A vacuum pump removes air, moisture and other non-condensable gases from an AC or refrigeration circuit before charging, and a rushed evacuation causes weak cooling, high head pressure or compressor failure months later. This guide explains micron readings and the 500-micron target, how to size a pump by CFM for splits, VRF and cold storage, and how to set up hoses and a micron gauge. It also covers the standing vacuum test, pump oil care, common mistakes and quick answers, so the system is dry and leak-free before refrigerant goes in.