Pressure Measurements
About pressure measurements
Pressure measurement is a very common
requirement for most industrial process control systems and many
different types of pressure-sensing and pressure measurement systems
are available. Before considering these in detail, however, it is
important to explain some terms used in pressure measurement and to
define clearly the difference between absolute pressure, gauge
pressure and differential pressure. The absolute pressure of a fluid
defines the difference between the pressure of the fluid and the
absolute zero of pressure, whereas gauge pressure describes the
difference between the pressure of a fluid and atmospheric pressure.
Absolute and gauge pressure are therefore related by the expression:
absolute pressure = gauge pressure + atmospheric pressure
The term differential pressure is used to describe the difference
between two absolute pressure values, such as the pressures at two
different points within the same fluid (often between the two sides of
a flow restrictor in a system measuring volume flow rate).
The range of pressures for which measurement is commonly required is
the span from 1.013 bar to 7000 bar (1 atmosphere to 6910
atmospheres). Apart from certain types of diaphragm-type instrument
which use either a piezoelectric crystal or a capacitive transducer to
measure the pressure-induced displacement, all the instruments listed
below are restricted to measuring only static pressures. Instruments
and techniques for measuring dynamic pressures have not been included
because their measurement is a very specialized area which is not of
general interest.
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Measurement of Mid-Range Pressures
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U-Tube
Manometer
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Dead-Weight
Gauge
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Well-Type
Manometer (Cistern Manometer)
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Diaphragm
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Bellows
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Bourdon Tube
-
Resonant-Wire Devices
-
Fiber Optic Pressure Sensors
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Low
Pressure Measurement
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Thermocouple Gauge
-
Pirani Gauge
-
Intelligent Pressure Transducers

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