What each number measures
Superheat is how many degrees the suction vapor is above its saturation temperature: suction line temp − saturation temp (read on the dew line for blends). It tells you how much of the evaporator is doing useful work. Subcooling is how many degrees the liquid is below saturation: saturation temp − liquid line temp(bubble line). It tells you how much liquid is stacked up in the condenser.
Why the metering device decides
A TXV or EXV actively holds superheat roughly constant by throttling flow. So on a TXV system, superheat barely moves whether you're a pound low or a pound high — it can't tell you about the charge. Subcooling can, because extra or missing refrigerant shows up as liquid backing up (or not) in the condenser.
A fixed orifice has no such regulation, so superheat tracks the charge directly: add refrigerant and superheat falls, remove it and superheat rises.
Reading the verdict
- High superheat (fixed orifice) → the evaporator is starved → undercharged, add.
- Low superheat / negative → liquid leaving the evaporator → overcharged or flooding, stop and recheck.
- Low subcooling (TXV) → not enough liquid in the condenser → undercharged, add.
- High subcooling → too much liquid stacked up → overcharged, recover.
Target superheat isn't a fixed number — it moves with indoor and outdoor conditions, so it comes from a field formula. Target subcooling comes from the equipment nameplate.