During evacuation of a moisture-rich system, which gas is used to counteract freezing?

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

Answer: (D)

Purging with a dry, inert gas to remove moisture is the main idea here. When you evacuate a moisture-rich system, water vapor can condense and even freeze on cold surfaces as pressure drops, creating ice that blocks flow and complicates the vacuum process. Introducing dry nitrogen sweeps out air and moisture without adding new water to the system, lowering the chance of ice formation as you pump down. Nitrogen is ideal because it’s inert, inexpensive, and readily available, so it won’t react with components or introduce additional moisture. Other gases could be more costly or riskier to handle, or they don’t offer the same practical balance of dryness and safety as nitrogen.