Every manifold pressure gauge set includes two color coded gauges. Typically, the high pressure gauge is read while the low pressure gauge is blue. High pressure gauges usually provide pressure readings up to four or five hundred psi, while low pressure gauges typically go to about 150 psi. The biggest difference in the low side gauge is that it is commonly called a compound gauge, because it not only shows pressure, but it also includes a scale for vacuum.

When servicing auto A/C systems, it is really important to assure you pull a deep vacuum on the system. The low side compound gauge will show the level of vacuum expressed in inches of mercury or in.Hg.

Before connecting your pressure gauges to any A/C system, it is a good idea to confirm that the gauges are calibrated. That is, when there is no pressure in the charge hose, the gauge must read zero. If the needle shows something other than zero, the gauge will have to be calibrated. That is accomplished by removing the front protective lens and using a blade screwdriver to adjust the calibration screw.

Gauges can go out of calibration frequently, depending on the amount of use. It is a good idea to check the calibration of the gauge every time you use the gauge set.