A technician pressurizes a system with dry nitrogen in the afternoon and comes back the next morning to find that the pressure has dropped noticeably. What has to be checked before calling that a leak?
- The ambient temperature at both readings, because a colder system holds less pressure even with no leak at all
- BWhether the nitrogen cylinder still had pressure left in it when the system was charged for the test
What remains in the supply cylinder says nothing about what happened inside the system after it was charged, valved off and disconnected. The cylinder could be empty and the standing test still be perfectly valid.
- CWhether the system was left with the compressor running overnight, which would have circulated the nitrogen charge
A system holding a nitrogen test charge is not running and must not be run. Circulating nitrogen with the compressor would be a separate and serious problem, not an explanation for an overnight pressure change.
- DThe oil level in the compressor crankcase, since oil absorbs nitrogen and lowers the standing pressure
Oil does not absorb enough nitrogen to move a gauge, and crankcase level is unrelated to what a sealed volume holds. The change here is about temperature, not about where the oil is sitting.
Why A is correct
Pressure and temperature move together for a gas held in a fixed volume. Without a temperature reading at both ends of the test, a pressure change cannot be attributed to a leak, and rooftop temperature swings between afternoon and early morning are easily large enough to move the gauge.
What this question is testing
This item tests understanding that a standing pressure test compares pressure at matched temperatures, and that an unexplained drop is not automatically evidence of a leak.
On the job
Technicians who skip the temperature reading chase leaks that do not exist, or worse, dismiss a real leak because the pressure drop looked small. The habit is simple: write down the gauge pressure and the ambient temperature when the test starts and again when it ends, right on the service ticket. On multi-day tests, a data-logging gauge takes the argument out of it entirely and gives the customer a record.
Memory technique
Cold gas reads low. Write down the temperature or the test is just a guess.
Exam tip
Standing pressure test questions almost always hinge on temperature. Record it at both readings or the test proves nothing.
Where to look it up
Leak repair verification under 40 CFR Part 82, Subpart F relies on valid test results; a standing pressure test requires matched temperature readings to be interpretable.