How to Read a PT Chart
What a pressure-temperature chart shows, how to read bubble vs dew columns, what temperature glide means in practice, and why some charts truncate above a critical temperature.
TL;DR: A PT chart maps refrigerant temperature to its saturation pressure (or back). Pure refrigerants get one curve. Zeotropic blends get two — bubble for the liquid side, dew for the vapor side. Use dew for superheat math, bubble for subcooling. Some refrigerants truncate above their critical temperature because no saturation state exists.
What a PT chart is
A pressure-temperature chart maps the saturation pressure of a refrigerant at each temperature in its operating range. The line drawn is the phase boundary between liquid and vapor at thermodynamic equilibrium.
In service, the chart is the bridge between a gauge reading and a temperature interpretation. The manifold reads pressure; the chart converts that to saturation temperature. The difference between actual line temperature and saturation temperature is superheat (suction) or subcooling (liquid).
The relationship is fundamental to every vapor-compression refrigeration cycle. Any point where liquid and vapor coexist — broadly, inside the evaporator and condenser — sits on the saturation curve. Knowing one (pressure or temperature) tells you the other. Outside the two-phase region the refrigerant is either pure liquid (above the curve at high pressure or low temperature) or pure vapor (below the curve at low pressure or high temperature).
Modern PT data is computed from Helmholtz-energy equations of state — the same thermodynamic framework NIST's REFPROP uses. The values on this site come from CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999), which implements REFPROP-compatible EOS for 122 pure refrigerants and predefined mixtures. Manufacturer-blend PT charts (R-448A, R-450A, R-1336mzz(Z), etc.) come from the named manufacturer datasheets and are cross-checked against ASHRAE 34-2022 composition specifications.
- Above the curve
Refrigerant is vapor (or supercritical above critical point).
- On the curve
Two-phase: liquid and vapor coexist at thermodynamic equilibrium.
- Below the curve
Refrigerant is subcooled liquid.
How to read one
Pick a temperature; read across to the corresponding saturation pressure. Or pick a pressure; find the corresponding temperature. The chart works in either direction.
On a paper chart, find your value on one axis and trace the curve. On a digital chart (like this site's calculators), enter the value and the lookup is instant. For the 1°F or 1°C increments most charts use, interpolation between adjacent entries is linear and accurate to better than ±0.1°F for the bubble or dew curves; the underlying Helmholtz EOS is smooth enough that linear interpolation introduces negligible error.
In the field, the practical use is converting manifold pressure (what you read on the gauge) to saturation temperature(what you compare against the line-temperature probe to compute superheat or subcooling). The chart isn't the answer — it's the unit conversion between two service measurements.
At 121 PSIG, R-22 exists as a liquid-vapor mixture at 70°F. Hold pressure and add heat — temperature stays at 70°F until all the liquid boils. Hold pressure and remove heat — stays at 70°F until all the vapor condenses.
Service technician reads 130 PSIG suction on an R-410A residential AC. Saturation temperature at 130 PSIG is 45°F. If the suction line measures 60°F, superheat = 60 − 45 = 15°F (in the standard TXV target range).
The bidirectional nature is what makes the PT chart the foundational reference in HVAC service. Any charging procedure, any diagnostic measurement, any retrofit comparison can be traced back to lookups on this curve.
Bubble vs dew columns
Pure refrigerants and azeotropes give a single saturation pressure per temperature, so one column suffices. Zeotropic blends — refrigerants whose components have different normal boiling points — give two columns:
Bubble
Saturated liquid pressure
The pressure at which the first vapor bubble appears as the liquid is heated.
- Use for
- Subcooling
- Side
- Liquid line
Dew
Saturated vapor pressure
The pressure at which the first liquid drop forms as the vapor is cooled.
