It is recommended that a quick acting indicating thermometer be used. Two thermometer probes must be firmly attached to pipework free of frost or ice to ensure good thermal conductivity, one at the evaporator inlet and the other as close as possible to the thermal element or bulb of the expansion valve (see Figure 18). The temperature of the refrigerant passing through the pipework at these two points can now be quickly and accurately taken.
The temperature difference between the two points is the amount of superheating taking place within the evaporator. A very wide differential means that the coil is starved of liquid and subject to excessive superheating, and that the evaporator is not fully active. Very little or no temperature difference means very little superheating; the evaporator is fully active over its entire surface, but some frosting back could occur at certain times during the system operation.
It is important that the expansion valve is adjusted to give the correct amount of superheating during commissioning, when a replacement has been made and, as good insurance, when a system has been charged.