If an air conditioning system has had 1) refrigerant removed to service or repair the system, 2) refrigerant loss caused by component failure or 3) shows evidence of refrigerant contamination, it must be evacuated to get rid of all moisture and air before it can be recharged with refrigerant.
Air and moisture are extracted from the system with a vacuum pump. Pumping draws air from within the closed system to create negative pressure, or vacuum. By lowering the pressure inside the system into a vacuum condition, the boiling point of water, or moisture, is also lowered to a point at which evaporation easily occurs. This vaporized moisture is then easily drawn out by the vacuum pump.
The vacuum necessary to vaporize moisture in the system for evacuation varies with the altitude. At sea level, where the air pressure is 14.7 psi, water boils at 212°F. To evacuate a system, it is necessary to lower the boiling point of water in the system to a point that is lower than the “ambient” air temperature (the temperature of the air surrounding the vehicle). At an ambient temperature of, say, 75″F, you must draw at least 29.5 inches of vacuum (in-Hg) to bring the boiling point of water down to 72°F.
As altitude increases, the vacuum gauge reading is affected significantly. For instance, it isn’t possible to obtain a vacuum reading of 29.5 inches once you’re above sea level. That’s because for each 1000 feet of altitude, the vacuum requirement decreases by 1 in-Hg. So the vacuum gauge must be corrected by 1 in-Hg for every 1000 feet of altitude to compensate for the change in atmospheric pressure. Thus, at 1000 feet, a gauge reading of 28.5 in-Hg will be the same as 29.5 in-Hg at sea level. The good news is that this will still accomplish the same job of lowering the boiling point enough to evacuate the moisture from the system
Once the desired vacuum reading is obtained, regardless of the altitude, continue pumping for about 30 minutes to completely rid the system of moisture