As soon as the piping is completed, it should be tested by means of an air pump; a manometer or mercury gage is used to indicate the pressure. In the case of large buildings, it is better to divide the piping into sections, and test each separately. All leaks revealed must be repaired at once, and the test repeated until the whole system is air tight at a pressure of from 15 to 20 inches of mercury, or 7 1/2 to 10 pounds per square inch.

The final test is of great importance. This test is to provide against future troubles and dangers from leaks resulting from sand holes in the fittings, split pipe, imperfect threads, loose joints or outlets left without capping. If the building is new, a careful inspection should first be made to see that all outlets are closed, then the valve in the service pipe closed and the air pump attached to any convenient side-light. To the same outlet or an adjacent one attach the mercury column gage used by gas fitters, and having a column from 15 to 20 inches in height. Care must be taken that there are no leaks in the gage or its connections; a tight-closing valve must be placed between the gas pipe and the temporary connections with the pump, so that it may be shut off immediately after the pump stops, thus preventing any leakage through the pump valves or hose joints. When all is ready, pump the system full of air until the mercury rises to a height of at least 12 inches in the gage; then close the intermediate valve between the pump and the piping. Should the mercury column "stand" for five minutes, it is reasonable to assume that the pipes are sufficiently tight for any pressure to which they will afterward be subjected.

If the mercury rises and falls with the strokes of the pump, it indicates a large leak or open outlet near the pump. But should there be a split pipe or an aggregation of small leaks, the mercury will run back steadily between the strokes of the pump, though more slowly than it rose. Should it rise well in the glass and sink at the rate of 1 inch in five seconds, small leaks in fittings or joints may be expected.

A leak that cannot be detected by the sound of issuing air may usually be found by applying strong soap-water with a brush over suspected joints or fittings; the leak in this case being indicated by the bubbles blown by the escaping air. Sometimes it is necessary to use ether in the pipes for locating leaks, if the pipes are in partitions or under floors. The ether is put into a bend of the connecting hose, or in a cup attached to the pump, and forced in with the air. By following the lines of the pipe, the approximate position of a leak may be determined by the odor of escaping ether.

If the house is an old one or has been finished, the meter should be taken out and the bottom of the main riser capped. Next remove all fixtures and cap the outlets. Then use ether to locate the leaks before tearing up floors or breaking partitions.