How to Read a Pressure Temperature Chart

Refrigerants are essential to the functioning of an air conditioner.
••• window air conditioner image by Aaron Kohr from

When repairing refrigerators, air conditioners and other machines that contain refrigerants, service technicians work with pressure temperature, or PT, charts. PT charts show the relationship between pressure and temperature of given refrigerants. By changing the pressure of the refrigerant, the technician can set its temperature to a given level.

    Put the pressure temperature chart in front of you.

    Determine whether temperature or pressure is provided in the left column.

    Determine the units in which temperature and pressure are measured. Temperature can be measured in either Fahrenheit or Celsius. Pressure, in this context, is typically measured in units called psi, which stands for "pounds per square inch."

    Read the values in the cells. If the left column values represent temperature, then the other cells provide the pressure at which a certain temperature is reached by the refrigerant. If the left column represents pressure values, then the other cells give the temperature readings at which a given pressure is reached.

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