An oxygen monitor is used to decide whether a chiller machine room is safe to enter after a refrigerant release. Below what oxygen reading is the atmosphere treated as oxygen deficient?
- ABelow twenty-one percent oxygen by volume
Twenty-one percent is normal air. Treating any reading below normal as deficient would put a room out of service for a trivial fluctuation.
- Below nineteen and a half percent oxygen by volume
- CBelow sixteen percent oxygen by volume
Sixteen percent is well into the range where judgment and coordination are already impaired. Waiting for that reading means people are affected before the alarm means anything.
- DBelow twelve percent oxygen by volume
Twelve percent is close to the level at which a person collapses. No usable warning threshold sits that low.
Why B is correct
Nineteen and a half percent is the line at which an atmosphere is considered deficient and unsafe to enter without supplied air. It sits only a percentage point and a half below normal because the effects of low oxygen come on gradually and are not obvious to the person experiencing them. Anything below it means ventilate and re-test, or enter with supplied air.
What this question is testing
This tests recall of the exposure threshold that governs entry decisions. The exam pairs oxygen deficiency with refrigerant releases because displacement, rather than toxicity, is what usually harms people in a machine room.
On the job
Chiller machine rooms are frequently below grade with poor natural ventilation, and the vapor from these refrigerants is heavy and settles. A monitor at the door is what tells a crew whether the exhaust has done its job. The number matters because operators will ask whether twenty percent is good enough, and the answer is that twenty is acceptable while nineteen is not, and that the difference is deliberately set with a margin.
Memory technique
Twenty-one normal, nineteen and a half is the line. Barely a point and a half of margin, which is why it is taken seriously.
Exam tip
Air is twenty-one percent oxygen. Deficient starts at nineteen and a half, not at some dramatic lower figure.
Where to look it up
An atmosphere below nineteen and a half percent oxygen by volume is oxygen deficient and unsafe to enter without supplied air.