Deep Technical Chapters · Evacuation
Evacuation & Micron Diagnosis
Evacuation is more than "pull to 500 microns". Refrigerant boil-off, retained oil, moisture, outgassing, temperature and setup all change what the micron gauge shows.
A micron gauge tells you what is happening. It doesn't tell you why. Pulling a number without understanding what moves the gauge leads to under-evacuated systems and false failures.
The procedure, properly
- Evacuation procedure and vacuum setup
- Hose restrictions — use large, short, clean hoses
- Isolation — valves closed to the system where appropriate
- Vacuum measurement with a micron gauge
- Micron gauge positioning (as close to the system as practical)
- Vacuum decay test after isolation
What moves the micron gauge
- Moisture and outgassing
- Refrigerant retained in the oil
- Refrigerant boil-off from the system and oil during evacuation
- Temperature effects
- System stabilisation
- Poor vacuum setup (restrictions, long small hoses)
Refrigerant boil-off
Refrigerant can continue boiling and evaporating from the system and the compressor oil during evacuation. Where appropriate and according to manufacturer procedure, compressor sump or crankcase heating helps drive retained refrigerant out of the oil. Excessive boil-off can make the micron reading rise, stall, fall slowly, or behave unpredictably — and it is not a leak.
Telling the difference
Learn to separate these before you conclude anything:
- An actual leak (decay continues after isolation)
- Refrigerant boil-off (settles as the oil gives up refrigerant)
- Refrigerant retained in compressor oil (helped by sump heat)
- Moisture and outgassing (slow, temperature-sensitive)
- Temperature effects
- Poor vacuum setup (restriction, not a system fault)
Field Rules
- 01
"A micron gauge tells you what is happening. It doesn't tell you why."
- 02
"Refrigerant boil-off is not a leak — and it is not "refrigerant glide"."
Diagnosis at a glance
Common mistakes & callbacks
- •Calling a slow micron rise a leak before isolating the vacuum pump.
- •Evacuating through long small hoses and blaming the system.
- •Not using sump heat where the manufacturer calls for it.
