R-508B PT Chart and Refrigerant Profile

Where you will find R-508B

R-508B is an ultra-low temperature refrigerant: an azeotrope of 46% R-23 and 54% R-116 by mass. Honeywell gives its range as roughly -40 to -101°C (-40 to -150°F). You find it in the low stage of cascade systems, chiefly ultra-low temperature freezers, and in other very low temperature process and test equipment. It replaced R-13 and R-503. EPA lists R-508B as acceptable in very-low-temperature refrigeration, for new and retrofit equipment, since July 28, 1995.

R-508B cannot condense against ambient air: its critical point is 52.5°F. A separate high stage condenses it in a cascade heat exchanger. The chart above stops at 50°F for that reason.

Compared with R-23, the other common low-stage refrigerant, R-508B runs at higher pressure at the same temperature, which means lower evaporating temperature at a given suction pressure. At -100°F R-508B reads 16.7 psig (bubble) against 9.0 psig for R-23. At -120°F it is still above atmospheric (2.9 psig bubble, 2.6 psig dew), where R-23 is in a vacuum.

A warm, idle low stage is near or above the critical temperature, so its standstill pressure is not a chart value. The system’s pressure rating has to cover it; never assume a stopped low stage sits at low pressure.

Charging R-508B

R-508B is an azeotrope, so its composition shifts very little between liquid and vapor. The chart shows bubble and dew columns because the blend model separates them slightly at the coldest temperatures (0.8°F glide at -130°F, essentially zero from -40°F up). For practical service it behaves like a single-component refrigerant. No manufacturer charging statement is quoted here; where the nameplate gives a charge weight, weigh it in to that number.

For superheat on the low stage, read suction pressure against the dew column and use the superheat calculator; the PT calculator converts any pressure to saturation temperature. The how to read a PT chart guide covers bubble and dew columns. Check the pressure rating of gauges, hoses and recovery cylinders before connecting to a warm circuit.

Oil and compatibility

BITZER lists POE for R-508B and notes that polyol ester oils are suitable for R-508A/B but must be matched to the special requirements of very low temperature operation. Use the compressor maker’s specified oil. The refrigerant oil types guide explains POE.

Safety, handling and recovery

A1 under ASHRAE 34: lower toxicity, no flame propagation. For Section 608, R-508B is a very-high-pressure refrigerant (critical temperature below 104°F; 40 CFR 82.152 names it as an example), so the work is Type II or Universal.

It is a non-exempt refrigerant: no venting, recovery to 0 in Hg for very-high-pressure appliances (40 CFR 82.156 Table 1), and sales restricted to certified technicians since January 1, 2018 (40 CFR 82.154(c)). Recover into a cylinder rated for the pressure.

Cylinders: R-508B’s legacy color was dark blue. Since January 1, 2020 new containers are light green-gray RAL 7044 under AHRI Guideline N-2017, with the legacy color kept on the label. See cylinder colors.

Status and replacements

R-508B has two GWP figures, and the difference matters. The full climate figure on the AR4 basis is 13,396 (46% × 14,800 for R-23 plus 54% × 12,200 for R-116), which is also what EPA’s SNAP page lists. For AIM Act allowance accounting only the HFC part counts: R-116 is a perfluorocarbon, not an AIM Act regulated substance, so the exchange value is 6,808 (46% × 14,800). The R-23 content is subject to the HFC phasedown (40 CFR 84.7): allowances at 60% of baseline in 2024–2028, 30% in 2029–2033, 20% in 2034–2035 and 15% from 2036.

No Technology Transitions sector limit for very-low-temperature equipment was found in 40 CFR 84.54. HFC leak repair under 40 CFR 84.106 applies from January 1, 2026 to appliances with 15 lb or more of HFC; many ultra-low freezers hold less, so check the nameplate charge.

No direct replacement is listed in the sources used for this page. EPA’s very-low-temperature list also includes R-508A, ethane (R-170, A3, acceptable with use conditions) and R-744. For R-23 alternatives see the R-23 profile; for GWPs see the GWP chart.

Common questions

What is the difference between R-508B and R-23?

R-23 is a single HFC; R-508B is an azeotrope of 46% R-23 and 54% R-116. R-508B runs at higher pressure at the same temperature and has a lower critical point (52.5°F against 79.1°F), and its AIM Act exchange value is lower because R-116 is not counted.