Manufacturers of refrigeration condensing units will supply technical data for their products on demand, but this will be based on ideal operating condi- tions in controlled ambient temperatures. The service engineer or technician, dealing with many different types of equipment, needs a simple quick method to enable him to determine the theoretical operating head pressure of a plant so that this can be compared with the actual operating pressure to assist in fault diagnosis. It is known that a shortage of refrigerant will produce a low suction pres- sure, and the fact that the compressor is doing very little work in handling vapour at a lower density will affect.
