What the chart is telling you
A PT chart lists saturation: the pressure at which a refrigerant's liquid and vapor sit together at a given temperature. Where refrigerant is boiling (the evaporator) or condensing (the condenser), pressure and temperature are locked together, so a gauge reading tells you the coil temperature without touching the coil. Where the refrigerant is all vapor or all liquid, the line temperature can differ from the chart. That difference is superheat on the vapor side and subcooling on the liquid side.
Temperature to pressure
Find the temperature in the left column and read across. An R-410A evaporator boiling at 40°F runs at 118.8 psig. An R-410A condenser at 110°F runs at 366.4 psig. The same two rows on an R-22 chart read 68.6 psig and 226.4 psig, which is why a chart for the wrong refrigerant gives nonsense. Use this direction when you know the temperature you want, for example the pressure a coil should hold at a design evaporating temperature.
Pressure to temperature
This is the direction you use with gauges on a running system. Find the pressure you measured and read the temperature beside it. On R-410A, a suction gauge reading of 130 psig means the refrigerant in the evaporator is boiling at 45.0°F. A liquid line reading of 365 psig means it condensed at 109.7°F.
Printed charts list set temperature steps, so a reading usually falls between two rows. Interpolate between them, or use the PT lookup, which accepts any pressure and returns the matching saturation temperature.
Reading a blend chart: two columns
Zeotropic blends such as R-407C change temperature as they boil or condense at a constant pressure, so their charts give a bubble point (saturated liquid) and a dew point (saturated vapor) for each row. ASHRAE defines superheat as vapor temperature minus the dew point and subcooling as the bubble point minus liquid temperature, which tells you which column to use:
- Suction side, for superheat: dew column. On R-407C at 63 psig, the dew point is 39.9°F while the bubble point is 28.8°F.
- Liquid side, for subcooling: bubble column. On R-407C at 260 psig, the bubble point is 109.9°F while the dew point is 118.5°F.
Picking the wrong column on a high-glide blend throws superheat or subcooling off by the full glide. Arkema describes R-410A as a near-azeotropic mixture; its glide is so small (0.2°F at a 40°F dew point) that the columns nearly match. More in bubble point, dew point and glide.
Using the chart on a running system
- Confirm the refrigerant from the rating plate and set your digital manifold to it.
- Run the system until pressures and temperatures settle, for as long as the installation manual calls for.
- Read suction pressure at the suction service valve and convert it to the dew-point temperature. Clamp a temperature probe on the suction line near the same valve. Line temperature minus saturation temperature is superheat.
- Read liquid pressure at the liquid service valve and convert it to the bubble-point temperature. Clamp a probe on the liquid line nearby. Saturation temperature minus line temperature is subcooling.
- Compare the results with the manufacturer's charging chart or target.
Steps 3 and 4 follow California's Title 24 charge verification procedure, which reads each pressure at a service valve and clamps the matching pipe sensor near that valve. The superheat and subcooling calculators do the math for steps 3 and 4. How to measure superheat and subcooling covers probe placement and the readings that mislead.
Gauge pressure, vacuum and altitude
Charts on FieldCharts read gauge pressure (psig) at sea level. Absolute pressure is gauge pressure plus atmosphere, about 14.7 psi at sea level in the U.S. Standard Atmosphere. Below zero gauge, values appear as inches of mercury of vacuum; R-134a at -20°F is 3.7 in Hg vacuum. The same standard atmosphere is about 2.5 psi lower at 5,000 ft, so a gauge there reads about 2.5 psi higher than a sea-level chart for the same temperature. Subtract the difference before you look up the reading. The PT chart index works through the numbers.
Common mistakes
- Reading the wrong column on a blend. Dew for suction, bubble for liquid.
- Trusting the gauge before the probe. Title 24 calls for pipe sensors in good contact with the line, and a Daikin installation manual says the suction thermometer should make adequate contact and be insulated. Superheat and subcooling are only as good as the temperature reading.
- Reading pressure far from the temperature point. The Title 24 procedure pairs each pressure reading with a line temperature taken near the same service valve.
- Running off the end of the chart. ASHRAE defines the critical point as where the liquid and vapor states of a substance become indistinguishable, so a PT chart ends at the critical temperature. R-744 (carbon dioxide) reaches its critical point at 87.8°F.