Refrigerant PT Chart
Saturation pressure and temperature, bubble and dew points, superheat and subcooling.
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Refrigerant PT Chart
A pressure–temperature (PT) chart for 13 refrigerants — R-22, R-32, R-134a, R-404A, R-407C, R-410A, R-507A, R-1234yf, R-1234ze(E), R-290 (propane), R-600a (isobutane), R-717 (ammonia) and R-744 (carbon dioxide) — that also does the arithmetic of a service call. Enter a gauge reading and a line temperature to get the superheat (from the dew point) or the subcooling (from the bubble point), look up the saturation temperature at a pressure or the pressure at a temperature, and get a table and a curve over the range you choose, in °C or °F and bar, kPa, MPa or psi, gauge or absolute.
For blends the page keeps both sides of the saturation curve: the bubble point, where liquid starts to boil, and the dew point, where the last liquid evaporates. Gauge readings can be corrected for altitude: at 1,500 m the air pressure is about 17 % lower than at sea level, which shifts every gauge reading.
Without a pass, the free preview shows your readings, the refrigerant’s published data, the temperatures of the chart’s first rows and a watermarked curve; the pressures, the superheat or subcooling, the working, Copy result and the CSV come with a Pro pass.
How to use it
- Pick the refrigerant, the temperature unit, the pressure unit and whether your readings are gauge or absolute pressure. Away from sea level, choose From my altitude or type the local atmospheric pressure.
- Choose what to work out: Superheat (suction pressure and suction-line temperature), Subcooling (liquid-line pressure and liquid-line temperature), the saturation temperature at a pressure, or the saturation pressure at a temperature.
- Set the chart range and step. The table and the curve follow it, with bubble-point and dew-point columns for blends; temperatures outside the refrigerant’s saturation curve show a dash.
- Copy the result or download the table as a CSV — with a Pro pass, or after unlocking this result.
Examples
Suction 118 psig at sea level, suction line 50 °F
Dew point 39.8 °F, superheat 10.2 °F
Suction 118 psig, line 50 °F, altitude 5,000 ft
Atmosphere 84.3 kPa, dew point 38.7 °F, superheat 11.3 °F
Thinner air makes the same gauge reading a lower absolute pressure.
Liquid line 26.5 bar(g), line temperature 38 °C
Bubble point 44.3 °C, subcooling 6.3 K
5.0 bar(g)
Bubble point 1.8 °C, dew point 7.9 °C, glide 6.1 K
2.0 bar(g)
0.8 °C
40 bar(g)
6.3 °C
Above its critical point, 31.0 °C, R-744 has no saturation temperature.
Common uses
- Checking superheat at the evaporator outlet or compressor inlet during a service call.
- Checking subcooling at the condenser outlet while charging a system.
- Reading R-407C and R-404A correctly from their bubble and dew points.
- Correcting gauge readings for altitude.
- Building a PT table in your own units and range for a refrigerant you work with.
- Seeing how saturation pressure rises with temperature for natural refrigerants such as R-290, R-717 and R-744.
Superheat and subcooling with blends
Superheat is how far the vapour is above its saturation temperature at the same pressure: the suction-line temperature minus the saturation temperature at the suction pressure. Subcooling is the same on the liquid side: the saturation temperature at the liquid-line pressure minus the liquid-line temperature.
A pure refrigerant, or an azeotrope such as R-507A, boils at one temperature at a given pressure. A zeotropic blend does not: at one pressure it starts to boil at its bubble point and finishes at its dew point. The difference is its glide: about 6 K for R-407C at 5 bar(g), about 0.5 K for R-404A and about 0.1 K for R-410A. Superheat is measured on vapour, so it is taken from the dew point; subcooling is measured on liquid, so it is taken from the bubble point. Reading R-407C from the wrong side of its glide puts superheat or subcooling out by several kelvin.
Gauge pressure, absolute pressure and altitude
A refrigeration gauge shows the pressure above the air around it (gauge pressure). Property data work in absolute pressure: the gauge reading plus the atmospheric pressure, 101.325 kPa (14.696 psi, 1.013 bar) at sea level. Higher up the air is thinner — the U.S. Standard Atmosphere puts it at about 84.6 kPa at 1,500 m and 79.5 kPa at 2,000 m — so the same gauge reading means a lower absolute pressure and a lower saturation temperature. From my altitude uses that standard atmosphere; type the local pressure instead if you have a barometer reading.
