Refrigeration Principles and System Components
Saturation is the whole game
At any given pressure a pure refrigerant has exactly one saturation temperature, and a P-T chart converts between the two in either direction. Superheat is measured suction-line temperature minus the saturation temperature at suction pressure. Subcooling is saturation temperature at liquid-line pressure minus measured liquid-line temperature. Evaporator superheat is read at the coil outlet; compressor superheat includes line gain, so it is always higher.
Components and what they imply
Compressor raises pressure and temperature. Condenser rejects sensible and latent heat. Metering device drops pressure and regulates flow. Evaporator absorbs heat. A TXV modulates flow and holds evaporator superheat roughly constant, so charge is verified by subcooling. A fixed orifice or piston cannot modulate, so charge is verified by superheat. Zeotropic blends have temperature glide and must be charged as liquid.
Traps
The biggest trap is judging charge from suction pressure alone. Low suction can come from restricted airflow, a kinked line, a plugged filter-drier, or a weak compressor. High superheat with normal subcooling suggests a restriction or a starving coil, not an overcharge. Another trap is confusing which measurement pairs with which metering device. A third is forgetting that a 90-degree day makes the condenser run hotter, raising head pressure legitimately.
Rule of thumb
On a design day, condensing temperature runs roughly 10-20 degrees above outdoor ambient, and the return-air to saturated-suction split across a comfort coil is near 35 degrees.
Worked example
R-410A suction pressure reads 118 psig, which the chart gives as 40 degrees saturation. Measured suction-line temperature is 52 degrees. Superheat is 52 - 40 = 12 degrees, inside the typical 8-14 degree target for a TXV system.