Measurement of Mid-Range Pressures
About measuring mid-range pressures
(1.013-7000 bar)
The only instrument commonly used for
measuring absolute pressures in the mid-range of 1.013 bar to 7000 bar
is the U-tube manometer, in the form where one end of the U-tube is
sealed and evacuated. The U-tube contains a fluid, and the unknown
pressure is applied to the open end of the tube. The
absolute pressure is measured in terms of the difference between the
mercury levels in the two halves of the tube. Quite apart from the
difficulty of judging exactly where the meniscus levels in the mercury
are, such an instrument cannot give a perfect measurement because of
the impossibility of achieving a total vacuum at the sealed end of the
tube. Although it is possible by modern techniques to design an
instrument which does give a reasonably accurate measurement of
absolute pressure, the problem is usually avoided in practice by
measuring gauge pressure instead of absolute pressure.
Gauge pressure is generally measured in one of two ways, either by
comparison with a known weight acting on a known area, or by
deflection of elastic elements. Instruments belonging to the former
class are a second form of the U-tube manometer and the dead-weight
gauge, whereas the latter class consists of either a diaphragm or some
form of Bourdon tube. Apart from these two standard classes of
measurement device, modern developments in electronics now allow the
use of other principles in pressure measurement, such as in the
resonant-wire device.
Choosing between the various types of instrument available for
measuring mid-range pressures is usually strongly influenced by the
intended application. U-tube manometers are very commonly used where a
visual indication of pressure levels is required, and dead-weight
gauges, because of their superior accuracy, are used in calibrating
other pressure measuring devices.
Where compatibility with automatic control schemes is required, the
choice of transducer is usually either a diaphragm type or a Bourdon
tube, with the former now being predominant. Bellows-type instruments
are also sometimes used for this purpose (but much less frequently),
mainly in applications where their greater measurement sensitivity is
required.
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Pressure Measurements
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