Superheat & subcooling (refrigerant charge check)

Superheat and subcooling are the two measurements a competent AC technician takes to prove whether the refrigerant charge is correct — instead of just guessing and topping up. Superheat is how many degrees the refrigerant vapour has been heated *above* its boiling point by the time it leaves the evaporator; subcooling is how many degrees the liquid refrigerant has been cooled *below* its condensing point at the condenser. Read together against the manufacturer's target figures (often a single-digit-to-low °C range), they reveal the real state of the system: high superheat / low subcooling typically points to an undercharge or a leak, while low superheat / high subcooling points to an overcharge. This is exactly why a good technician doesn't respond to weak cooling by blindly adding gas — checking superheat and subcooling tells them whether the charge is genuinely low (and therefore leaking, since a sealed system shouldn't lose gas) or whether the fault is elsewhere. It's the diagnostic behind doing a leak test rather than an annual refill, and getting the charge dialled in correctly is part of a properly set-up and sized cooling system.