After evacuating a system that was full of moisture, a technician changes the oil in the vacuum pump before starting the next job. What does moisture-contaminated pump oil do?
Correct Answer
D) It holds moisture that boils back out inside the pump, so the pump can no longer reach a deep vacuum
A rotary vane vacuum pump seals with its oil, and that oil absorbs the water vapor it pulls out of a system. Once the oil is loaded with moisture, water boils back out of it on the low pressure side of the pump, which sets a floor on how deep a vacuum the pump can produce. The pump keeps running and sounds normal, but the micron reading plateaus no matter how long it runs. Fresh oil is cheap, and on a wet system the oil often gets changed more than once during the job.
Why This Is the Correct Answer
The oil is part of the pump's sealing and pumping mechanism, and it carries away whatever the pump removes. Water dissolved in that oil evaporates again inside the pump at low pressure, adding vapor right where the pump is trying to create a vacuum. The result is a hard floor on achievable vacuum that no amount of run time will beat.
Why the Other Options Are Wrong
Option A: It makes the pump run hot enough to trip its own thermal overload protector partway through
Wet oil rarely causes an overload trip. The failure is quiet: the pump runs and sounds normal and simply cannot pull the system any deeper than the moisture in its oil allows.
Option B: It thickens in the pump sump and blocks the intake screen, so the pump loses suction and stops pulling
Moisture makes the oil milky and thin rather than thick, and the pump keeps pulling. Expecting a mechanical blockage misses that the limit is a vapor pressure inside the pump itself.
Option C: It carries acid back into the next system through the service hoses on the following job
Acid travels out of a burned system in its oil and refrigerant, not backward out of a vacuum pump. Hoses and the pump's own check valve keep pump oil out of the next appliance.
Memory Technique
Wet oil, wet vacuum. Milky in the sight glass means change it before you blame the system.
Reference Hint
40 CFR 82.156 requires reaching specified evacuation levels; pump maintenance is what makes that achievable in practice.
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