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

Mcleod Gaues

The McLeod gauges is used for measuring very low pressures down to one hundered-thousandth of an inch of mercury (or 10m bar down to 10-3m bar). The pressure can be calculated by using the following equations:

P=KH Ho ­­­(1-KH)

P= measured pressure

K= a constant, determined by the geometry of the gauge

H= difference in hights of the two mercury columns

Ho =hight of the top of the closed capillary tube above the line marked on the tube.

The McLeod gauge is a very accurate pressure-measuring device and often serves as a standered for calibrating other low-pressure gauges. If can be designed to measure pressure as low as 0.05 microns(0.00005 torr).

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