R-134a PT Chart: Pressure and Vacuum, −40°F to 150°F

Give the lookup a gauge pressure, typed or stepped with the buttons and slider, and it shows the saturation temperature of R-134a, which has one curve and no glide. Below atmospheric pressure, set Gauge reads to inHg vacuum and type the vacuum the way your compound gauge shows it. Change I have a to Temperature to read a whole row instead; between two rows, both are shown. For metric, check Also show °C and kPa. Below the tool is the whole chart, including the cold rows where R-134a sits under atmospheric pressure.

Worked example: R-134a at 75°F and at 35 psig

Show the math

The temperature box starts at 75°F, one of the chart's own rows, and R-134a's pressure on that row is 78.65 psig.

The pressure box starts at 35 psig. R-134a reaches 34.09 psig at 39°F and 35.05 psig at 40°F, so 35 psig lies 0.91 psi into a step that's 0.96 psi wide. That ratio is a little under 0.95, which makes the saturation temperature 39.948°F. Rounded to one decimal place, the tool shows 39.9°F, just short of the 40°F row.

R-134a is a single-component refrigerant, so at any pressure it boils and condenses at one temperature. Its bubble and dew columns in CoolProp's data match on every row, and the chart prints them as one.

Reading the vacuum rows

On the cold end of this chart, R-134a's saturation pressure drops below the pressure of the atmosphere. That happens below about −15°F, so a gauge shows it as vacuum, in inches of mercury, rather than as psig.

The chart follows the same convention. Each of those cells ends in inHg vac, from 14.80 at −40°F down to 0.05 at −15°F. From −14°F, where R-134a is at 0.35 psig, the cells go back to ordinary gauge pressure. As an example, the −30°F row sits at 9.84 inHg vacuum.

The pressure box accepts 0 psig and upward, and you can look up the vacuum range in the table. What does 0 psig itself correspond to? The −15°F row is 0.05 inHg of vacuum, which is about 0.025 psi below zero, and the −14°F row is 0.35 psig above it. Zero sits roughly a fifteenth of the way from the colder row, so the lookup returns -14.9°F.

With Also show °C and kPa ticked, vacuum values also come out as negative kPa, because the metric figures are gauge readings as well.

Superheat and subcooling with R-134a

With one saturation curve, there's no dew-or-bubble decision for R-134a: both line readings go through the same column. The superheat and subcooling calculator includes R-134a among its five refrigerants. What a given system should read is set by its manufacturer, not by this chart.

How this chart was made

Each row is a saturation point from CoolProp 8.0.0, calculated at a whole degree Fahrenheit. The absolute pressure was converted to gauge by subtracting 14.696 psia, a standard atmosphere at sea level; where the result was negative, it's shown as inches of mercury of vacuum instead.

For comparison, the rows were checked against Hudson Technologies' R-134a distributor chart at a handful of temperatures. From 40°F upward, each compared point differs by under 1% of the absolute pressure, and the largest difference there was 0.32 psi. None of the vacuum rows were part of that comparison. More on the checks is on the page about how the charts are made.

Readings between rows are interpolated in a straight line and rounded to 0.1°F. Readings beyond the chart's range return no result.