Superheat = suction line temp − saturation (dew) temp at suction pressure. High = evaporator running dry (undercharge, restriction, low load). Low = liquid getting too close to the compressor.

Subcooling = saturation (bubble) temp at liquid pressure − liquid line temp. High = refrigerant stacking in the condenser (overcharge, restriction). Low = not enough liquid (undercharge).

On a TXV/EEV system the valve holds superheat, so diagnose mainly on subcooling. On a fixed orifice, superheat moves with conditions — use the WB/DB target.

P-T data is seeded from published refrigerant tables, cross-validated against NIST/CoolProp. Diagnostic guide only — verify against manufacturer data for critical work; your gauges and judgement decide.