Manometers

The manometer is the simplest measuring instrument 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.

U-tube Manometer

The U-tube is the simplest form of manometer and is used for experimental work in laboratories. By suitable choice of liquids, a wide range of pressure can be recorded.

Construction It consists of a transparent (glass) tube constructed in the form of an elongated U and is partially filled with a liquid, most commonly water or mercury. Water and mercury are used because their specific weights for various temperatures are known exactly and they do not stick to the tube. One ned of the tube is connected to one pressure tap and the other end is connected to the other pressure tap, or it may be left open to the atmosphere.

Working When there is a pressure difference between the two ends of the tube, the liquid level goes down on one side of the tube and up on the other side. The difference in liquid levels from one side to the other indicates the difference in pressure.

Well-type Manometer

The well-type manometer is widely used because of convenience; the reading of only a single leg is required in it. It consists of a very large-diameter vessel (well) connected on one side to a very small sized tube. Thus the zero level moves very little when pressure is applied. Even this small error is compensated by suitably distorting the length of scale. However, such an arrangement is sensitive to non-uniformity of the tube cross-sectional area and is thus considered somewhat less accurate.

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.

Barometer

Since manometers inherently measure the pressure difference between the two ends of the liquid column, if one end is at zero absolute pressure, then the difference in height of the liquid from the zero reference indicates the absolute pressure. This is the principle of the barometer.

A barometer is a well-type absolute pressure gauge whose pressure range is from zero absolute to atmospheric pressure. Its readings are generally in millimeters 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.

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 10 degree. The inclination is done to expand the scale and thereby to increase the sensitivity.

The inclined manometer is used to measure very small pressure differences (in hundredth of an inch of water). The manometer is tipped so that the liquid moves a longer distance through the tube as it rises. The distance y that the liquid moves through the tube is greater than the distance x that the liquid rises.