207. Gas is measured for pressure and for volume. The pressure of gas is usually measured by a water gauge, and the reading is taken in inches of water, which represents the vertical height of a column of water which the gas pressure can sustain. This is the pressure of the gas above that of the atmosphere; it is really the working pressure, and not the absolute pressure.

The common water gauge is shown in Fig. 76. The tube a is made of metal, and is provided with a socket d which will screw on to an ordinary fixture in the place of a burner. The tubes b and c are made of glass, and are filled with water up to the zero of the scale. The scale is graduated in inches and convenient fractions of an inch. The tube c is open to the air at the top. When pressure is admitted to the tube a, the water will sink in the tube b, 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. The depression below zero in b should be added to the rise above zero in c. The fall in one tube will not exactly equal the rise in the other, unless the tubes are of exactly equal bore.

Gas Measurement 87

Fig. 76.

For measuring heavy pressures, mercury may be used in the tubes, instead of water.

208. Pressures which have been 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:

One inch of water at 62° = 5.2 lb. per square foot.

One inch of water at 62° = .0361 lb. per square inch.

One inch of mercury at 62° = .4897 lb. per square inch.

Pressures 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:

One pound per square foot = .1923 inch of water at 62°.

One pound per square inch = 27.7 inches of water at 62°.

One pound per square inch = 2.042 inches of mercury at 62°.

209. If the specific gravity of a gas is less than that of air at the same temperature, then the pressure will always be greatest at the top of the pipe or chamber which contains the gas. If the gas is heavier than air, then the greatest pressure will be at the bottom of the chamber which contains it.

The upward pressure of gas having a less density than air, is caused by the deficiency in its weight and its consequent inability to balance the pressure of the atmosphere.

The increase of pressure in each 10 feet of rise in pipes with gas of various densities, is as follows:

Table 9

Rise in pressure.

(Inches of Water)

0

.0147

.0293

.044

.058

.073

.088

.102

Density of gas.. .

1

.9

.8

.7

.6

.5

.4

.3

Example

The pressure in the basement, at the meter, is 1.2 inches of water; what will be the pressure at the sixth story, 70 feet above, the density of the gas being .4?

Solution

The table shows that the increase will be .088 inch for each 10 feet of rise, therefore, .088 X 7 = .616 inch increase. Then, pressure at sixth story = 1.2+ .616 = 1.816 in. Ans.

210. To measure the volume of gas passing through a large pipe, it is necessary, first, to determine the velocity by means of a Pitot tube, or some other suitable instrument, and then multiply the mean velocity by the sectional area of the pipe.

For ordinary purposes, however, the volume of gas passing through a pipe is measured by an apparatus called a gas meter. A gas meter measures the volume only, and its indications are not affected by any change that may occur in the pressure of the gas. Gas is used at the burners at a nearly uniform low pressure, while the pressure in the street mains often varies 3 or 4 inches per day. Consequently, a gas meter is a very inaccurate instrument for measuring the actual quantity of gas supplied.

Usually the gas meter is adjusted to measure correctly at a certain pressure, and that pressure is intended to be the average pressure of the gas in the mains in the locality where the meter is used. Although the mechanism of the meter may be perfectly accurate, yet if the pressure for which it is adjusted does not correspond with the actual average pressure in the mains, it will necessarily give incorrect measurement