A helper assumes the four inches of mercury vacuum that satisfies small appliance recovery is the same depth of vacuum used to dry a system out before recharging. How do those two levels compare?
- AThe recovery level is deeper by roughly half, which is why a two stage pump is called for
Nothing about the recovery level requires a two stage pump, and recovery is not performed with a vacuum pump at all because a pump discharges to the room.
- The recovery level is far shallower, standing only about two pounds below atmospheric pressure
- CThe two levels match, since each one is read on the same compound gauge in inches of mercury
The two readings are not the same depth, and a dehydration target is far too deep to resolve on a compound gauge at all, which is why a separate micron gauge exists.
- DThe recovery level is far deeper, reaching well below one hundred microns of absolute pressure
This reverses the relationship. A hundred microns is a deep dehydration vacuum, roughly a ten thousandth of the pressure the recovery requirement asks for.
Why B is correct
Four inches of mercury vacuum is measured downward from atmospheric pressure, and each inch is worth roughly half a pound per square inch. The appliance therefore sits about two pounds below room pressure, which a compound gauge shows as a needle barely into the vacuum side. Dehydration targets are hundreds of times deeper and are read on a micron gauge, not on a compound gauge.
What this question is testing
This item tests whether you can place the small appliance recovery level on a real pressure scale instead of treating it as an unattached number. It also checks that you keep the recovery step and the dehydration step separate, since they use different tools and are performed at different points in the job.
On the job
A technician who thinks four inches of mercury is a deep vacuum will keep a small appliance on the machine long past the point the rule was satisfied, and may conclude a good recovery machine is failing. The opposite mistake is worse: someone who thinks the recovery level equals a dehydration vacuum may reach for a vacuum pump, which exhausts refrigerant straight into the shop. On the bench the sequence is simply recovery machine to reach the required level, then vacuum pump after the repair to dry and de-air the system before the weighed charge goes in.
Memory technique
Four inches is a fingertip below the line. Microns are the bottom of the well.
Exam tip
Recovery pulls a shallow vacuum to prove the refrigerant is out. Dehydration pulls a deep vacuum to prove the water is out. Different targets, different gauges, different tools.
Where to look it up
Four inches of mercury vacuum is about two psi below atmospheric pressure, roughly one fifteenth of a full vacuum.