CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
After repairing a refrigerant leak on a residential split system, a technician evacuates the system using a vacuum pump and a micron gauge before charging. To adequately remove moisture and non-condensable gases, the system should be evacuated to approximately:
- A1000 psig
- B50 psig
- C29.9 in. Hg
- D500 microns or below
Show answer & explanationAnswer & explanation
Correct answer: D. 500 microns or below
Industry best practice calls for pulling a deep vacuum to 500 microns or lower (measured with a micron gauge, not a standard compound gauge) before charging, ensuring adequate removal of moisture and air. Standard compound gauges reading 29.9 in. Hg lack the resolution to confirm this deep vacuum level.
Why the other options are wrong
- A. Positive psig readings are irrelevant to vacuum evacuation.
- B. 50 psig is a positive pressure reading, not applicable to vacuum evacuation.
- C. 29.9 in. Hg on a standard gauge is not precise enough to confirm a deep vacuum.
Deep Vacuum Evacuation
The process of removing air and moisture from a refrigeration system using a vacuum pump until the system reaches 500 microns or below, verified with a micron gauge.
- Standard compound gauges cannot accurately measure deep vacuum levels
- 500 microns is the widely accepted industry target before charging
- A rising micron reading after pump shutoff (decay) indicates a leak or moisture
Memory trick: Five hundred microns or bust — dry system is a must.