Arkansas HVACR NewsMagazine January 2026

HVACR NewsMagazine January 2026

Tech News

In this case? •

Pressure shot past 1,000 microns • Never stabilized (flattened out) • Proved the system was still wet Even after HOURS of evacuation.

That’s why a high -performance vacuum rig can pull the system to 135 microns while ice remains inside. The ice has gone dormant. Its vapor pressure has collapsed below what the gauge is reading. The pump wins the tug-of war not because the system is dry, but because the ice can’t push hard enough to register.

ASHRAE Standard 147 and Guideline 6 address this directly: vacuum decay testing, not the absolute micron reading, is the verification method for dehydration.² The industry standard requires pressure to hold below 500 microns for a minimum of 10 minutes with the pump isolated.³ As I’ve discussed before in identifying true leaks in a vacuum, distinguishing between moisture and actual leaks is critical for proper diagnosis. The decay curve tells you which you’re dealing with. A leak produces a linear rise toward atmospheric pressure. Moisture produces a rapid rise that plateaus at the vapor pressure of water at the coldest point in the system.

4. The Truth Only Appears When You Isolate the Pump (The Decay Test)

When you close the isolation valve: • The pump is no longer “winning” • The system’s internal vapor pressure builds • Ice begins to warm and starts sublimating faster • The gauge shows the REAL condition of the system