Superheat and subcooling, against the plate
Work a charge reading properly: the right measurement for the metering device, compared to the target for the conditions on the day rather than to a number somebody remembers.
The reading
Saturation temperature comes off the gauge, which has the scale printed on it for the refrigerant in the system. No pressure table is assumed here, because assuming the wrong refrigerant's table is a standard way to get this badly wrong.
No chart to hand?
The approximation taught for fixed-orifice systems. It is an approximation, it is only for pistons, and the manufacturer's chart wins wherever the two disagree.
Reading it properly
Which number belongs to which metering device
A fixed orifice or piston has no way of regulating how much refrigerant it feeds, so the superheat leaving the evaporator changes with conditions and with charge. Superheat is therefore the charging measurement, and it has to be compared against a target for the weather on the day — which is why a charging chart has two axes.
A thermostatic or electronic expansion valve holds superheat roughly constant on purpose. That makes superheat useless as a charge measurement on those systems, because the valve will hold it steady while the charge is wrong. Subcooling is the measurement instead, and the target is usually a single number on the plate.
Using the wrong one of the two is the most common way a correct reading produces a wrong conclusion, so this page will not let you.
Why a reading with no target is not a verdict
Ten degrees of subcooling is not good or bad on its own. It is good against a target of ten and bad against a target of eighteen, and both of those are real targets on real equipment.
Without the target this page reports the reading and says so. That is not caution for its own sake: charging to a remembered number is how systems end up overcharged, and an overcharged system runs high head pressure, costs more to run and fails earlier than an undercharged one.
What a reading outside tolerance is telling you
Low subcooling usually means undercharge, but can be a restriction before the metering device. High subcooling usually means overcharge, or a restriction after the condenser backing liquid up into it.
Low superheat means liquid may be reaching the compressor, which is how compressors die — that one is urgent. High superheat means the coil is starved: undercharge, a restriction, or a metering device not feeding.
In all four cases, check airflow first. A coil with no air over it produces readings that look exactly like a charge problem, and adding refrigerant to fix an airflow fault makes it worse.
Related
Airflow, split and static pressure · The annual service checklist · All HVAC tools