Water boils at 100 °C at 1 atm; as the pressure drops it boils at a lower temperature. Refrigerants follow the same rule but at much lower temperatures: R404A boils at −46 °C at 1 bar absolute, at −8 °C at 5 bar and at 31 °C at 15 bar. The temperature corresponding to each pressure is the saturation temperature; the reverse also holds, every temperature has a corresponding saturation pressure.
The refrigeration technician’s gauge measures pressure but reads it as temperature: suction pressure gives the evaporating temperature, discharge pressure the condensing temperature. Superheat and subcooling calculations are the difference between the measured pipe temperature and these saturation temperatures. On gauge dials the saturation temperatures of common refrigerants are printed next to the pressure.
Pure refrigerants have a single saturation curve; zeotropic blends have two curves, one for liquid (bubble) and one for vapour (dew), and the difference between them is the glide. Gauges usually read gauge pressure (bar g); absolute pressure is 1.013 bar more than this. Our refrigerant pressure–temperature chart converts from temperature to pressure and from pressure to temperature for 12 refrigerants and offers bar/psi and absolute/gauge options.