Superheat.

Superheat is how many degrees above its boiling point the refrigerant gas is as it leaves the evaporator coil and heads back to the compressor.

It is measured by taking suction pressure, converting that to the temperature at which the refrigerant boils, and subtracting it from the actual measured suction line temperature.

Why it exists

A compressor pumps gas. Liquid does not compress, and liquid arriving at the compressor washes oil off bearings and can do immediate mechanical damage.

Superheat is the safety margin. A reading comfortably above zero means all of the refrigerant finished boiling inside the coil, with room to spare, so what is coming back is gas.

Zero superheat means liquid is leaving the coil. That is the condition an accumulator exists to survive, and it is what running an iced system produces.

What the readings mean

High superheat means refrigerant is boiling off too early in the coil, so the last part of the coil is doing nothing. Usually undercharge, a restriction, or a TXV stuck closed.

Low superheat means refrigerant is not finishing boiling, so liquid is heading back. Usually overcharge, a TXV stuck open, or very low airflow across the coil.

Why both measurements, always

Subcooling reads the high side. Superheat reads the low side. Several genuinely different faults produce similar readings on one side and are separated by the other.

The common misdiagnosis in this trade is treating a metering device problem as a low charge, because the symptoms overlap almost perfectly. Adding gas to a restricted system makes things worse rather than better, and the system comes back.

What you should get

If somebody touched the refrigerant in your system, ask what superheat and subcooling were before and after. Both are quick to take with gauges and a clamp thermometer that are already on the truck.

A technician who measured is happy to hand the numbers over. It is part of what a check-up here writes down, along with pressures, amp draws and static pressure, because next summer the reading that moved is worth more than any single reading on its own.