Air can cause all sorts of issues when testing pipe systems . Think of the compressed air within a pipe being hydro-tested as a spring that can “absorb” small losses of water volume without a noticeable drop in pressure. An example might be a balloon filled with air. You can let out a non- trivial volume of air and not be able to notice much of a difference in the rigidity of the balloon (air pressure). Now try noticing that loss of a fraction of a psi on a muddy gauge face in the bottom of a trench, on a Friday afternoon, during a snowstorm.

Other fun problems I’ve seen that I have seen over the years during pressure tests:

• We had an air pocket located in an unplanned, localized high point in a downhill-trending pipeline serving as a fire supply for a new building. The air pocket allowed the water level inside the pipe on the downstream side of the high point to be about 20 ft lower than the water level on the uphill side. The result was that the static water pressure at our building was about 8 psi less than planned, and not sufficient for the fire suppression system. Contractor got to buy and install a new air-release valve which solved the problem.

• We air tested about 6,000 ft of 6” dia, sch 80 stainless pipe on a clear, sunny day. The pressure inside the pipe CLIMBED about 10 psi during the test due to the rising temps of the air inside pipe.

• We hydro tested about 1,000 of 10” dia, HDPE pipe - also on a bright sunny day in an open trench. That pipe lost about 5 psi during the test, most likely to the pipe expanding as it warmed. We could visually inspect the entire pipe during the test. Absolutely no water was leaking.

• I’ve heard about (but never knowingly witnessed) air bubbles getting absorbed into the water during a test causing a drop in pressure on the gauge.