Often confused · Required evacuation levels
Deep vacuum and dehydration: microns, moisture and the rule
Dehydration means pulling a system into a deep vacuum after service so the moisture inside boils off at room temperature and the vacuum pump carries it out as vapor. It's a separate step from recovery: the evacuation levels in 40 CFR 82.156 (eCFR as of October 5, 2026) decide how much refrigerant comes out, and none of them dries anything.
Part of Required evacuation levels in the Study guide
Recovered isn't dry
A technician replaces the compressor on an R-22 split system holding 30 lb. Recovery with a machine built in 2015 takes the system to 0 inHg, which is what the evacuation table asks of a high-pressure appliance under 200 lb (40 CFR 82.156(a), eCFR as of October 5, 2026). The refrigerant is out. The system now sits at atmospheric pressure with the circuit open, taking in room air and the water vapor in it.
Nothing in that table has dried anything. Its levels decide how much refrigerant has to come out before a system is opened; dehydration is a separate job that starts after the repair, with a vacuum pump and a target measured in microns. Charge straight from 0 inHg and the moisture stays sealed in with the new refrigerant and oil.
Why a vacuum dries anything
Water's boiling point falls as pressure falls. Pull the pressure low enough and the water clinging to tubing and dissolved in oil boils at room temperature, turns to vapor, and goes out through the pump. That's why depth matters and time matters: the pump removes vapor only as fast as the water can boil.
Where the micron target comes from
The figure you'll meet on the exam is 500 microns. Treat it as an exam key / industry practice, not in the rule text: 40 CFR 82.156 states its levels in inches of mercury and millimeters of mercury absolute, and none of them is a dehydration target.
A vacuum pump isn't recovery equipment
A vacuum pump exhausts whatever it pulls into the room. Run one on a system that still holds refrigerant and you've vented that charge, which 40 CFR 82.154(a) prohibits (eCFR as of October 5, 2026). The venting prohibition covers what counts as a release.
Recovery levels against a drying vacuum
The same vacuum in three units
Recovery levels from 40 CFR 82.156 next to the exam-key dehydration figure
| Level | What it's for | Inches of Hg vacuum | Absolute, in microns |
|---|---|---|---|
| 4 inHg vacuum | Type I recovery option | 4 | ≈ 658,000 |
| 10 inHg vacuum | Type II recovery, e.g. medium-pressure under 200 lb, post-1993 machine | 10 | ≈ 506,000 |
| 15 inHg vacuum | Type II recovery, medium-pressure 200 lb or more, post-1993 machine | 15 | ≈ 379,000 |
| 25 mm Hg absolute | Type III recovery, low-pressure appliances | ≈ 28.9 | 25,000 |
| 500 microns | Dehydration after service (exam key / industry practice, not in the rule text) | ≈ 29.9 | 500 |
Recovery levels: 40 CFR 82.156(a) Table 1 and (b), eCFR as of October 5, 2026. Conversions use 29.92 inHg = 760 mm Hg = 760,000 microns at sea level. The full recovery table is on required evacuation levels.
Vacuum and moisture, exam-style
Decide whether each stem is about recovery or about drying before you read the options.
0 right · 0 of 7 answered
During a standing vacuum test, the reading rises after the pump is isolated and then levels off. What does this pattern usually indicate?
Why each option is right or wrong
Answer: C. Moisture still boiling off inside the system
- AWrong: a leak keeps the reading climbing steadily toward atmospheric pressure instead of leveling off.
- BWrong: pump size affects how long evacuation takes; once the pump is isolated, it plays no part in the reading.
- CCorrect: moisture boils off until the water vapor reaches its own pressure, so the reading rises and then levels off; keep evacuating and test again.
- DWrong: a dry, tight system holds close to the reading it was isolated at; a rise of any shape means it isn't ready.
A rise that levels off points to moisture: water keeps boiling until the vapor reaches its own pressure, then the reading stops climbing. A leak keeps the reading rising steadily. Continue the evacuation and repeat the test.
What is the correct sequence for evacuating a high-pressure system?
Why each option is right or wrong
Answer: D. Recover refrigerant, connect vacuum pump to both high and low sides, pull vacuum to required level
- AAdding nitrogen before recovery contaminates the refrigerant and makes it harder to recover.
- BRunning a vacuum pump on a system that still holds refrigerant would discharge that refrigerant to the air; recover first.
- CEvacuating from the low side only is slower and can leave pockets on the high side.
- DCorrect: recover the refrigerant first, then evacuate through both the high and low sides to the required level.
