Manufacturers of refrigerants and controls, and other suppliers distribute hundreds of thousands of pressure-temperature (P-T) charts to the trade every year. It would be rare indeed to find a service technician who could not put his hands on a pressure-temperature card at a minute's notice.
In spite of the widespread availability and apparent reference to the pressure-temperature relationship, the number of service technicians who use the P-T chart properly in diagnosing service problems is very few.