Auto A/C Terms - Glossary
Pump down

Another term that refers to evacuating or purging the A/C system of air. Typically performed with a vacuum pump, removing air from an A/C system is an important part of any A/C service.

Knowing that, you should also understand that it has recently been estimated that over 90% of the vacuum pumps in service really do not work properly or perform the deep vacuum that they should.

Understand that vacuum is typically measured in inches of mercury (in-Hg). On a compound pressure gauge used for low pressure readings, the vacuum scale is so small that it’s impossible to tell the difference between 28.9 inches of vacuum or 29.9 inches of vacuum.

So what’s the difference? 29.9 inches of vacuum (in-Hg) is a perfect vacuum. On the other hand, 28.9 in-Hg is a level of vacuum of about 25,000 microns. Microns (or millionths of a millimeter) is a far more accurate way of measuring vacuum. Understand that water only boils when a vacuum level of under 1,500 microns is attained. Therefore, using the compound pressure gauge and inches of vacuum to determine if you’re really boiling water is never an accurate measurement. Serious service technicians will check their vacuum pumps on a regular basis to assure they continue to pull deep vacuum (usually considered to be under 500 microns).