Auto A/C Terms - Glossary
Inches of Mercury - ( in-Hg )

n auto A/C service, vacuum is an extremely important part of regular service. Just before the system is recharged, a vacuum pump is connected to pull vacuum. That level of vacuum is expressed as in-Hg.

As an example, a perfect vacuum is -29.92 in-Hg. Another way to express the level of vacuum is by using microns. Microns are a unit of measurement that are one millionth of a millimeter. They are a far more accurate measurement when it comes to vacuum.

Understand that when reading vacuum on a typical manifold pressure gauge, the scale is typically so small that the thickness of the gauge needle alone is almost as much as one in-Hg of vacuum. You never really know what level of vacuum to are attaining.

If you use a micron vacuum gauge, you find exactly what level of vacuum the system is under. Why are micron vacuum gauges so important? First, understand that water only boils in a vacuum when the level of vacuum attained is 1,500 microns or less (with microns, the lower the number, the deeper the vacuum).

When using a compound vacuum gauge to read vacuum (expressed in-Hg) it’s difficult to tell the difference between 28.9 and 29.9 in-Hg. However, at 28.9 in-Hg, you are only attaining about 25,000 microns, at 29.9 in-Hg, you are attaining about 50 microns. Understand that at 25,000 microns, you are not performing any service to the A/C system.

Most service information refers to pumping down the A/C system or ‘pulling deep vacuum’. Deep vacuum is a level of vacuum that is typically under 500 microns.