the system pressure can be reduced to a near total vacuum.  The reason for doing this is to remove all of the water in  the system. As mentioned, water can freeze, causing system  malfunction and can also combine with the refrigerant to  create corrosive compounds. Once the system has been purged of its refrigerant and it  is at atmospheric pressure, the vacuum pump is operated.  It gradually reduces the pressure in the system. As it does,  the boiling point of any water in the system is also reduced.  Water boils off or is vaporized under the reduced pressure and  is pulled from the system by the pump, leaving the system  moisture free to be recharged with refrigerant.  [Figure 16-90] The strength and efficiency of vacuum pumps varies as does  the amount of time to hold the system at reduced pressure  specified by manufacturers. Generally, the best-established  vacuum is held for 15–30 minutes to ensure all water is  removed from the system. Follow the manufacturer’s  instructions when evacuating a vapor cycle air conditioning  system.  [Figure 16-91] connected. A built-in compressor enables complete system  refrigerant purging. A built-in vacuum pump performs  system evacuation. A container and recycling filters for  the refrigerant and the lubricating oil allow total recovery  and recycling of these fluids. The pressure gauges used on  the service unit panel are the same as those on a manifold  set. Top-of-the-line units have an automatic function that  performs all of the servicing functions sequentially and  automatically once the hoses are hooked up to the vapor cycle  air conditioning system and the system quantity of refrigerant  has been entered.  [Figure 16-88] R134a comes in containers measured by the weight of the  refrigerant they hold. Small 12-ounce to 2 1 ⁄ 2-pound cans are  common for adding refrigerant. Larger 30- and 50-pound  cylinders equipped with shutoff valves are often used to  charge an evacuated system, and they are used in shops that  service vapor cycle systems frequently.  [Figure 16-89]  These  larger cylinders are also used in the full servicing carts  described above. The amount of refrigerant required for any  system is measured in pounds. Check the manufacturer’s  service data and charge the system to the level specified  using only the approved refrigerant from a known source. Vacuum pumps used with a manifold set, or as part of a  service cart, are connected to the vapor cycle system so that  Even the smallest leak in a vapor cycle air conditioning  system can cause a loss of refrigerant. When operating  normally, little or no refrigerant escapes. A system that  requires the addition of refrigerant should be suspected of  having a leak. Electronic leak detectors are safe, effective Leak Detectors Vacuum Pumps Refrigerant Source 16-54 Figure 16-88.  A modern refrigerant recovery/recycle/charging service unit. Electronic control of solenoid activated valves combine with  a built-in system for recovering, recycling, and recharging. A built-in vacuum pump and heated refrigerant reservoir are also included.