Refrigerant Superheat

Inside an evaporator, liquid refrigerant boils at a temperature fixed by its pressure — 40 °F for R-410A at about 118 psig — and stays there while any liquid remains. Only once the last droplet has vaporised can the gas get warmer, and that extra warmth is superheat. It is therefore proof of a dry suction line: a reading of 10 °F says the refrigerant finished boiling shortly before the coil outlet, which is exactly where you want it. Zero superheat says liquid is still present and heading for the compressor, and compressors do not compress liquid — they break.

Take the pressure at the suction service port, convert it to saturation temperature with the refrigerant's P-T chart (or your gauge's scale), then subtract it from a thermometer clamped and insulated on the suction line. Targets: 8–12 °F at the evaporator outlet for a TXV system, and for a fixed-orifice system the charging chart's value, which varies with indoor wet bulb and outdoor dry bulb and can legitimately be 5 °F or 25 °F. High superheat usually means undercharge, a restriction, or a starved TXV; low superheat means overcharge or a flooding valve. The trap is measuring superheat at the compressor instead of the evaporator on a long line set — the extra pickup can add 10 °F and send you chasing a charge problem that does not exist. Example: a 52 °F suction line over a 40 °F saturation temperature is 12 °F of superheat, comfortably in range.