To read a refrigerant pressure chart, first ascertain the condition of the refrigerant at each stage of the refrigeration cycle: liquid, vapor or a mixture thereof. Pay attention to the system's evaporator, condenser and receiver, as these are the only places where the relationship between temperature and pressure is guaranteed.
Typically, the highside of a refrigeration system contains refrigerant in all three of its stages while the discharge line contains only vapor. In the refrigeration condenser, refrigerant vapor becomes a mixture of liquid and vapor, while the line between it and the receiver generally contains only liquid.
When reading a refrigeration pressure chart, keep in mind that the points in the system that contain only vapor will have an actual temperature higher than the saturation temperature. Conversely, points in the system where only liquid is present will typically have a measured temperature below the saturation temperature.
When reading a refrigeration pressure chart that includes measured temperatures, use this knowledge to infer the condition of the refrigerant at each point in the refrigeration process. Often, engineers and technicians must make deductions about the condition of the system's refrigerant because it is not feasible to put temperature and pressure dials on every line and valve in the system.