Pressure

The pressure of gas is measured by the common water gauge, which is shown in Fig. 16. The tubes 6 and c are glass, and are filled with water up to the zero of the scale, which is graduated in inches and fifths or tenths of an inch. The tube c is opened to the air at the top. When pressure is admitted to a, the water will sink in the tube 6, and will rise in c. The difference in the height of the water in the two tubes, measured in inches, is the measure of the pressure exerted in inches of water. For measuring heavier pressures, mercury is used instead of water.

Pressures measured in inches of water or mercury may be translated into pounds per square inch or square foot, by multiplying the reading by the following figures: 1 in. of water at 62° F. = 5.2020 lb. per sq. ft. 1 in. of water at 62° F. = .0362 lb. per sq. in. 1 in. of mercury 62° F. = .4897 lb. per sq. in.

Pressure per square inch or square foot may be converted into inches or feet of water, or inches of mercury, by multiplying the pressures by the following figures:

1 lb. per sq. ft. = .1923 in. of water at 62° F. 1 lb. per sq.in. = 27.70 in. of water at 62° F. 1 lb. per sq. in. = 2.042 in. of mercury at 62° F.

Measurement Of Pressure And Flow 367

Fig. 16.

Flow

The volume of gas, passing through a pipe in a given time, is computed by multiplying the velocity by the area of the pipe. The velocity may be measured by a Pitot tube, as shown in Fig. 17. This consists of two tubes, a and 6, inserted in a plug c, the lower end of a being square, and that of 6 curved to face the current; the upper ends are connected to a water gauge d. Gas entering through 6 depresses the water column as shown; the velocity corresponding to the reading is found from tables which are generally furnished with the instrument.

The actual quantity of the gas is computed by correcting the volume for temperature and pressure, reducing it to a volume at standard temperature of 32° F. and standard pressure of 1 in. of water. The correction for temperature may be made as follows:

Rule I

Multiply the measured volume by 492 and divide the product by 460 plus the actual temperature. The quotient will be the volume at 32° F.

The correction for pressure may be made as follows: Rule 2. - Multiply the volume at 32° F. by the pressure in inches of water plus 407, and divide the product by 408. The quotient will be the volume at 1 inch pressure, and at 32° F.

Example

A pipe passes 1,000 cu. ft. of gas per hour, under a pressure of 8 in. of water and at a temperature of 60°. What will the volume be when the pressure is reduced to 1 in., and the temperature to 32°?

Solution

By the first rule, the volume at 32° is length is known, that supplied through a longer or shorter pipe is to the known volume as the square root of the given length is to the square root of the required length. With pipes of the same length and diameter, the volumes delivered at any proposed pressure is to that supplied at any other pressure as the square root of the proposed pressure is to the square root of the given pressure.

(1000x492)/(460+60)=946.1 cu.ft. By rule 2, the volume under 1 in. pressure and at 32° is [946.1x(8+407)] /408=962.3 cu.ft. If the quantity of gas delivered through a pipe of given

Measurement Of Pressure And Flow 368

Fig. 17.