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The measurement of pressure by manometer or a Barometer

Pressure measurement by shipboard machinery
Shipboard machinery must operate within certain desired parameters. Instrumentation enables the parameters—pressure, temperature, and so on—to be measured or displayed against a scale. A means of control is also required in order to change or alter the displayed readings to meet particular requirements. Control must be manual, the opening or closing of a valve, or automatic, where a change in the system parameter results in actions which return the value to that desired without human involvement.

Pressure measurement

The measurement of pressure may take place from one of two possible datums, depending upon the type of instrument used. Absolute pressure is a total measurement using zero pressure as datum. Gauge pressure is a measurement above the atmospheric pressure which is used as a datum. To express gauge pressure as an absolute value it is therefore necessary to add the atmospheric pressure.



Manometer

A U-tube manometer is shown in Figure . One end is connected to the pressure source; the other is open to atmosphere. The liquid in the tube may be water or mercury and it will be positioned as shown.

U-tube manometer

Fig:U-tube manometer

The excess of pressure above atmospheric wil be shown as the difference in liquid levels; this instrument therefore measures gauge pressure. It is usually used for low value pressure readings such as air pressures. Where two different system pressures are applied, this instrument will measure differential pressure.


Mercury barometerFig:Mercury barometer


Barometer

The mercury barometer is a straight tube type of manometer. A glass capillary tube is sealed at one end^ filled with mercury and then inverled in a small bath of mercury . Almost vacuum conditions;exist above the column of mercury, which is supported by atmospheric pressure acting on the mercury in the container. An absolute reading of atmospheric pressure is thus given.

Aneroid barometerFig:Aneroid barometer

The aneroid barometer uses an evacuated corrugated cylinder to detect changes in atmospheric pressure . The cylinder centre tends to collapse as atmospheric pressure increases or is lifted by the spring as atmospheric pressure falls. A series of linkages transfers the movement to a pointer moving over a scale.





Bourdon tube

This is probably the most commonly used gauge pressure measuring instrument and is shown in Figure . It is made up of an elliptical section tube formed into a C-shape and sealed at one end. The sealed end, which is free to move, has a linkage arrangement which will move a pointer over a scale. The applied pressure acts within the tube entering through the open end, which is fixed in place.

Bourdon tube pressure gauge
Fig: Bourdon tube pressure gauger

The pressure within the tube causes it to change in cross section and attempt to straighten out with a resultant movement of the free end, which registers as a needle movement on the scale. Other arrangements of the tube in a helical or spiral form are sometimes used, with the operating principle being the same.

While the reference or zero value is usually atmospheric, to give gauge pressure readings, this gauge can be used to read vacuum pressure values.


Diaphragm pressure gauge
Fig: Diaphragm pressure gauge

Other devices

Diaphragms or bellows may be used for measuring gauge or differential pressures. Typical arrangements are shown in Figure above. Movement of the diaphragm or bellows is transferred by a linkage to a needle or pointer display.

Bellows pressure gauge
Fig: Bellows pressure gauge

The piezoelectric pressure transducer is a crystal which, under pressure, produces an electric current which varies with the pressure. This current is then provided to a unit which displays it as a pressure value.



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