Which gas is used to counteract freezing while evacuating a system with large amounts of moisture?

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

Answer: (A)

When evacuating a system that has a lot of moisture, the goal is to displace that moisture with a dry, nonreactive gas so water vapor doesn’t condense and freeze on cold surfaces or within the pump. Dry nitrogen fits perfectly: it’s inert and dry, it won’t react with system materials, and it’s inexpensive and readily available. By backfilling with dry nitrogen, you sweep moisture out of the system as you pump down, reducing the chance of ice forming that can clog passages or impair vacuum.

Oxygen would introduce reactive conditions and water vapor could still condense. Argon is inert and can purge, but it’s typically more expensive and not as practical for large-volume moisture removal. Helium, while useful for leak testing and certain cryogenic tasks, is costly and not the typical choice for drying during evacuation.