Which gas should be used to pressurize a system after installation to test for leaks?

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When testing a system for leaks after installation, you want an inert, dry gas that won’t react with refrigerant or lubricants and won’t introduce moisture. Nitrogen fits this role perfectly: it’s inert, nonflammable, inexpensive, and widely available, so you can pressurize the system safely to the test pressure and look for pressure loss without complications. Helium can be used for very sensitive leak detection, but it requires special equipment and is more costly for general field testing. Argon is also inert but less practical due to cost and availability, and oxygen is not used because it supports combustion and can react with oils. So nitrogen is the practical, safe choice for leak testing after installation.