Manometer

The original manometer is a clear U-shaped tube half full of water. Pressure on one side pushes the column down and up the other, and you read the difference in inches. That is where the unit inches of water column, written in. w.c., comes from. It is a small unit on purpose, since one psi equals about 27.7 inches of water column and house gas runs at a fraction of a psi. Most crews now carry a digital version, but the unit and the idea did not change.

Section G2417.4 of the 2021 International Residential Code names the tool. Test pressure gets measured with a manometer or another device calibrated to show a pressure loss from leakage during the test. The same section caps the gauge: the top of its scale can be no more than five times the test pressure. A 3 psi test therefore tops out at a 15 psi gauge.

That cap is the whole point of the reading. Put a 100 psi gauge on a 3 psi test and a real leak moves the needle a hair, which reads as a pass. Sized right, a leak is obvious. Passing means the number does not move at all across the full hold. Any drop is a leak, and the code answer is to replace the bad section rather than chase it with dope.

For example, Pinal County requires a 10 psi test on a gauge no larger than 30 psi, held and watched by the inspector for at least 10 minutes, with the system already under pressure when he pulls up. One Arizona wrinkle to plan around: air in a sealed pipe gains pressure as it warms and loses it as it cools. A line pumped up in cool morning air and read at noon in Phoenix sun will drift, so set the gauge in shade and let the pipe settle before you call the reading.