How do you know when enough vapor has entered the appliance before you charge refrigerant liquid?

• A. Refrigerant saturation temperature increases above 32°F
• B. Compressor head pressure rises
• C. Vacuum gauge reads zero
• D. Sight glass shows liquid

Answer: (A)

The key idea is making sure the refrigerant entering the compressor is in vapor form, not liquid. A practical way to tell you’ve got enough vapor is by watching the refrigerant’s saturation temperature: when it rises above about 32°F, you know the vapor phase is dominant in the system and liquid slugging into the compressor is unlikely. This reflects the system pressure and the state of the refrigerant — higher saturation temperature means the refrigerant is no longer principally liquid at the seen pressure.

Relying on head pressure rising isn’t reliable because that can change for reasons other than having enough vapor, and a vacuum gauge reading zero just means there’s no vacuum present, not anything about the vapor/liquid state. A sight glass showing liquid indicates liquid is still present, which means vapor hasn’t fully displaced the liquid yet.