When the pipework is finished, nobody knows what is inside the circuit: a brazed joint that was overlooked, a flare nut that was not tightened, a cracked flare. Looking for these with refrigerant is both expensive and lets gas escape to the atmosphere. Instead the circuit is filled with dry nitrogen. Nitrogen is cheap, non-flammable, contains no moisture and does not react with oil. Oxygen and compressed air are never used: oxygen explodes on contact with oil, and compressed air brings moisture into the circuit. The nitrogen used must be dry and of high purity.
The test pressure is chosen according to the maximum allowable pressure of the system (the PS value on the nameplate); the EN 378 standard requires the tightness test at this value and the strength test a little above it. In R404A and R449A systems the common practice on site is 25–28 bar for the high-pressure side and 12–18 bar for the low-pressure side. The pressure is raised in steps: first gross leaks are looked for at 3–5 bar, then it is taken up to the target value. The cylinder is always connected through a regulator.
The circuit is held under pressure for at least a few hours, preferably 24 hours, and the starting and finishing pressures are noted together with the ambient temperature. If the temperature has changed, the pressure changes too: at 25 bar every 1 K amounts to about 0.09 bar, so in pipework that cools by 10 K overnight a drop of 0.9 bar does not mean a leak. If there is a drop after the temperature correction, the joints are scanned with foam. For a sensitive search nitrogen with 5% hydrogen and a hydrogen detector are used.
Points to watch: whether the pressure switches, the safety valve and some sensors withstand the test pressure is checked in advance, and if necessary they are isolated from the circuit. The manufacturer’s limit for the low-pressure side of the compressor is not exceeded. When the test is over the nitrogen is released slowly and in a safe direction; in an enclosed machinery room there is a danger of asphyxiation. The sequence never changes: nitrogen test, evacuation, then refrigerant charge. Passing nitrogen through the pipe at a low flow rate during brazing also prevents scale from forming on the inner surface.