What gas is used for leak testing after reassembly of the system?

• A. Nitrogen
• B. Helium
• C. Oxygen
• D. Argon

Answer: (A)

When checking for leaks after reassembly, you want to pressurize the system with an inert gas so you can see where it escapes without causing chemical reactions or safety issues. Nitrogen fits this role well: it’s inert, non-reactive with refrigerants and oils, inexpensive, and widely available. You can pressurize to a safe level and look for leaks with soap bubbles or a leak detector, which makes it practical for routine service.

Helium could reveal smaller leaks more sensitively, but it requires specialized equipment and can be costly for everyday use. Oxygen isn’t used because mixing oxygen with lubricants and refrigerants can create safety hazards, and argon, while inert, is less convenient and more expensive to obtain in typical service settings.