When the air test is applied to the roughing, air should be forced into the system through a force pump, until a pressure of 10 lbs. is reached, 10 lbs. representing 20 in. of mercury. The air test is not so convenient and satisfactory to the plumber as the water test, for the location of a small leakage of air is not so easily found as a small leakage of water. Generally, in the case of a small air leak, the plumber goes over the pipe with a lather of soap applied with a brush. The escaping air will form a bubble, thus showing the location of a defect.

However, the air test subjects all parts of the system to the same uniform pressure, while the pressure in the water test varies from zero pressure at the top to a pressure at the bottom depending upon the height of the stack. In applying the air test, all openings are closed. Through any convenient plug, a gas pipe is connected, to which a mercury gauge is attached, and hose connection made to the force pump. The air pumped into the system exerts a pressure on the mercury, forcing it upward in the tube about two inches for each pound of air pressure.