How the charts and questions are made

This page sets out where each number and each practice question on the site comes from, how they were checked, and what the site leaves out.

The charts

Every pressure here was computed with CoolProp 8.0.0, an open-source library of fluid properties, using its HEOS backend: the Helmholtz-energy equations of state that CoolProp carries for each fluid. For each refrigerant, the research script asked for the saturated-liquid pressure (the bubble point) and the saturated-vapor pressure (the dew point) at every whole degree Fahrenheit from −40°F to 150°F. That's 191 rows per chart.

R-32, R-22 and R-134a are single compounds with their own models in CoolProp, so their bubble and dew columns are identical. R-410A is a blend that CoolProp ships ready-made. R-454B isn't one of CoolProp's ready-made fluids, so it was set up as a mixture of its two parts at the standard composition: 68.9% R-32 and 31.1% R-1234yf, by mass. As a test of that mixture method, the same setup was run for R-404A and R-407C, two blends CoolProp does ship, and the results agreed with CoolProp's own versions of them to 0.01 kPa.

CoolProp reports absolute pressure. A gauge reads relative to the air around it, so 14.696 psia, the standard atmosphere at sea level, was subtracted from every value. Where that subtraction goes negative, the refrigerant is below atmospheric pressure, and the chart shows inches of mercury (inHg) of vacuum instead of a negative psig figure, the way a compound gauge reads. Of the charted refrigerants, only R-134a has rows like that, from −40°F up to −15°F.

On the first pass, CoolProp's mixture solver failed to converge at 13 points across the blends in the research. Nine of them were R-454B rows between 134°F and 149°F. A second script solved each missing point again at exactly its own temperature, starting the solver from the nearest row below that had converged. It then confirmed every new point sits on a smooth curve: the rise from the row beneath differs from the rise before it by less than 0.1 psi. None of the rows were filled by interpolation, so R-454B's chart is complete to 150°F.

How the charts were checked

The computed rows were compared with published charts at a few points each: Honeywell's for R-454B and R-448A, and Hudson Technologies' distributor charts for the other eight. The text of both Honeywell charts was saved during the research, but the web address each came from wasn't recorded, so they're named here and not linked. Five of the ten, R-404A, R-407C, R-448A, R-449A and R-1234yf, appear only in the printable chart pack and have no chart page on this site. Each pack chart sends its reader here, so their checks are listed alongside the others.

The points compared, fluid by fluid:

• R-454B: Honeywell's Solstice 454B chart, bubble and dew at six pressures from 20 psig to 450 psig, 12 points.
• R-448A, pack only: Honeywell's Solstice N40 chart, bubble and dew at six pressures from 0 psig to 397 psig, 11 points. The twelfth, Honeywell's bubble point at 0 psig, is −51.0°F, below the bottom of our table, so it couldn't be compared.
• R-32, R-410A and R-22: Hudson, seven temperatures from −40°F to 150°F, 7 points each.
• R-134a: Hudson, seven temperatures from −10°F to 150°F, 7 points.
• R-404A, pack only: Hudson, seven temperatures from −40°F to 150°F, 7 points.
• R-407C and R-449A, pack only: Hudson, six temperatures from −40°F to 120°F, 12 points each. Hudson prints a liquid (bubble) and a vapor (dew) column for these two blends, and both columns were compared.
• R-1234yf, pack only: Hudson, four temperatures from 0°F to 90°F, 4 points.

Each comparison is made at the reference chart's own point. Hudson's charts list a pressure for each whole degree, so those line up with a row of ours directly. Honeywell's charts go the other way: they list bubble and dew temperatures, to a tenth of a degree, at round pressures. For those, our pressure at Honeywell's exact temperature is found by straight-line interpolation between the two neighboring rows and set against Honeywell's pressure. Vacuum values are compared as pressure below atmospheric, never as positive numbers.

For the five charted refrigerants, taken as a share of absolute pressure (the gauge figure plus 14.696 psi), the rows are within about 1% of the reference charts checked at every point compared but one. Absolute pressure is the basis because a gauge reading near zero turns any small gap into a large percentage: at −40°F, R-22's gap is 0.07 psi on a chart value of just 0.5 psig.