In psi, pressures below atmospheric are shown as inches of mercury of vacuum, as US charts print them (1 inHg = 3.386389 kPa).
Formula and data
- Saturation curve: ln(p / p_r) = (T_r / T) · Σ Nᵢ θ^tᵢ, with θ = 1 − T / T_r; at a pressure the page solves it for the temperature.
- Pure refrigerants: the vapour-pressure equations [CoolProp](http://www.coolprop.org/) fits to each fluid’s reference equation of state. Checked against the NIST Chemistry WebBook for R-22, R-32, R-134a, R-290, R-600a, R-717 and R-744 (within 0.05 % of pressure) and, for R-1234ze(E), against the vapour-pressure equation of Thol and Lemmon.
- Blends: the bubble-point and dew-point equations of Lemmon’s pseudo-pure equations of state for R-410A, R-404A, R-507A and R-407C, valid from 200 K (−73.15 °C).
- Names and compositions: the ASHRAE refrigerant designations of Standard 34.
- Units: the pound-force per square inch and the inch of mercury from NIST SP 811; the atmosphere at altitude from the U.S. Standard Atmosphere.
Equations of state behind each refrigerant
- R-22: Kamei, Beyerlein and Jacobsen
- R-32: Tillner-Roth and Yokozeki
- R-134a: Tillner-Roth and Baehr
- R-404A, R-407C, R-410A, R-507A: Lemmon (pseudo-pure)
- R-1234yf: Richter, McLinden and Lemmon
- R-1234ze(E): Thol and Lemmon
- R-290: Lemmon, McLinden and Wagner
- R-600a: Bücker and Wagner
- R-717: Gao and co-authors
- R-744: Span and Wagner
Limitations
- Saturation only: no properties of superheated vapour or subcooled liquid (enthalpy, density) and no pressure–enthalpy diagram.
- R-454B and other newer blends are not included: their bubble and dew points need a mixture model checked against the maker’s data.
- Blend data are for the nominal composition. A blend that has leaked from the vapour side, or that was topped up with another refrigerant, has a different composition and different pressures.
- Within a few kelvin of the critical point pressures change quickly and the fitted equations are least certain; R-744 above 31.0 °C has no saturation state at all.
- An aid for checking and learning, not a substitute for the equipment maker’s charging chart or a qualified technician. Refrigerants differ in flammability, toxicity and pressure: follow the safety data sheet and the rules where you work.
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Frequently asked questions
What do I get without a pass?
Without a pass, Refrigerant PT Chart shows your readings, the temperatures of the chart’s first rows and a watermarked curve, with every pressure, the superheat, the subcooling and the working hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
How do I calculate superheat?
Measure the suction pressure and the suction-line temperature at the same place. Find the saturation temperature at that pressure — for a blend, its dew point — and subtract it from the line temperature. R-410A at 118 psig has a dew point of 39.8 °F, so a 50 °F suction line has 10.2 °F of superheat.
How do I calculate subcooling?
Measure the liquid-line pressure and temperature. Subcooling is the saturation temperature at that pressure — for a blend, its bubble point — minus the line temperature. R-32 at 26.5 bar(g) has a bubble point of 44.3 °C, so a 38 °C liquid line has 6.3 K of subcooling.
What is the difference between the bubble point and the dew point?
For a zeotropic blend they are the two ends of boiling at one pressure: at the bubble point the first vapour forms in the liquid, at the dew point the last liquid evaporates. The difference is the glide — about 6 K for R-407C at 5 bar(g), about 0.1 K for R-410A. Pure refrigerants and azeotropes such as R-507A have a single saturation temperature.
Why should I correct gauge readings for altitude?
A gauge measures above the surrounding air. At 1,500 m the air is at about 84.6 kPa instead of 101.3 kPa, so a gauge reads about 16.8 kPa (2.4 psi) more for the same absolute pressure, and an uncorrected chart gives a saturation temperature that is too high. Choose From my altitude, or type the local pressure.
Why is there no saturation temperature for R-744 above 31 °C?
That is its critical temperature (30.98 °C, at 73.8 bar absolute). Above it there is no liquid–vapour boundary, so an R-744 system with a warm gas cooler runs transcritical: the high side has a pressure and a gas-cooler outlet temperature but no condensing temperature.
Does the chart use psig or psia?
Whichever you choose. psig is psia minus the atmospheric pressure (14.7 psi at sea level). In psig, pressures below atmospheric are shown as inches of mercury of vacuum, as US PT charts print them.