Ken,

You should be aware that there are two superheat's in the low side of the system. One is for evaporator superheat, the other is suction superheat at the compressor suction valve.

To measure evaporator superheat, record the actual line temperature at the outlet of the evaporator. Hopefully, there is a pressure tap there to record the evaporator outlet pressure.

Take the pressure reading and use a pressure-temperature chart for the refrigerant you are using in the system. Look up the pressure in the chart, then cross-reference that pressure to the corresponding temperature for the refrigerant. This is the saturation temperature for the refrigerant at that pressure.

Subtract the saturation temperature from the actual temperature (from the evaporator outlet). This is the evaporator superheat. You should check the evaporator rating to see what is the correct superheat.

The evaporator manufacturer should have designed the evaporator to provide the required cooling capacity at a specific superheat. That is the number you want.

It could be 6 to 8 degrees F, so if the TXV is set for 10 degrees of superheat, you will loose a little capacity on the evaporator and compressor.

The suction line superheat should be monitored also. High suction superheats relate directly to higher discharge temperatures.