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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