Using dry nitrogen to pressure test a system is a very effective method of verifying a system is leak-free. In fact, I believe it is more reliable than using a standing vacuum test typically performed after evacuation.

Pressure testing a system with dry nitrogen tests the system under a positive pressure, which is its normal operating state. Also, if there is a loose joint when in a vacuum state, it might be possible for the filler material to be drawn into the joint, creating a temporary seal and a false positive result — but it will leak when pressurized.

When leaving a system under a nitrogen pressure test, if the temperature of the nitrogen drops, so will its pressure. This could cause you to believe there is a small leak when, in fact, the system is tight. The slight drop in pressure is actually the result of a drop in the temperature and not a system leak.

Using the Ideal Gas Law and a little math, you can determine the accepted drop in pressure as the result of a drop in temperature. The Ideal Gas Law states:

PV / nRT

Where
P = Absolute pressure of the gas, measured in psia
V = Volume of the gas
n = Amount of the gas measured as moles
R = Ideal gas constant
T = Temperature in Rankin (°R)

When comparing two states of a gas, we can rewrite the formula to:

(P1V1 / nRT1) = (P2V2 / nRT2)

When we use dry nitrogen to pressurize a refrigeration system, we can assume volume (V), n, and R are all fixed values and will not change, so we can again rewrite the formula to:

P1 / T1 = P2 / T2

We can solve for P2, and now the new formula becomes:

P2 = T2 * (P1 / T1))

So, for example, consider a system that is pressurized to 150 psig with an ambient temperature of 80°F and is left under this pressure overnight. The next day, the ambient temperature drops to 70°F, so we would expect the nitrogen pressure to also drop slightly, not because of a leak, but due to the change in temperature. Using our new formula and converting the temperature from Fahrenheit to Rankin and psig to psia, we can predict the slight change in pressure:

P2 = (70°+459.67) * ((150+14.7)/(80°+459.67))

P2 = 529.67 * (164.7/539.67)

P2 = (161.64 PSIA – 14.7) = 146.95 psig

Seeing a slight drop in pressure would not be the result of a leak but of a change in temperature. However, if the pressure had dropped below 146.95 psig, then there is likely a leak that would need to be located and repaired.

Some digital pressure gauges have a test pressure function, which use a temperature probe to compensate for the change in temperature during the test. These will display a true loss of pressure as the result of a potential leak and not a change in temperature over a specific period of time. This can be a very useful feature to use on a digital manifold, as it will save you from having to do any math during the test.

So the next time you pressure test a system with dry nitrogen and the ambient temperature changes, you should calculate the accepted change in pressure before concluding that there is a small leak within the system.