these devices the micromachined silicon diaphragm is the most commonly used industrial
pressure sensor for the generation of electrical signals.

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