R-32 PT Chart: Saturation Pressure from −40°F to 150°F

Start from what the gauge says: type the pressure, or step it with the buttons and the slider, and the box beside it gives the temperature where R-32 saturates. R-32 has a single saturation curve, so that one figure serves both superheat and subcooling. To go the other way, set I have a to Temperature and enter a temperature from −40°F to 150°F; one that falls between two rows returns both of them. Check Also show °C and kPa to add metric figures in brackets. The complete chart, one row per degree, is printed underneath.

Worked example: R-32 at 75°F and at 130 psig

Show the math

The first box starts at 75°F. Because 75°F has a row of its own, the result is simply that row's value: 223.15 psig.

The second box starts at 130 psig, and no row lands exactly there. The nearest rows are 43°F at 128.09 psig and 44°F at 130.53 psig, so a single degree covers 2.44 psi at this point on the curve. A reading of 130 psig is 1.91 psi above the 43°F row. Dividing 1.91 by 2.44 gives about 0.78 of a degree, which puts saturation at 43.78°F. The tool rounds that to a tenth and shows 43.8°F.

One pressure per temperature

CoolProp models R-32 as a pure fluid, not a mixture. When a pure fluid boils at a fixed pressure, the first bubble and the last drop of liquid appear at one and the same temperature, so the bubble point and the dew point coincide. That's why this chart has a single pressure column, and why the lookup gives back one temperature instead of a pair with a glide between them.

Blends don't work that way. Put the 75°F row next to the same row on the R-454B chart: R-32 reads 223.15 psig, while R-454B has 208.40 psig on its bubble side and 199.51 psig on its dew side. The blend's two columns are 8.89 psi apart at that temperature. R-32 has no second column to be apart from.

Superheat and subcooling with R-32

Since R-32 has only one saturation curve, a suction reading and a liquid reading are both converted with the same column. There's no choice between dew and bubble to get wrong. The superheat and subcooling calculator includes R-32 and does the conversion and the subtraction in one step.

Neither this chart nor the calculator tells you what a system should read. Charging figures belong to the equipment manufacturer, and handling guidance belongs in the manufacturer's installation instructions and the refrigerant's safety data sheet.

How this chart was made

CoolProp 8.0.0, an open-source thermodynamic property library, produced one saturation point for each whole degree in the range. Its absolute pressures were turned into gauge pressures by taking off 14.696 psi, the standard atmosphere at sea level.

As a cross-check, the rows were set against Hudson Technologies' R-32 distributor chart at seven temperatures from −40°F to 150°F. At each of those seven points they differ by less than 1% of the absolute pressure, and the largest difference was 0.05 psi. The page on how the charts are made describes the comparison for every refrigerant.

A pressure that sits between two rows is converted by drawing a straight line from one row to the next, then rounding to 0.1°F. A pressure beyond either end of the chart gets no answer at all, rather than an extrapolated one.