- Use for
- Superheat
- Side
- Suction line
Live values at 70°F — pure vs blend
| Refrigerant | Type | Bubble | Dew | Glide |
|---|---|---|---|---|
| R-22 | Pure / azeotrope | 121.4 | 121.4 | 0.0°F |
| R-134a | Pure / azeotrope | 71.1 | 71.1 | 0.0°F |
| R-410A | Pure / azeotrope | 201.8 | 201.1 | 0.2°F |
| R-407C | Zeotrope | 140.5 | 117.3 | 11.0°F |
| R-454C | Zeotrope | 141.2 | 112.2 | 13.9°F |
| R-455A | Zeotrope | 170.1 | 121.1 | 21.6°F |
Values pulled live from the verified dataset (CoolProp 7.2.0). Glide column shows the bubble-minus-dew spread at 0°C from r.physical.temperatureGlideF.
Temperature glide
Glide is the spread between bubble and dew temperatures at the same pressure. For a zeotropic blend at constant pressure, the refrigerant doesn't boil or condense at a single temperature — it does so across a range. The first vapor appears at the bubble temperature; the last liquid disappears at the dew temperature.
Practical implications
- Superheat → use dew
Suction-line measurement. Vapor above dew temperature is superheated. Using bubble understates superheat by the glide.
- Subcooling → use bubble
Liquid-line measurement. Liquid below bubble temperature is subcooled. Using dew overstates subcooling by the glide.
- Charge → check spec
System nameplate may specify bubble, dew, or mean saturation. Get this wrong and the charge target is off by the full glide.
Critical-point truncation
Every pure refrigerant has a critical temperatureabove which the liquid/vapor distinction disappears. No saturation state exists; the substance is supercritical. PT charts end there — there's nothing physical to plot above.
- R-744 (CO₂)Tcrit87.8°FChart truncates at87°F
Commercial CO₂ refrigeration runs transcritically above this point — the 'condenser' becomes a 'gas cooler'.
- R-13 (legacy CFC)Tcrit83.7°FChart truncates at83°F
Used historically as the low-stage refrigerant in cascade systems where the high side stays below ambient.
- R-1150 (ethylene)Tcrit48.6°FChart truncates at48°F
Industrial cascade refrigerant for sub-zero process applications.
See it in action — R-744 (CO₂)
R-744's saturation curve ends at 87°F. Above critical, CO₂ systems operate transcritically — the high side has no condensing pressure to read off a PT chart.
For zeotropic blends the critical point is replaced by a critical locus— a curve along which the critical temperature varies with composition. The dataset's individual refrigerant pages note where this applies.
Transcritical operation matters most for R-744 (CO₂) commercial refrigeration in warm climates, where the gas cooler routinely runs above 87.8°F. In transcritical mode the high side has no condensing pressure; gas-cooler outlet pressure is controlled by a high-pressure throttle valve and gas-cooler outlet temperature is the meaningful service metric (target 8-10°F above ambient at design optimum). Other refrigerants with low critical temperatures rarely operate transcritically in HVAC because their equipment is sized to keep operating points below critical even at peak ambient.
Chart scope & sources
A standard PT chart on this site covers −40°F to 150°F at 1°F increments— 191 data points. Refrigerants with low critical temperatures truncate; refrigerants outside CoolProp's validity range also truncate. The chart shows what's physically real and skips what isn't, rather than extrapolating fabricated values.
The chart range is chosen to cover the operating envelope of common HVAC applications: residential AC (typical evap 40°F, cond 95-110°F), commercial refrigeration MT (15-30°F evap), commercial LT (-40°F to -10°F evap), heat pumps in heating mode (outdoor coil 10-25°F evap). Specialized applications (cryogenics below -150°F, high-temperature process refrigeration above 200°F) need refrigerants outside this range and are addressed on per-refrigerant pages.
Source provenance.Every value on this site traces to a published source: CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999) for pure refrigerants and predefined mixtures; ASHRAE 34-2022 for composition specifications and safety classifications; AHRI Standard 700 for refrigerant specifications; manufacturer technical datasheets (Honeywell Solstice / Genetron, Chemours Opteon, Arkema Forane, AGC AMOLEA) for the 11 blends not in CoolProp's library. CoolProp data is REFPROP-compatible (validated against NIST's reference database) with typical accuracy better than ±0.5% across the operating range.
