Commonly referred to as the absence of air, vacuum is a controlled, contained system or condition, where the actual pressure is lower than ambient atmospheric pressure.
Vacuum is expressed in inches of mercury (in-Hg). Note that a perfect vacuum is 29.92 in-Hg (a vacuum above a column of mercury will support the column to a height of 29.92 inches). Although low side compound gauge on the manifold pressure gauge set shows vacuum levels to 30 in-Hg, the perfect vacuum remains at 29.92 in-Hg.
Vacuum is important to auto A/C systems for two reasons. First of all, it is important to remove any air from the A/C system before charging. That’s because air is a non condensable gas and will cause high side pressures to be higher than normal. Those pressures will also reduce cooling performance.
Also understand that water boils in a vacuum. When the A/C system is pulled into a vacuum, any moisture that may be in the system is boiled and pulled out of the system by the pump. That’s important because of the effects moisture has when it mixes with refrigerant and causes harmful acids that corrode condensers, evaporators and other aluminum components of the A/C system.
It is also important to understand that vacuum can be measured more accurately in microns. One Micron is equal to one-millionth of a millimeter. In order to measure vacuum that way, you will require a micron vacuum gauge.
The importance of this is best explained when looking at the different levels of vacuum. Understand that your low pressure gauge could be showing 29" of vacuum. However, that scale is so small, it’s almost impossible to determine what the ‘real’ level of vacuum is.
What’s the difference?? A level of vacuum of 28.92 in-Hg is about 25,000 microns. At 29.9 in-Hg, it’s about 50 microns. The difference in those gauge readings is impossible to see on a low pressure compound gauge. But what is important is that it water requires a level of vacuum under 500 microns to boil. Therefore, if you rely on your low pressure gauge only, you may never know if your vacuum pump is really boiling water or not.