How is pressure–temperature data used when charging a system?

• A. Saturation pressure corresponding to evaporating/condensing temperatures guides correct charge; deviations indicate over/under charge.
• B. The system is charged by time.
• C. Only ambient humidity matters.
• D. Charge is independent of temperatures.

Answer: (A)

Using pressure–temperature data provides a reference point for charging. For a given refrigerant, each evaporating and condensing temperature corresponds to a specific saturation pressure on the refrigerant’s P–T chart. When you charge the system, you measure the actual suction (evaporator) pressure and the condensing pressure and compare them to the saturation pressures for the observed temperatures. If the pressures match the saturation values, the charge is appropriate for those operating conditions. If the measured pressures are higher than the saturation pressures, it indicates overcharge; if lower, undercharge. This method uses real operating data to guide charging rather than guessing. Charging by time, focusing on ambient humidity, or assuming charge is independent of temperature don’t reflect how the refrigerant’s pressure–temperature relationship governs the proper amount of charge.