Six points fall outside that 1%, and five of them are on pack-only charts:

• R-410A at −40°F: 0.84 psi on a chart value of 11.6 psig, about 3.2%. It's the only charted point over 1%.
• R-404A, pack only, at 0°F: 0.51 psi on a chart value of 32.1 psig, about 1.1%.
• R-407C, pack only, at −40°F on the bubble side: 0.25 psi on a chart value of 3.0 psig, about 1.4%.
• R-448A, pack only, on the dew side: 0.20 psi at 0 psig (about 1.3%), 0.38 psi at 20 psig (about 1.1%) and 0.82 psi at 67 psig (just over 1.0%). Across the six pressures checked, the dew temperatures computed for R-448A run about half a degree warmer than Honeywell's.

The biggest gaps in psi come high on the charts, where they're a small share of the pressure: R-410A differs by 5.02 psi at 150°F, about 0.8% of a 608 psig chart value, and R-454B by 1.92 psi on the bubble side at 450 psig, about 0.4%. R-454B's closest call is 0.33 psi on the bubble side at 20 psig, just under 1%. Elsewhere the largest gaps are 0.05 psi for R-32, 0.44 psi for R-22 and 0.32 psi for R-134a. On the pack-only charts they're 2.54 psi for R-448A, 2.01 psi for R-404A, 1.63 psi for R-449A, 1.43 psi for R-407C and 0.05 psi for R-1234yf, each under 0.7% of the absolute pressure at its point. Read in degrees instead, R-454B's bubble temperatures sit 0.3°F to 0.4°F below Honeywell's, and its dew temperatures within 0.15°F of them.

Three things about the Hudson comparisons are worth knowing:

• Hudson prints one pressure per temperature for R-410A and R-404A, so each point was compared with whichever of the bubble and dew columns was closer.
• Hudson's R-134a chart doesn't make clear whether its values below about 15°F are psig or vacuum. Points below −10°F were left out, and the −10°F and 0°F points carry less weight than the rest.
• At −40°F, Hudson's vapor values for R-407C and R-449A are printed as 4.4 and 0.0, with no sign, where our dew rows are below atmospheric pressure. Both were read as inches of mercury of vacuum. That reading is an assumption, so those two dew points carry less weight too.

The gaps are listed so you can weigh them yourself. None of them is a promise about any particular reading.

The lookups

The temperature box reads a whole-degree row straight off the chart. The pressure box finds the two rows whose pressures bracket the value typed, draws a straight line between them, and rounds the temperature it lands on to 0.1°F. A pressure or temperature outside the −40°F to 150°F range gets a note and no answer: nothing is extrapolated past either end.

The superheat and subcooling calculator uses the same rows. It reads superheat against the dew-point temperature at the suction pressure, and subcooling against the bubble-point temperature at the liquid pressure. Each saturation temperature is rounded to 0.1°F before the subtraction, so every figure on screen can be checked against the one beside it.

The practice questions

The Core and Type II tests have 25 questions each. They're original, written for this site from saved copies of 40 CFR 82.157 and 82.161, Appendix D to Subpart F, the class II list in Appendix B to Subpart A, EPA's Section 608 certification page, and EPA's 2024 fact sheet on the Emissions Reduction and Reclamation program.

Each explanation cites a paragraph and quotes it word for word. An automated test compares every quoted passage with the saved copy of the page it cites, ignoring only quote-mark style, spacing and capital letters, and fails if a single word is missing or changed. Other tests check that each cited paragraph exists, each answer points to a real choice, and no prompt appears twice.

EPA's own question bank isn't public: Appendix D has the agency release it only to approved certification programs. So nothing here comes from that bank, and these tests aren't the certification exam. They score against the 70% pass mark for a closed-book test.

Updates

The pressure–temperature data is physics and doesn't go out of date, but it's reviewed once a year. The EPA rule content is checked against eCFR and EPA's pages every quarter; the next check is due in December 2026. Every change is logged on What changed.

What isn't covered

• Type I and Type III practice questions.
• Target superheat and subcooling figures, which come from the equipment manufacturer.
• Refrigerants without a chart page yet, including R-404A, R-407C, R-448A, R-449A and R-1234yf.