Auto A/C Terms - Glossary
Hg (in-Hg)

The chemical symbol for Mercury. It is used when referring to vacuum as inches of Mercury (‘in-Hg’). Every auto air conditioning service must include a vacuum before recharging the air conditioning system. The level of vacuum attained would be expressed in in-Hg.

As an example, the typical properly working vacuum pump would be able to reach about 29.9 in-Hg of vacuum after about 1/2 hour of operation. Pulling vacuum on the air conditioning system assures that all moisture and air is removed. Those contaminants will cause problem gauge readings and will not allow the air conditioning system to provide the maximum cooling performance that it’s been designed for.