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>» 2.5 MEASUREMENT OF PRESSURE, , The pressure of a fluid is measured by the following devices :, , 1. Manometers 2. Mechanical Gauges., 2.5.1 Manometers. Manometers are defined as the devices used for measuring the pressured, a point in a fluid by balancing the column of fluid by the same or another column of the fluid. They at, classified as :, , (a) Simple Manometers, (b) Differential Manometers.

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[aie Pressure and its Measurement 43, , 2.5.2 Mechanical Gauges. Mechanical gauges are defined as the devices used for measuring, the pressure by balancing the fluid column by the spring or dead weight. The commonly used mechanical pressure gauges are :, , (a) Diaphragm pressure gauge, (b) Bourdon tube pressure gauge,, , (c) Dead-weight pressure gauge, and (@) Bellows pressure gauge., , p 2.6 SIMPLE MANOMETERS, , A simple manometer consists of a glass tube having one of its ends connected to a point where, pressure is to be measured and other end Temains open to atmosphere. Common types of simple manometers are :, , 1. Piezometer,, , 2. U-tube Manometer, and, , 3. Single Column Manometer,, , 2.6.1 Piezometer. It is the simplest form of manometer used for, measuring gauge pressures. One end of this manometer is connected to, the point where pressure is to be measured and other end is open to the, atmosphere as shown in Fig. 2.8. The rise of liquid gives the pressure, head at that point. If at a point A, the height of liquid say water is hin, piezometer tube, then pressure at A, , , , =pxexh a», m, Fig. 2.8 Piezometer., 2.6.2 U-tube Manometer. It consists of glass tube bent in U-shape, one end of which is, connected to a point at which pressure is to be measured and other end remains Open to the, , atmosphere as shown in Fig. 2.9. The tube generally contains mercury or any other liquid whose, specific gravity is greater than the specific gravity of the liquid whose pressure is to be measured., , , , (a) For gauge pressure (b) For vacuum pressure, Fig. 2.9 U-tube Manometer., , (a) For Gauge Pressure. Let B is the point at which pressure is to be measured, whose value is p., The datum line is A-A., Let h, = Height of light liquid above the datum line, hy = Height of heavy liquid above the datum line, S, = Sp. gr. of light liquid, P, = Density of light liquid = 1000 x S,, 5S, = Sp. gr. of heavy liquid, P2 = Density of heavy liquid = 1000 x Sy

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44 Fluid Mechanics, , bove the horizontal datum, , . . ssure a, As the pressure is the same for the horizontal surface. Hence pre hould be same., , line A-A in the left column and in the right column of U-tube manometer s, , Pressure above A-A in the left column =pt+p,x8x Ay, Pressure above A-A in the right column = P.X Bx hy, Hence equating the two pressures P+P gh; = P28hy, , ...(2.7), , = h a x 8 x hy). 4 7, P= (Prgh2 — Pi heavy liquid in the, , (6) For Vacuum Pressure. For measuring vacuum pressure, the jewel a Ee, manometer will be as shown in Fig. 2.9 (b). Then, , Pressure above A-A in the left column = pogh, + pigh; +P, Pressure head in the right column above A-A =0, Pgh, + Pgh, +p=0, D=— (Poghz + P18hj). (2.8)