MANOMETERS

The manometer is the simplest measuring instument used for gauge pressure (low-range pressure) measurements, by balancing the pressure against the weight of a column of liquid. The action of all manometers depends on the effect of pressure exerted by a fluid at a depth. The different types of manometers are discussed below.

1) The U-tube Manometer :-

The U-Tube is the simplest form of manometer and is used for experimental work in laboratories.

Limitations :-

In the U-tube manometer,the application of pressure causes the liquid in one leg to go down while hat in the other leg goes up, so there is no fixed referece. This tends to make the measurement of the height more difficult then it would be if one surface could be maintained at some fixed level.

2) WELL-TYPE MANOMETER :-

The Well-type manometer is widely used because of the convenience in its

requiring the reading of only a single leg.

In a single-leg instrument, high accuracy is achieved by setting the zero level of the well at the zero level of the scale before each reading is taken.

3) THE BAROMETER :-

A barometer is well-type absolute pressure gauge whose pressure range is from zero absolute to atmospheric pressure. Its readings are generally in millmeters of mercury (mm Hg), With a barometer, high vaccums are not measured. The pressure in the evacuated portion of the barometer is not really absolute zero but rather the vapour pressure of the filling fluid, mercury, at ambient temperature.

4) INCLINED MANOMETER :-

The inclined tube manometer or slant manometer is an enlarged leg manometer with its measuring leg inclined to the vertical axis by some angle. The angle of inclination is of the order of 100 .

The inclined manometer is used to measure very small pressure differences (in hundredth of an inch of water).

5) THE MICROMANOMETER :-

A micromanometer is used for the accurate measurement of extremely small pressure differences.

Wednesday, October 8, 2008

Construction And Working

A C-type Bourdon tube consists of a long thin-walled cylinder of non-circular cross-section, sealed at one end, made from materials such as phosphor bronze,steel and beryllium copper and attached by a light line work to the mechanism which operates the pointer. The other end of the tub is fixed and is open for the application of the pressure which is to be measured. The tube is soldered or welded to a socket at the base, through which pressure connection is made. Figures 5.8 (a) & (b) show the schematic arrangement of a complete Bourdon tube gauge.

As the fluid under pressure enters the Bourdon tube, it tries to change the section of the tube from oval to circular and this tends to straighten out the tube. The resulting movement of the free end of the tube causes the pointer to move over the scale. The tip of the Bourdone tube is connected to a segmental lever through an adjustable lever link. The lever length also may be adjustable. The semental lever end on the segment side is provider with a rack which meshes to a suitable pinion mounted on a spindle. The segmental lever is suitably pivoted and the spindle holds the pointer, as shown in fig 5.8 (b). A hairspring is provide the necessary tension for proper meshing of the gear teeth, thereby freeing the system from backlash. Any error due to friction in the spindle bearing is known as “lost moion”.

Bourdon tubes are made of a number of materials, depending upon the fluid and the pressure for which they are used, such as phosphor bronze, alloy steel, stainless stel, “Monel”, metal, and beryllium copper. For adequate reliability, the materials for Bourdon tubes must have good elastic or spring characteristics. Bourdon tubes are generally made in three shapes:

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