The two numbers, defined

Microns (micrometers of mercury) measure the absolute pressure remaining in a system after the vacuum pump has done its work. Lower numbers mean fewer molecules left in the system — and specifically, fewer water molecules.

• 500 microns is the EPA-recognized moisture-removal target under §82.156. It is the threshold for declaring an evacuation "complete" from a regulatory standpoint.
• 200 microns is a deeper evacuation target used by many equipment manufacturers for long-term reliability with POE oil and HFC refrigerants.

The Universal exam asks which number is the regulatory minimum (500 microns, with a 10-minute standing test) and why the lower number exists (deeper moisture removal for systems with hygroscopic oil).

Why the 500-micron threshold exists

At 500 microns absolute pressure, water boils at approximately -55 °F. That is below the temperature of any practical evaporator coil, which means the residual water in a system evacuated to 500 microns will not boil off during operation — it will stay dissolved in the oil and circulate harmlessly, or it will remain in non-condensable form where it cannot cause acid formation.

The §82.156(b) procedure for verifying a 500-micron evacuation:

1. Pump the system down with a properly-rated vacuum pump (typically a two-stage rotary vane or a hybrid pump).
2. Isolate the pump from the system by closing the manifold valve.
3. Watch the micron gauge. The reading must drop to 500 microns or lower.
4. Wait at least 10 minutes (some authorities recommend 15-20 for confidence). If the gauge rises by more than a small amount during the standing period, there is either moisture still boiling off in the system, a leak drawing air back in, or a gauge problem.

The standing period is what catches sloppy work. A vacuum pump can pull a system to 500 microns for a moment — but if the gauge creeps up while the pump is isolated, the evacuation is not done.

Why some manufacturers want 200 microns

POE oil used with R-410A, R-454B, and other modern refrigerants is more hygroscopic than the mineral oil used with R-22. POE absorbs water faster from the atmosphere when a system is open. To reduce that absorbed moisture before charging, manufacturers like Carrier, Trane, and Lennox often specify a 200-micron deep evacuation.

At 200 microns, the boiling point of water drops to roughly -70 °F, providing a much larger margin against residual moisture causing acid formation, copper plating, or freeze-up at the metering device.

200 microns is a manufacturer specification, not an EPA rule. If your system is under warranty and the installation manual calls for 200 microns, that is the legal requirement under your warranty terms — but the EPA's compliance threshold remains 500 microns with a 10-minute stand.

Equipment that matters

Pulling 200 microns takes a better vacuum pump than pulling 500 microns:

• Two-stage rotary vane pumps reach 15 microns or lower at the inlet. They are the field standard for HFC and HFO work.
• Single-stage pumps typically bottom out at 50 to 100 microns — adequate for 500 microns, marginal for 200 microns.
• Hybrid pumps combine a rotary vane first stage with a high-vacuum second stage and can reach single-digit microns.
• Micron gauges (Thermocouple or Pirani type) are mandatory for verifying either target. A manifold pressure gauge reading in inches of Hg is not accurate enough below 1,000 microns.

The 500-micron vs 200-micron decision on the job

| Scenario | Target | Reason | |----------|--------|--------| | Standard residential R-410A split system | 500 microns | EPA requirement; manufacturer may call for more | | New installation with POE oil and long line set | 200 microns | POE absorbs more water during installation | | Warranty repair on a manufacturer's named equipment | Per manufacturer | Warranty terms override default | | Refrigerant changeout with system open > 1 hour | 200 microns | Extended exposure to atmospheric humidity | | Burnout repair (moisture + acid contamination) | Triple evacuation, then 200 microns | Moisture is not removable by evacuation alone after burnout — also use a filter drier | | Simple top-off, system never opened | Not applicable | No evacuation needed if no repair was made |

Common exam traps

• Confusing 500 microns with 500 millitorr — they are the same number, but the unit is microns of mercury absolute, not gauge pressure. 500 microns ≈ 29.88 inches Hg vacuum ≈ 99.66 percent vacuum.
• Skipping the standing test — pulling to 500 microns is not the same as holding 500 microns for 10 minutes.
• Believing a digital manifold can replace a micron gauge — manifold gauges read in psig or inches Hg and are not accurate below 1,000 microns. A dedicated micron gauge is required equipment for verification.

A practical closing note

The 500-micron figure is the line between "regulation-compliant evacuation" and "non-compliant." The 200-micron figure is the line between "manufacturer-recommended evacuation" and "could-have-done-better." Knowing which number applies to your specific service context — and being able to defend your choice to an inspector, a customer, or an exam proctor — is the difference between a passing grade and a failing grade.