OK so technically evaporator pressure should be measured,preferably as close to the evap as possible especially if long pipe runs are in place,then suction line temperature.Use your comparator to establish stauration temperature then subtract SAT temp from actual Temp this gives evaporator superheat.I mention pipe runs because a long suction line will induce pressure drop giving a lower pressure entering the compressor and depending on the insulation you will pick up heat along the line.The consequence of this is increase in temperature entering the compressor.TEV are usually factory set for a predetermined superheat.Chillers for instance that are tested in the factory will no doubt have the superheat checked and adjusted accordingly but and the big but is at what condensing temperature were they set at in the factory.Usual design on an aircooled chiller is 35 C ambient for 7 CLWT however if the fan control is set at a default and the ambient temperature is say 15 C in the factory the superheat will change as the ambient temp rises and increases the pressure diff over the TEV.So putting this to commissioning stakes if you set a unit to work and the ambient is either high or low and go back in the mid temp ambient chances are the superheat will be different.It is also important to check superheat at low load/low ambient conditions as well.
I could go on about this for ages but there you go just a feeler.