ASHRAE recommends evacuation to below 1000 microns, and after isolation a system must not rise above 2500 microns within several hours. Some equipment manufacturers specify deeper vacuum such as 400 microns to ensure that harmful water vapor is removed from the system.

1000 microns equal only .039 inches of mercury, a measurement that cannot be made with a mechanical gauge, or determined by evacuation time or the sound of the pump. The only tool that can measure vacuum at these levels is an electronic vacuum gauge.

The best place to measure vacuum is at the system, not at the pump. With a combination vacuum/charging valve (link to product), you can attach the gauge directly at the system and isolate it from the pump, hoses and manifold for a true indication of the vacuum in the system. You can see the last evidence of moisture being removed and know that the system has dried out.