Which gas should be used to pressurize a system for leak testing?
Dry nitrogen, used with a pressure regulator and relief valve. EPA's outline says to use nitrogen rather than oxygen or compressed air, including on R-410A systems.
- Oxygen
- Compressed shop air
- Acetylene
- Dry nitrogenCorrect answer
Why “Dry nitrogen” is correct
Dry nitrogen is inert and contains no moisture, so it is the gas used for pressure testing. Oxygen or compressed air mixed with compressor oil under pressure can explode, and shop air adds moisture.
Source: EPA: Test Topics
Why the other answers are wrong
- Oxygen — Oxygen with oil under pressure can explode.
- Compressed shop air — Shop air contains oxygen and moisture; under pressure with oil it can also explode.
- Acetylene — Acetylene is a fuel gas and highly flammable.
The source, word for word
Use of nitrogen rather than oxygen or compressed air for leak detection
Use of pressure regulator and relief valve with nitrogen
EPA's outline names the gas and the regulator. The reasons given here, that oil and oxygen under pressure can explode and that shop air adds moisture, are trade knowledge.
Order of preference
For high-pressure systems, EPA's outline ranks the test gases. Nitrogen alone is best, nitrogen with a trace of R-22 is next, and pure refrigerant is the last choice. Why a trace of R-22 is added explains the middle option.
After the test
Nitrogen is exempt from the venting prohibition in any application, so pure nitrogen can be released (40 CFR 82.154(a)(1)(ii)). Stay within the test pressure on the nameplate. Low-pressure chillers are different: they use hot water or a built-in heater first, and a much lower pressure ceiling.
Keep practicing
- Why a trace of R-22 goes in the nitrogen
- The leak test pressure limit on an R-123 chiller
- What may be released without breaking the venting ban
- EPA 608 Core practice test
Unofficial, not affiliated with the EPA or any EPA-approved certifying organization. Questions are original practice written from public EPA material, not real exam questions.