This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
All plumbing work of any importance should be given two tests; the first, called the "roughing test," applies only to the soil, waste and vent pipes, and is made before the fixtures are connected. The best method of making this test is to plug the main drain pipe just outside the running trap, and also all openings for the connections of fixtures, etc., and then fill the entire system with water. This may be done in small systems through the main vent pipe on the roof, and in larger ones by making a temporary connection with the water main. If any leaks are present they are easily detected in this way. In cold weather, when there would be danger of freezing, compressed air under a pressure of at least ten pounds per square inch may be used in place of water. Leaks in this case must be located by the sound of the issuing air. The water test is to be preferred in all cases, as it is easier to make, and small leaks are more easily detected.
The final test is made after the fixtures are in and all work is completed. There are two ways of making this test, one known as the "peppermint test," and the other as the "smoke test." In making either of these, the system should first be flushed with water, so that all traps may be sealed. If peppermint is used, 4 to 6 ounces of oil of peppermint, depending upon the size of the system, are poured down the main vent pipe, and then a quart or two of hot water to vaporize the oil. The vent pipe is then closed, and the inspector must carefully follow along the lines of piping and locate any leeks present by the odor of the escaping gas. Another and better way is to close the vent pipe and vaporize the oil in the receiver of a small air pump, and then force the gas into the system under a slight pressure. The receiver is provided with a delicate gage, so that after reaching a certain pressure (which must not be great enough to break the trap seals) the pump may be stopped and the pressure noted. If, after a short time, the pressure remains the same, it is "known that the system is tight; if, however, the pressure drops, then leaks are present and must be located, as already described. Ether is sometimes used in place of peppermint for this purpose.
In making the smoke test the system is sealed, and the vent pipes closed in the same manner as for the test just described; smoke from oily waste or some similar substance is then forced into the pipes by means of a bellows. When the system is filled with smoke, and a slight pressure produced, the fact is shown by a float, which rises and remains in this position if the joints are tight. If there are leaks, the float falls as soon as the bellows are stopped. Leaks may be detected in this way, both by the odor of the smoke and by the issuing jets from leaks of any size. Special machines are made for both the peppermint and smoke tests. The water test is preferable for roughing in, and the smoke test for the final. Every system of plumbing should be tested at least once a year.