What is the correct measure of Vacuum, is it the level of vacuum or the time the Vacuum pump is kept running?
The level of Vacuum, as measured in microns of Hg in a Vacuum gauge (that is of the Pirani or Thermocouple type) is the right measure of Vacuum. The recommended is at least 200 microns (100 microns desirable) Hg for 134a and 500 microns Hg at least for R12. After achieving this level of Vacuum, the Vacuum pump should be isolated with the valve and stopped and the Vacuum holding capacity of the system should be checked. If the pressure rises slowly and then stabilises at a higher level of microns, it means that there is some moisture still remaining and the Vacuum pump should be restarted and reconnected and run again for about 10 minutes and the rise in pressure checked again and this process should be repeated till the rise in pressure is as low as possible, suggesting that almost all the water vapour has been removed. After this has been achieved, the Vacuum pump should be run for another 5 to 10 minutes before proceeding to the next step of charging refrigerant.
