To remove as much moisture as possible from a system, what evacuation procedure is recommended?

• A. 1000 microns
• B. 500 microns
• C. 10 microns
• D. 50 microns

Answer: (B)

Removing moisture from a refrigerant system relies on creating a deep vacuum so water can boil off and be pumped away. At room temperature, water will boil off when the surrounding pressure is low enough, so you target a vacuum around 500 microns (or lower) and hold it while outgassing continues. This level is practical to achieve with typical vacuum pumps and gauges and is sufficient to purge residual moisture from oil and crevices in the system. A shallower vacuum, like 1000 microns, won’t purge moisture as effectively, leaving moisture behind. Deeper vacuums (10 or 50 microns) are more demanding to reach reliably in the field and aren’t usually necessary for standard service, though they can be used with proper equipment and time. So, evacuating to about 500 microns is the recommended procedure to remove as much moisture as possible.