What is the operating procedure for refrigerant charging?

[VonEmerson]

Before charging the refrigerant, evacuate the system for 1 hour and ensure there are no leaks and the system is under absolute vacuum. Open the cylinder with the valve facing downward, loosen the middle hose connection of the pressure gauge, open the high-pressure side manual valve, and charge liquid refrigerant from the high-pressure side. When the pressures on the high and low-pressure gauges balance and no longer rise, close the high-pressure side manual valve, turn the R134a cylinder valve upward, and let it stand for more than five minutes. Start the air conditioner, open the low-pressure side manual valve, and charge gaseous refrigerant (not liquid) from the low-pressure side. The engine speed should be maintained at 800-1200 rpm. During the refrigerant charging process, observe the refrigerant flow through the sight glass. When the bubbles disappear, close the low-pressure side manual valve and remove the service pressure gauge.