A A/C vacuum pump has several uses for the auto air conditioning mechanic and automotive shop. The main purpose is to remove air and moisture from your automobile air conditioning system. Moisture is a serious threat to your car’s air conditioning system. Moisture will move through the refrigerant line as fine mist and crystallize wherever it expands. Also, when moisture mixes with refrigerants it will become extremely corrosive. You won’t notice this damage until it’s too late. A vacuum pump removes moisture by creating a vacuum in the air conditioning system. The low pressure will boil off the moisture at very low temperatures. This will vaporize the water and vent it out through the system. Now the next issue is what is the best type and size of vacuum pump to use. First, there are 2 types you will come across. The cheapest is a venturi type that runs from your shop air compressor.  A Venturi type are popular with the home mechanic crowd since they are cheap and easy to use. However, they are limited by the fact that they it is very hard to boil off much moisture with one of these types. They just will not pull a good a vacuum as you need. The best and most common types that are found in a professional auto shop is a good electric model.( Click here to see a Robinair Vacuum Pump.) Every professional air conditioner mechanic and auto shop have at least one. You now have to decide what make and model to purchase or rent. Most air conditioning vacuum pumps have a CFM rating on the box. This indicates how much air volume it can remove from the system. The more air the pump can move, the quicker it will achieve the proper vacuum. There is also two stage and single stage pumps. Also, the two stage pumps are quicker and more efficient, but a single stage will get the job done.  This chart below is handy reference to help you choose the right size for your needs.