Releasing refrigerant into the air is illegal and harms the environment. Professional shops use recovery machines to service A/C systems. This machine connects to the car's ports, sucks out the refrigerant, and separates the compressor oil. It filters out moisture and acid, storing the clean refrigerant in an internal tank. It then pulls a deep vacuum to boil off moisture before recharging the system with a precise weight of refrigerant.
Modern machines are fully automated. You input the vehicle year and model, and the machine looks up the factory refrigerant weight in its database. It weighs the gas using an electronic scale to ensure exact charging, which is critical for small modern systems that hold less than 15 ounces of gas.