While troubleshooting or installing refrigeration systems using a thermostatic expansion valve, it is a relatively common task to measure the superheat value of the refrigerant leaving the evaporator. This could be a very useful measurement to determine how a system is operating.
However, there is a major consideration when using this measurement to analyze the operation of a system. If the heat load on the evaporator is very high — in other words, if the air temperature entering the evaporator is very high — the refrigerant’s superheat value leaving the evaporator will be high. The thermostatic expansion valve will not be able to feed enough refrigerant into the evaporator.