Refrigeration and air conditioning

Le sous-refroidissement is subcooling, and the match is so clean that it invites a rule: sous- for sub, sur- for super, and the vocabulary of this trade will translate itself. It very nearly does. La surchauffe is superheat, le surdimensionnement is oversizing, le sous-dimensionnement is undersizing. Then the rule breaks once, and it breaks where the mistake costs something. « Supercooling » in English is not the mirror of subcooling and has nothing to do with a liquid a few degrees below its saturation temperature: it names a liquid taken below its freezing point that has not frozen, because nothing has given the first crystal a place to start. For that state French does not say surrefroidissement and has no such word in ordinary use. It changes the base instead, and says la surfusion — built not on cold at all but on la fusion, melting, and naming something that ought to have changed phase and has not. So the two prefixes will carry a reader safely from this entry to the gauge at the condenser outlet, and desert that reader the moment the physics stops being about saturation.