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Pressures in, diagnosis out.
Charging by "beer can cold" is how compressors die. The correct method reads pressures and line temperatures, converts them through the refrigerant's pressure-temperature relationship, and compares superheat and subcooling against the manufacturer target. This calculator does the PT conversion for R-410A and R-22, flags values outside field bands, and names the likely fault from the classic diagnosis matrix — undercharge, overcharge, or restriction.
HVAC techs use it on every service call — gauges and clamp thermometers in, charge verdict out, without flipping through a PT card. The diagnosis matrix catches the classic misread: high superheat with high subcooling is a restriction, not an undercharge, and adding refrigerant to it kills compressors.
It depends on the metering device. TXV systems hold superheat constant by design, so you charge to the manufacturer's subcooling spec (commonly 8–12°F). Fixed-orifice (piston) systems let superheat float with load, so you charge to a superheat target from the manufacturer chart based on indoor wet-bulb and outdoor dry-bulb temperatures.
The classic undercharge signature: not enough refrigerant mass, so the evaporator starves (high superheat) and the condenser cannot stack liquid (low subcooling). The reverse — low superheat with high subcooling — points to overcharge. High on both suggests a restriction or low evaporator airflow, not a charge problem.
Ten to fifteen minutes minimum at stable conditions — pressures drift while the coil pulls down. Reading early is the most common way techs misdiagnose charge. Also insulate your temperature clamps and wait for them to settle; a bad line-temp reading corrupts both numbers.
Not ideally. R-410A is a near-azeotropic blend; significant leaks can shift its composition, so best practice is recover and recharge by weight. R-22 is a single-component refrigerant and tolerates topping off — though as a phased-out HCFC, it is expensive and reclaimed-only. Either way, refrigerant handling requires EPA Section 608 certification.