How to Read a PT Chart (Superheat, Subcooling, and Blends)

A pressure-temperature chart maps one physical relationship: for a given refrigerant, saturation pressure and saturation temperature lock together. Saturation is the condition where liquid and vapor exist side by side — inside a running evaporator, inside a condenser, or in a cylinder sitting on the truck. Wherever both phases are present, pressure tells you temperature and temperature tells you pressure. That is the whole chart.

Two things to keep straight:

• Field charts read in psig, not psia. Your manifold reads gauge pressure and the chart is printed in gauge pressure, so they match directly at sea level. At altitude, atmospheric pressure drops roughly 0.5 psi per 1,000 ft, which starts to matter above about 5,000 ft.
• The chart only applies at saturation. Superheated vapor in the suction line and subcooled liquid in the liquid line do not follow the chart — and that is exactly why superheat and subcooling measurements work. The chart hands you the saturation point; your clamp thermometer tells you how far the actual line temperature has moved away from it.

Every field use of a PT chart is one of three moves.

1. Predict or set coil pressure. A comfort-cooling evaporator is commonly designed to run about 35°F colder than return air (the standard design TD rule of thumb), so with 75°F return air you expect a coil near 40°F — on R-410A that is 118.9 psig on the low-side gauge. On the high side, condensing temperature typically runs 15°F to 30°F above outdoor ambient depending on condenser design. If your gauges land far from those predictions, the chart just told you where to start looking.

2. Superheat (suction side). Superheat = measured suction-line temperature − saturation temperature at suction pressure (definitions per ASHRAE Fundamentals; targets come from the manufacturer's charging chart). It proves the vapor kept absorbing heat after it finished boiling — meaning no liquid is headed for the compressor. Fixed-orifice systems are charged to a target superheat off the manufacturer's charging chart.

3. Subcooling (liquid side). Subcooling = saturation temperature at liquid-line pressure − measured liquid-line temperature. It proves the liquid kept rejecting heat after it fully condensed — a solid liquid column feeding the metering device. TXV systems are charged to the manufacturer's subcooling target, commonly 8–12°F on residential equipment; the number on the unit's data plate always wins.

R-22 and R-32 are single-component refrigerants: one molecule, one boiling temperature per pressure, one column on the chart. R-410A is technically a blend (50% R-32 / 50% R-125), but it is near-azeotropic — its glide is well under 0.5°F — so charts print it as a single column and you treat it like a pure refrigerant.

Zeotropic blends are different. R-454B (68.9% R-32 / 31.1% R-1234yf) and R-407C boil and condense across a temperature range at constant pressure. That range is temperature glide — per HVAC School, the difference between the bubble point and the dew point. So the chart carries two columns per pressure:

• Bubble point — where liquid starts to boil (equivalently, where condensing finishes).
• Dew point — where the last droplet finishes boiling (equivalently, where condensing starts).

Which column goes with which reading:

Reading	Where you measure	Column	Why
Superheat	Suction line	Dew point	Superheat begins only after boiling is complete
Subcooling	Liquid line	Bubble point	Subcooling begins only after condensing is complete
Static / standing pressure	Cylinder or off system	Bubble (liquid present)	Liquid-vapor equilibrium sits near the bubble line

R-454B's glide is small — roughly 1.5°F to 2.5°F depending on operating range — so the wrong column costs you a degree or two. On R-407C, with roughly 8–10°F of glide, the wrong column wrecks the diagnosis outright. Build the habit on the small-glide blends so it holds on the big ones.

R-410A saturation values, per the LearnMetrics R-410A PT chart and cross-checked against AC Direct's published table:

Temp (°F)	Pressure (psig)
20	78.7
30	97.5
35	107.9
40	118.9
45	130.7
50	143.3
90	274.3
95	295.0
100	316.9
105	339.9
110	364.1
115	389.6
120	416.4
125	444.5

Superheat check. The suction gauge reads 119 psig — the chart says 40°F saturation. Your insulated clamp probe on the suction line at the condensing unit reads 50°F. Superheat = 50°F − 40°F = 10°F. Vapor left the coil fully boiled and picked up 10°F of extra heat on the way to the compressor.

Subcooling check. Head pressure reads 340 psig — the chart says 105°F condensing temperature. The liquid-line probe reads 95°F. Subcooling = 105°F − 95°F = 10°F, inside the common 8–12°F TXV window — but confirm against the data plate.

Sanity-check the whole picture: with 85°F outdoor air, condensing at 105°F is a 20°F condenser split — normal territory for a mid-efficiency condenser.

R-454B bubble and dew values, per the LearnMetrics R-454B PT chart; a second published table at HVAC PT Charts agrees within about 1 psig. Confirm final numbers against the equipment manufacturer's chart (Honeywell publishes the Solstice 454B PT chart with the same bubble/dew layout).

Temp (°F)	Bubble (psig)	Dew (psig)
20	75.0	70.9
30	92.9	88.1
40	113.3	107.7
45	124.6	118.5
50	136.6	130.1
90	262.5	251.9
100	303.6	291.6
105	325.7	313.6
110	348.9	336.4
120	399.1	385.8
125	426.0	412.4

Superheat — dew column. The suction gauge reads 108 psig. In the dew column, 107.7 psig sits at 40°F. The suction line reads 49°F. Superheat = 49°F − 40°F = 9°F.

Subcooling — bubble column. Head pressure reads 326 psig. In the bubble column, 325.7 psig sits at 105°F. The liquid line reads 96°F. Subcooling = 105°F − 96°F = 9°F.

What the wrong column costs you. At 326 psig the dew column corresponds to roughly 107.5°F. Use it for subcooling by mistake and you would report about 11.5°F instead of 9°F — a 2.5°F error on a target window only 4°F wide. Make the same mistake on a big-glide blend like R-407C and the error grows to 8–10°F, enough to misdiagnose the charge completely.