Evacuation and Charging
Evacuation is measured in microns
A compound manifold gauge cannot resolve a deep vacuum; use an electronic micron gauge placed at the system, not at the pump. The standard target for a tight high-pressure system is 500 microns. If the system will not hold, suspect moisture, a leak, or contaminated vacuum-pump oil.
Triple evacuation
The practical method is triple evacuation: pull a vacuum, break it with dry nitrogen, repeat at least twice more, then pull the final vacuum. Nitrogen sweeps carry moisture out and let you pressure-test at the same time. Never use oxygen or air to pressurize a system.
Standing vacuum test
After reaching 500 microns, isolate the pump and watch the gauge. A slow rise to a few hundred microns that then levels off suggests moisture; a rise back toward atmosphere means a leak.
Charging
Charge by weight on a scale whenever the data plate gives a charge. Liquid refrigerant should enter the liquid line; adding liquid into the suction of a running compressor can slug it. Vapor charging is slower but safer for small trim adjustments. Verify charge with superheat on a fixed orifice or subcooling on a TXV.
Rule of thumb
500 microns, held, with the gauge at the system. If in doubt, evacuate again - moisture is far cheaper to remove now than after a burnout.