Why should oxygen or shop compressed air never be used to pressurize a small sealed system for a leak check?
Correct Answer
D) Oxygen can react violently with the compressor oil and explode, and shop air carries moisture into the system
Oxygen under pressure in contact with refrigeration oil can ignite and explode, and technicians have been killed doing exactly this. Shop compressed air is a different problem: it carries water vapor and oil mist from the compressor into a system that has to be dry, seeding the corrosion and freeze-up problems that a drier and a proper vacuum are meant to prevent. Dry nitrogen through a regulator is the only correct choice for pressurizing a system.
Why This Is the Correct Answer
Oxygen and hydrocarbon oil are an explosive pairing under pressure, which makes oxygen categorically unsafe for pressure testing. Shop air introduces moisture and oil carryover that contaminate a system meant to be clean and dry. Dry nitrogen delivered through a regulator avoids both problems.
Why the Other Options Are Wrong
Option A: Both gases slip past Schrader valve cores far faster than refrigerant does, so the test pressure never holds long
Leak rate past a valve core is a nuisance, not a safety objection, and neither gas behaves that way. The reason oxygen is banned is that it can ignite the compressor oil.
Option B: Both gases will corrode an aluminum evaporator within a few hours of being left in the system
Corrosion from moisture is a slow process measured in months rather than hours, and it says nothing about why oxygen in particular is dangerous to use as a test gas.
Option C: Neither gas can build enough pressure to reveal a small leak in a sealed system of this size
Both gases reach test pressure without any trouble, which is exactly why the shortcut keeps getting suggested. The objection is the explosion hazard and the moisture, not a lack of pressure.
Memory Technique
Oxygen plus oil equals fire. Shop air plus a sealed system equals water. Nitrogen is dry and inert, which is why it is the only one left.
Reference Hint
EPA Section 608 safety material on pressure testing; dry nitrogen with a pressure regulator is the accepted method.
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Recovery from a small appliance has slowed down and the pressure is barely moving. What accepted technique helps drive the remaining refrigerant out?
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