If the liquid keeps passing through the fins after the vapour has condensed in the condenser, it cools below the saturation temperature; this difference is called subcooling. Example: if the R404A condensing pressure is 15 bar g, the saturation temperature is about 34 °C; if 29 °C is measured on the liquid line, the subcooling is 5 K.
Subcooling guarantees two things: that full liquid reaches the expansion valve without vapour bubbles (otherwise valve capacity falls and bubbles appear in the sight glass) and that capacity increases (every 1 K of subcooling raises capacity by about 0.5–1%). Low subcooling (0–2 K) indicates a low charge; very high subcooling (above 10 K) indicates overcharge or liquid backing up in the condenser (receiver outlet valve throttled, non-condensable gas).
The service technician confirms the charge with this value: if subcooling is also at 3–8 K while superheat is normal, the system is correctly charged. For zeotropic refrigerants the condensing temperature is read from the "bubble" column of the chart. Liquid–suction line heat exchangers and mechanical subcoolers deliberately increase subcooling to gain capacity on long lines and in hot climates.