How to read a pressure-temperature chart
What the chart is telling you
A refrigerant boiling in an evaporator is at saturation: liquid and vapour together, and the pressure fixes the temperature. Measure the pressure and you know the temperature of the boiling refrigerant without putting a probe inside the pipe. That is the whole trick.
What the chart cannot tell you is the temperature of the pipe once the refrigerant has fully evaporated or fully condensed. Beyond saturation, pressure and temperature come apart, and that gap is exactly what superheat and subcooling measure.
Superheat and subcooling
Superheat = actual suction line temperature minus saturation temperature at suction pressure. It tells you the refrigerant has fully evaporated before reaching the compressor. Too little and liquid reaches the compressor. Too much and the evaporator is starved.
Subcooling = saturation temperature at discharge pressure minus actual liquid line temperature. It tells you the refrigerant fully condensed and confirms there is a solid column of liquid feeding the metering device.
Both need a pressure reading and a temperature reading taken at the same point. A pressure reading alone tells you nothing about charge.
Why blends have two columns
A zeotropic blend does not boil at one temperature. It starts boiling at the bubble point and finishes at the dew point, and the difference between them is the glide. R-407C glides several degrees; R-404A glides less than one; R-410A is near-azeotropic and glides almost not at all.
The rule: use dew point for superheat, because superheat begins once the last liquid has evaporated. Use bubble point for subcooling, because subcooling begins once the first liquid has formed.
Get this backwards on a high-glide blend and your superheat calculation is out by the glide, which is enough to make a correctly charged system look starved. See glide explained.
Gauge pressure and absolute pressure
Service gauges read gauge pressure, which is pressure above atmospheric. Published property data is often in absolute pressure. The two differ by roughly one atmosphere, and mixing them up shifts every reading.
Use a chart or an app that states which it is using, and match it to your gauges. On a manifold marked in psig, use a psig chart.
Where to get the actual numbers
We do not publish a PT table here, deliberately. Saturation data is a property of the fluid as manufactured, and the authoritative version is the manufacturer's technical data sheet for the product in your cylinder. Charging equipment against a number transcribed from a third-party table is a small risk with an expensive failure mode.
The Technical Data Sheets for the lines we hold are on the data sheets page, published by the manufacturer and reproduced unaltered. Most modern service manifolds also carry the common refrigerants built in.
Quick answers
What is a PT chart used for?
Converting a gauge pressure reading into the saturation temperature of the refrigerant, so superheat and subcooling can be calculated without probing inside the pipe.
Should I use bubble point or dew point for superheat?
Dew point. Superheat begins once the last liquid has evaporated. Use bubble point for subcooling, which begins once the first liquid has formed.
Why do blends show two temperatures for one pressure?
Zeotropic blends boil across a range rather than at a single temperature. The lower figure is the bubble point, the higher is the dew point, and the difference is the glide.
Where do I get accurate PT data for my refrigerant?
The manufacturer's technical data sheet for the product in your cylinder. Sheets for the lines we hold are on our data sheets page, reproduced unaltered.
- Gujarat Fluorochemicals · Technical Data Sheets with saturation property data, hosted unaltered on this site.
- ASHRAE Standard 34 · blend designation, glide and dew and bubble point definitions · ashrae.org.
- AHRI Standard 700 · refrigerant composition tolerances · ahrinet.org.
This guide is general information compiled from the cited sources — not legal, safety or engineering advice. Read our full disclaimer.
Related: Manufacturer data sheets · Glide explained · Operating pressures compared · Charging a chiller