Charging Rules & Pre-Charge Procedures (Essential Points)

Rule 1: System must be running

You must run the system to accurately check charge, verify superheat and subcooling.

Charge by verifying subcooling.
Subcooling is measured on the high side in the condenser (red indicates subcooling).

If the system is critically charged, weigh in the refrigerant as specified by the manufacturer (in ounces).
After adding the prescribed amount, verify both superheat and subcooling.

Use a gauge manifold with low-side (blue) to the low side and high-side (red) to the high side. The middle hose is open to atmosphere.
Open valve handles to eliminate pressure inside the manifold so the middle hose reads atmosphere.
Calibrate gauges to read 0 at atmospheric pressure: this ensures readings are accurate going forward. 0~\text{psig} reference.
Close the valve handles on both sides so the pressures in the blue and red hoses are isolated when readings are taken.
Whatever pressure is in the blue hose (when the valve is closed) is read by the blue gauge; the red hose pressure is read by the red gauge.

Crack the low-side and high-side service valves to allow the system pressure to fill the hoses; gauges will move as pressure equalizes.
The white color inside the hose ends indicates vapor that was trapped in the hose; it has been compressed.
Purge air from the hoses to avoid air entering the system.

Under the manifold, at the flare connection where the hose meets the manifold, loosen the hose with your thumb and forefinger.
Allow refrigerant to push any trapped air and vapor out to the atmosphere.
EPA 608 de minimis release: releasing a small amount of refrigerant is allowed; the risk of not purging is worse than the small release.