225. As soon as the pipes are all in place and are properly secured, the system should be tested to find whether it is perfectly gas-tight. The instruments required for making the test are a pump and a mercury gauge.

Air should be forced into the pipe system until the gauge indicates 15 or 20 inches of mercury, or 7 to 10 pounds per square inch. The pump should then be shut off, leaving the gauge under pressure. The pressure should be continued in the pipes for about an hour, and if the gauge shows a falling off in pressure of more than one-half inch of mercury, or one-quarter of a pound per square inch, then the system cannot be passed as perfect.

The extent of the leak may be judged by the rapidity of the fall in pressure, but its location must be found by the sense of smell. For this purpose, a small quantity of ether should be introduced into the pipes. The odor of the ether will diffuse throughout the system of piping and will escape from the leak, thus revealing its location.

If any fittings are cracked, they should in all cases be removed. Cracked or split pipes should always be removed; it is useless to try to patch them. No patched or cemented pipes or fittings should be allowed to pass.

In case of large buildings, it is advisable to test the piping in sections, say one floor at a time, since in this way it is much easier to locate leaks. After each section is tested, they may be connected, and then subjected to a final test.

The pipes should not be covered until the tests are completed. Usually the gas companies or the city authorities require that the testing be done in the presence of their inspector. If no such regulations are in force, then the owner or architect should witness the tests, so as to avoid any possible disputes.