A technician is sweeping an electronic leak detector along the brazed joints of an R-134a rack looking for a small leak. How should the probe be handled to give the best chance of finding it?
- AHeld about a foot back from the joints and swept quickly along the line, so the sensor does not saturate on background vapor and false-trigger
Distance and speed both work against the sweep. A foot of separation lets the plume disperse below what the sensor can detect, and a fast pass gives it no time to respond, so a small leak is walked right past.
- BPassed a few inches above each joint and moved slowly, since refrigerant vapor is lighter than air and drifts upward
This treats refrigerant as a light gas. Common refrigerant vapors are heavier than air and sink away from the joint, so a probe held above it samples clean air while the leak is escaping downward.
- Moved slowly along the underside of each joint about an inch away, since these refrigerant vapors are heavier than air and settle downward
- DParked in one spot in the machine room for several minutes to sample the whole space, then moved to the far wall
A stationary probe reads general room background instead of a specific fitting. It may confirm that refrigerant is escaping somewhere in the room, but it will never say which joint needs the repair.
Why C is correct
Refrigerant vapor is denser than air and falls away from the leak, so the sensor has the best chance of picking it up just below and very close to the joint. Slow movement gives the detector time to respond, which is what catches small leaks.
What this question is testing
This tests hands-on leak detection technique rather than definitions, specifically the physical behavior of refrigerant vapor around a leak and how probe speed, probe distance, and probe position affect whether a small leak is caught. It rewards the field habits that separate a productive leak search from a wasted afternoon.
On the job
On a rack with a hundred brazed joints, technique is the difference between a two-hour leak search and a callback next week. Techs shut off ceiling fans and condenser fans where they can, let the room settle, then work bottom to top and slow. A detector that is too sensitive alarms constantly in a room with background refrigerant, so most techs step outside to zero the unit, then come back in. Confirming every detector hit with bubbles before firing up a torch is standard discipline.
Memory technique
Refrigerant falls, so hunt low and go slow.
Exam tip
Remember that refrigerant vapor sinks. Any answer that has you scanning above a joint or hurrying the probe along is wrong.
Where to look it up
Electronic detector practice: probe below the joint, roughly one inch away, moving about one to two inches per second.