Verification policy: every value is recorded in data/refrigerants.json(the generated data layer), validated against a Zod schema at build time, and cross-checked against AHRI 700 specifications where applicable. The previous WordPress version of this site shipped with approximately 25,000 fabricated quantitative errors including PT values wrong by 2-15×, some above critical pressure (a physical impossibility), and several A2L / A3 / B2L refrigerants classified as "A1 non-flammable". The current rebuild was structured specifically to make those failure modes impossible: refrigerant data comes from primary sources, safety class is a Zod enum, and any value outside the chart range returns "out of range" rather than an extrapolated number.
For the 11 blends CoolProp doesn't model (R-448A, R-450A, R-1336mzz(Z), etc.), PT charts come directly from the named manufacturer datasheet. Where transcription is pending, the chart is empty and the page says so — never invented.
Common pitfalls
PSIG vs PSIA confusion
Manifold gauges read PSIG (above atmospheric). Charts here are PSIG unless explicitly stated. PSIA = PSIG + 14.696.
Single curve on a zeotrope
R-407C, R-454C, R-455A, R-448A, R-449A all have meaningful glide. Single-curve math introduces error equal to the glide — up to 22°F on R-455A.
Fabricated source values
The previous WordPress version of this site shipped with PT values wrong by 2–15×, including several physically impossible values above critical pressure. The chart's source matters. Every value here comes from CoolProp 7.2.0 or a cited manufacturer datasheet.
Saturation ≠ operating pressure
A PT chart gives the saturation pressure at thermodynamic equilibrium. Operating pressure on a running system depends on charge, ambient, load, superheat, and subcooling. See what should R-22 pressures be for the operating-pressure perspective.
FAQ
›What is a PT chart actually showing me?
Pressure on one axis, temperature on the other. The line drawn between them is the saturation curve — the boundary between liquid and vapor phases at thermodynamic equilibrium. Picking a temperature and reading off the corresponding pressure tells you 'this refrigerant exists as both liquid and vapor at this temperature only when held at this pressure'.
›Why do some PT charts have two pressure columns?
Zeotropic refrigerant blends boil and condense across a temperature range at constant pressure. The two columns are bubble (saturated-liquid) and dew (saturated-vapor) — the start and end of the phase transition. Pure refrigerants and azeotropes have a single saturation pressure per temperature, so one column suffices.
›Why does the R-744 (CO2) chart stop at 87°F?
Carbon dioxide's critical point is 87.8°F. Above the critical temperature, no saturation state exists — the refrigerant becomes supercritical and the liquid/vapor distinction disappears. CO2 commercial refrigeration systems often operate transcritically (above this point on the high side); below it, the standard PT chart applies. Other refrigerants with low critical temperatures (R-13 at 84°F, R-1150 ethylene at 49°F) similarly truncate.
›What does temperature glide mean in practical terms?
For a zeotropic blend, at constant pressure the refrigerant doesn't boil or condense at a single temperature — it does so across a range. R-407C at typical evaporator pressure has ~11°F glide; R-454C has ~14°F; R-455A has ~22°F. This affects EXV sizing, charge measurement, and superheat measurement. The blends have their place but require treating saturation conditions as ranges rather than points.
›Should I use bubble or dew for superheat math?
Dew. Superheat is measured on the suction line where the refrigerant has fully passed through evaporation — the relevant saturation boundary is the dew temperature (the point where the last drop of liquid disappears). The site's superheat calculator uses the dew curve automatically.
›Should I use bubble or dew for subcooling math?
Bubble. Subcooling is measured on the liquid line where the refrigerant is fully condensed — the relevant saturation boundary is the bubble temperature (the point where the first vapor bubble appears when boiling, or where the last vapor disappears when condensing). The subcooling calculator uses the bubble curve automatically.