In Health Blog

Pressure Temperature Chart: About, Pressure and More

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The Pressure Temperature chart is a valuable tool used by service technicians to verify proper system operation. PT diagrams are most often used for three purposes: to establish a coil pressure for the refrigerant to produce the desired temperature and to check the amount of superheat above the saturated vapor condition at the evaporator outlet.

Then check the amount of subcooling below. Here is the saturated liquid condition at the end of the condenser. The Traditional PT charts list the saturated refrigerant pressure, in psig, with a column for temperature on the left-hand side.

However, single-component refrigerants and azeotropes boil and condense at one temperature for a given pressure. Therefore, only one column is needed to display the pressure-temperature relationship for any phase change process in a system.

The properties of the new zeotropic mixtures are somewhat different from those of traditional coolants. Zeotropic combinations change composition during the boiling or condensation process.

As the mixture changes phase, more than one component will transfer to the other stage faster than the rest.

This property is called fractionation. The changing composition of the liquid also causes the boiling point temperature to change. The complete temperature transform from one side of the heat exchanger to the other is called temperature glide.


Temperature is a degree of how hot or cold the thing is; precisely, it measures the average kinetic energy of the atoms in an object, which is a kind of energy related to motion.

For example, the temperature of a small cup of water is the same as that of a tub of water, but the tub of water is hotter because it has more water and, therefore, total heat energy.


Pressure is an expression of the force exerted on a surface per unit area. If a force F is applied to area A, the pressure P=F/A.

A simple example of pressure can be seen by holding a knife to a piece of fruit. If you have the knife flat against the fruit, it will not cut the surface. The force is distributed over a large area.

Using a two-column PT chart

  • When checking the superheat or subcool temperature, the procedure is the same as for a single-component coolant.
  • Superheat is verified by measuring the steam line temperature, measuring the pressure, and subtracting the saturated temperature from the measured temperature.
  • In the case of a mixture, read the saturation temperature next to the pressure in the vapor column (dew point) from the graph.
  • When checking the subcooling condition, the technician will measure the liquid line temperature and the pressure at that point and subtract the measured temperature from the saturated temperature at the end of the condenser.
  • With mixing, read the saturation temperature and the pressure in the liquid column (bubble point) from the graph.
  • For an azeotropic or single-component refrigerant, the operating pressure for the low side of a system can be found by cross-referencing the desired coil temperature on the PT chart.
  • However, for high-slip mixes, the desired coil temperature is the coil’s average (or midpoint) temperature.

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