Reaching a low number on the micron gauge is half of an evacuation. The other half is what the number does when the pump stops pulling. A decay test tells you whether the system is tight and dry, still wet, or leaking.
Why can't the compound gauge tell you?
A micron is one thousandth of a millimetre of mercury of absolute pressure; atmosphere is roughly 760,000 microns. Everything that matters in evacuation happens in the last few thousand microns, which is a sliver at the end of a compound gauge's inches-of-mercury scale. A system at 400 microns and one at 5,000 microns both read "full vacuum" on the manifold. Only a micron gauge shows the difference.
How should the evacuation be set up?
- Remove Schrader cores with a core removal tool. The cores are the biggest restriction in the path.
- Use short, large-diameter hoses dedicated to vacuum. Flow at deep vacuum falls off sharply with small, long hoses.
- Put the micron gauge at the system, not on the pump. A gauge at the pump reads the pump.
- Change the pump oil regularly. Moisture-laden oil limits how low the pump can pull.
What does the decay test look like?
Evacuate to the target the equipment manufacturer gives (500 microns is common), then valve off the pump and watch.
| What the reading does | What it means | Next step |
|---|---|---|
| Stays at or near the target | Tight and dry | Break the vacuum with refrigerant and charge |
| Rises, then levels off and holds | Moisture or refrigerant in the oil boiling off | Keep evacuating; consider heating the system gently |
| Rises steadily and keeps climbing | A leak: air has no stopping point short of atmosphere | Find the leak, repair, pressure test again |
The acceptable rise and hold time come from the manufacturer or your employer's procedure.
Why does a proper evacuation matter?
Moisture left in a system reacts with refrigerant and oil to form acids, especially with hygroscopic POE oil, and can freeze at a metering device. Air left behind becomes non-condensables that raise head pressure. Both shorten compressor life and both are avoidable.
Where is this on the 313A exam?
Evacuation tools (B-4.05) and evacuating systems (B-7.03) are both listed in Major Work Activity B, routine trade activities, which is 16% of the Red Seal exam. The free sample and the practice bank include decay-test interpretation questions.
Sources
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.