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What should never be used in a "standing pressure test" for leak-checking to avoid creating an explosive mixture?

  1. Nitrogen

  2. Oxygen

  3. Carbon Dioxide

  4. Helium

The correct answer is: Oxygen

One should avoid using oxygen in a "standing pressure test" for leak-checking as it is highly flammable and can create an explosive mixture. Option A, Nitrogen, is a commonly used inert gas and option C, Carbon Dioxide, is used as a refrigerant and might not be readily available for leak-checking. Option D, Helium, is also a common choice for leak-checking, but it may be costly and not available in all facilities. Therefore, option B, Oxygen, should never be used as it poses a safety risk.