With the variety of refrigerants a technician can encounter, remembering all the different acceptable system pressures can be difficult or even overwhelming. To make this task more manageable, think in terms of temperature and then convert to a pressure. Focus on remembering and understanding the various design temperatures of a system. Then, with the use of a basic pressure/temperature card or app, determining the required pressure will become much easier.
For example, instead of remembering that a typical reach-in cooler using R-134a should have an operating evaporator pressure of around 15-18 psig, remember the relationship between the coil temperature and the entering air temperature, referred to as the evaporator’s temperature difference (TD). Once you know the evaporator’s operating coil temperature, you can easily convert it to a pressure. This enables you to determine the correct operating pressure no matter what refrigerant is in the system.