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.

Thursday, October 9, 2008

MEASUREMENT OF VACUUM

Vacuum pressures are those which are below atmospheric. With modern vacuum pressure systems, it is possible to obtain pressures from 1000m bar (approximately 1 atmosphere) down to 10m bar. There is no single transducer available which covers this full range. Down to 1m bar, it is possible to use some of the techniques, e.g. manometers and diaphragm type transducers. Some of the methods of vacuum measurement are discussed below.

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 H_o(1-KH)

P= measured pressure

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

H= difference in hights of the two mercury columns

H_o =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).

DISADVANTAGES

I) lack of good vibration and shock resistance
II) difficult to repair
III) limited to relatively low pressures

ADVANTAGES

i) their moderate cost

ii) high over range characteristics

iii) adaptability to absolute and differential pressure measurement

iv) good linearity

v) availability in several materials for good corrosion resistance

vi) small size

vii) adaptability to slurry services

SLACK DIAPHRAGM GAUGE

It is more difficult to measure pressure below the atmospheric pressure because the changes are small. The full range from atmospheric pressure to a perfect vacuum is only 14.7 psi (1 kg/cm²)

A slack diaphragm gauge with a weak spring and a large area can be used over pressure ranges as low as 0.01- 0.40 mm Hg (torr). It is possible to achieve accuracies of 1-2%.

METALLIC DIAPHRAGM GAUGE

It consists of a thin flexible diaphragm made of materials such as brass or bronze. The motion of the diaphragm operates an indicating or a recording type of instrument.

This type of gauge is capable of working in any position and is portable, and therefore well adapted for use or for installation in moving equipments such as aircrafts.

DIAPHRAGM PRESSURE TRANSDUCERS

Diaphragms are widely used for pressure (gauge pressure) and draft measurements, particularly in very low ranges. They can detect a pressure differential even in the range of 0 to 4 mm.

The diaphragms can be in the form of flat , corrugated or dished plates and the choice depends on the strength and amount of deflection desired.

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