The core idea
Two temperatures tell you almost everything about a running refrigeration circuit. Superheat is measured at the suction line: how many kelvin the refrigerant vapour is above its saturation temperature at that pressure. Subcooling is measured at the liquid line: how many kelvin the liquid is below its saturation temperature there. Both are simply T_actual − T_saturation, taken on the correct side of the compressor, and both are read with the same kit: a pressure gauge and a thermometer clamped to the pipe.
Superheat exists to protect the compressor, which can compress only vapour — liquid slugging destroys it. The right amount of superheat is a small margin, typically 5–8 K: enough that the last droplets evaporate before the compressor, but little enough that the evaporator stays full of boiling liquid, where the useful heat transfer happens. Roughly speaking, every extra kelvin of superheat idles about 3 % of the evaporator's capacity.
Subcooling plays the opposite role on the way out: it guarantees the liquid reaching the expansion device is pure liquid. If it flashes to vapour early ("flash gas"), the valve's capacity collapses, since vapour occupies volume while moving almost no heat. A healthy condenser typically produces 4–8 K of subcooling.
Real-world example
A supermarket's under-counter freezer warms to −12 °C instead of −18 °C. The technician clamps on a thermometer: suction line −5 °C, gauge set reads saturation −8 °C → superheat 3 K, and a foamy sight glass — the classic signature of undercharge. Some vapour reaches the compressor and the coil is starved. After topping up the charge, the same line measures −14 °C against saturation −20 °C → superheat 6 K, liquid line now 4 K subcooled, and the cabinet pulls back down. For scale: a 4 kW freezer moves ≈ 2 450 × 4.19 ≈ 10 000 W of heat at a modest COP of 2 — the charge itself is measured in grams, which is why a slow leak shows up first as a small temperature drift, not a breakdown.
Common pitfall
Measuring the wrong pressure or the wrong spot, or treating one reading alone as a verdict. Superheat must be checked at the evaporator outlet / compressor suction, subcooling at the condenser outlet, each paired with the saturation temperature from the same side's pressure. A low-side gauge reading at the compressor inlet misses the pressure drop along the suction line — understating true superheat. And a single number in isolation misleads: low superheat with a cold, flooded compressor points to overcharge or a stuck TXV, while the same low superheat with a foamy sight glass points to undercharge. Read them as a pair, with the temperatures of both lines under your hand.