For a high-pressure system, recover all refrigerant first, then connect the vacuum pump to both the high and low sides and run it until the required vacuum level is reached. Pulling from both sides removes moisture and non-condensables more thoroughly.
Which tool is used to measure the deep vacuum required for small appliance evacuation?
Why each option is right or wrong
Answer: C. Micron gauge
- AAn anemometer measures airflow, not vacuum.
- BA pressure test gauge is too coarse to read a deep vacuum.
- CCorrect: a micron gauge reads the very low absolute pressure of a deep vacuum.
- DAn infrared thermometer measures surface temperature, not vacuum.
A micron gauge is specifically designed to measure deep vacuum levels accurately. Standard compound gauges cannot precisely measure the low pressures (measured in microns) required to ensure proper evacuation of moisture and non-condensables.
What is a key benefit of the triple evacuation method when servicing a high-pressure system?
Why each option is right or wrong
Answer: C. It removes moisture more effectively by using nitrogen to dislodge moisture between vacuum cycles
- ATriple evacuation takes longer, not less time; its benefit is drier results.
- BThere is no standard '75% faster' figure; the method adds steps for more thorough moisture removal.
- CCorrect: breaking each vacuum with dry nitrogen picks up remaining moisture, so the next vacuum pulls out more of it.
- DFilter-driers are still needed; evacuation does not replace them.
The triple evacuation method, which involves pulling a vacuum, breaking it with nitrogen, and repeating multiple times, helps remove moisture more effectively than a single evacuation. The nitrogen helps dislodge and carry away moisture that would otherwise be difficult to remove under vacuum alone.
A technician breaks a deep vacuum on a chiller using ambient air rather than dry nitrogen. What is the main consequence?
Why each option is right or wrong
Answer: D. Significant moisture ingress
- ASuperheat is an operating measurement; breaking vacuum with air does not directly change it.
- BAtmospheric air brings the system back up to roughly atmospheric pressure, which will not rupture a disc.
- CSubcooling is an operating measurement; the real problem here is moisture.
- DCorrect: ambient air carries water vapor, so breaking vacuum with air puts moisture back into the system you just dried.
Breaking a vacuum with ambient air pulls humid air into the system. Dry nitrogen carries no moisture, so it is used instead.
A technician opens an R-410A system containing POE oil for three minutes to replace a component. What should be done with the liquid-line filter-drier?
Why each option is right or wrong
Answer: C. It must be replaced immediately before system evacuation.
- APurging a drier with nitrogen does not restore its moisture capacity, so it should not be reused.
- BA moisture indicator lags behind; POE oil can absorb water before the sight glass shows wet.
- CCorrect: POE oil readily absorbs moisture, so whenever the system is opened, replace the filter-drier before evacuating.
- DThere is no safe exposure window; POE starts absorbing moisture as soon as it is open to air.
POE oil is highly hygroscopic. Standard practice is to replace the filter-drier whenever a POE system is opened to the atmosphere, before evacuation.
A technician performs a deep vacuum on a rooftop unit at 30°F ambient temperature. The system holds at 450 microns during a 10-minute decay test. Why might this result be unreliable?
Why each option is right or wrong
Answer: C. Cold moisture can freeze, and ice gives off vapor too slowly to show.
- AMicron gauges are not known to shift drastically below 40°F; the issue is the moisture, not the gauge.
- BCold reduces outgassing; it does not increase it.
- CCorrect: at 30°F trapped water can freeze, and ice releases vapor so slowly that a short decay test can hold while the system is still wet.
- DPump oil viscosity affects how fast the pump pulls down, not whether a held reading is trustworthy.
In cold weather, moisture in the system can freeze. Ice gives off vapor far more slowly than liquid water boils under vacuum, so a 10-minute decay test can hold at 450 microns while water is still inside. Warming the system and repeating the test gives a reliable reading.
More practice across all four sections: Practice test · Timed mock
Key takeaways
- The evacuation table in 40 CFR 82.156 sets recovery levels; reaching one doesn't dry the system.
- 500 microns is an exam key / industry practice, not in the rule text.
- The rule's deepest recovery level, 25 mm Hg absolute, is 25,000 microns.
- Leak checks and system work for high-pressure equipment are on Type II high-pressure systems.
Sources
- 40 CFR 82.156, Proper evacuation of refrigerant — eCFR as of October 5, 2026eCFR
- 40 CFR 82.154, Prohibitions (venting) — eCFR as of October 5, 2026eCFR
- Section 608 test topics: Core dehydration, Type II and III — checked October 7, 2026EPA