How Pressure is Measured in Buildings
General Properties of Gases
In the preceding Section we saw that undue pressure of a gas may cause explosion. It is important, therefore, that authorities keep strict watch on gases confined within pipes and reservoirs, never allowing the pressure to exceed that which the walls of the reservoir will safely bear.
Pressure in a gas pipe may be measured by a simple instrument called the pressure gauge: The gauge consists of a bent glass tube containing mercury, and so made that one end can be fitted to a gas jet. When the gas cock is closed, the mercury stands at the same level in both arms, but when the cock is opened, the gas whose pressure is being measured forces the mercury up the opposite arm. If the pressure of the gas is small, the mercury changes its level but very little. It is clear that the height of a column of mercury is a measure of the gas pressure. Now it is known that one cubic inch of mercury weighs about half a pound. Hence a column of mercury one inch high indicates a pressure of about one half pound to the square inch; a column two inches high indicates a pressure of about one pound to the square inch, and so on.
This is a very convenient way to measure the pressure of the illuminating gas in our homes and offices. The gauge is attached to the gas burner and the pressure is read by means of a scale attached to the gauge.
In order to have satisfactory illumination, the pressure must be strong enough to give a steady, broad flame. If the flame from any gas jet is flickering and weak, it is usually an indication of insufficient pressure and the gas company should investigate conditions and see to it that the consumer receives his proper value.
FIG. - A pressure gauge.