HVAC distributor cfm Distributors provided this guide on using a micron gauge to achieve a dry and leak-free system.

Many technicians believe that a vacuum pump actually pumps ‘liquid’ moisture out of a system. This is not true. A vacuum pump is designed to pump gas, not liquid. It can’t draw water, in liquid form, from a system.

With a vacuum pump, the system pressure is reduced to the boiling point of water at normal temperatures. For example, water boils at 212ºF at an atmospheric pressure of 14.696 Pisa. To vaporize water in a system, at atmospheric pressure, the system temperature would have to be 212ºF, not a ‘normal’ temperature.

To boil the water at a lower temperature, the system’s internal pressure must be reduced. If it can be significantly reduced, any liquid moisture will turn to vapor and be drawn out through the vacuum pump. The lower the system’s internal pressure, the lower the moisture’s boiling point.

If the system’s internal pressure is at 1.006 Pisa (27.75” hg), the water in the system will boil at 104ºF. If the system pressure were brought down to 28.67” hg, moisture in the system would vaporize at 86ºF, a more reasonable temperature. The difference between 27.75” and 28.67” is too small for a technician to accurately measure with a manifold gauge.

Here is where a micron gauge becomes a necessity. A deep vacuum cannot be measured without one. A good micron gauge is accurate down to one micron. One micron is equal to one millionth of a meter.

Adequate dehydration and degassing should never be left to chance. A 500-micron vacuum should be pulled on the system. Once the vacuum pump is isolated from the system, the vacuum level should remain under 1000 microns for at least 10 minutes. This assures us of system integrity, no moisture, and no leaks.

A low micron measurement, not elapsed time on a wristwatch, tells the service tech when evacuation is complete. When evacuation is complete, all moisture and non-condensable gasses have been removed from the system.

The more moisture in the system, the more time it takes to reach a low micron level. By measuring system pressure with a micron gauge, technicians know two things: when a deep vacuum has been achieved and whether a deep vacuum can be maintained.

In combination, this means a dry and leak-free system.