This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
If the location of the pipes is not shown by the architect, then the gas-fitter must use his own judgment in determining their position. He should be governed by the following considerations:
1. The pipes should run to the fixtures in the most direct manner practicable.
2. The pipes must be graded to secure proper drainage without excessive cutting of floorbeams, or otherwise damaging the building.
3. Pipes which run crossways of floorbeams should be laid not more than 1 ft. from the wall, to avoid serious injury to the floor.
4. Fixtures should be supplied by risers rather than by drop pipes, as far as practicable.
5. All pipes should be located where they can be got at for repairs, with the least possible damage to the floors or walls.
6. The fittings should be malleable iron galvanized, beaded fittings being preferable to plain ones. Plain black iron fittings should never be used on important work.
As soon as the pipes are all in place and are properly secured, the system should be tested to find if it is gas-tight. Air should be forced in the system until the gauge indicates 15 or 20 in. of mercury, or 7 to 10 lb. per sq. in., the pressure being continued for about an hour, and if the gauge shows a falling off in pressure of more than 1/4 in. of mercury, or 1/8 lb. per sq. in., then the system cannot be passed as perfect. The mercury-column differential proving gauge is well adapted for testing gas pipes, etc., which are to be made air-tight; common spring gauges are unreliable.
The extent of a 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 vapor of the ether will diffuse throughout the system and escape from the leak, where it will be detected by its odor. The gauge should be provided with an ether cup especially for testing purposes.
In case of large buildings, it is advisible to test the piping In sections, say one floor at a time, because 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. The owner, architect, or inspector should witness the tests.