HVAC Charge ProGauge pressure and altitude
Field gauges read pressure relative to the surrounding atmosphere (psig). A printed P-T chart normally assumes a standard atmosphere at sea level. At higher elevations the atmosphere is thinner, so the same absolute pressure reads a little differently on a gauge. HVAC Charge Pro’s P-T data uses a standard sea-level reference (101,325 Pa) and applies no altitude correction, so check how your chart or tool handles elevation.
Bubble point and dew point
At a given pressure, a refrigerant has two saturation temperatures. The bubble point is where the liquid begins to boil, which is the saturated liquid temperature. The dew point is where the vapor first begins to condense, which is the saturated vapor temperature. For a pure refrigerant the two are the same.
Temperature glide on blends
Zeotropic blends do not boil or condense at one temperature, so the bubble and dew points differ at the same pressure. In the data bundled in HVAC Charge Pro, R-407C at 70 psig has a dew point of about 44.5 °F and a bubble point of about 33.5 °F, a difference of about 11 °F. Using the wrong column would shift a superheat or subcooling result by that much. R-410A and R-454B show a much smaller difference.
Reference excerpt
Gauge pressure (psig) at selected saturation temperatures, from the P-T data bundled in HVAC Charge Pro. This is a short excerpt for orientation, not a complete chart.
On narrow screens, scroll the table horizontally to see all columns. Keyboard users can focus the table and use the arrow keys.
| Saturation temp | R-410A dew | R-410A bubble | R-454B dew | R-454B bubble |
|---|---|---|---|---|
| 40 °F | 118.4 | 118.8 | 107.5 | 113.2 |
| 60 °F | 170.1 | 170.7 | 155.3 | 162.7 |
| 80 °F | 235.7 | 236.5 | 216.0 | 225.4 |
| 100 °F | 317.6 | 318.5 | 291.9 | 303.3 |
| 120 °F | 418.3 | 419.5 | 385.5 | 398.6 |
Data generated with CoolProp 8.0.0 for the app’s offline reference. Verify against the refrigerant manufacturer’s published chart for service decisions.
Which column for which calculation
- Superheat: use the dew point at suction pressure.
- Subcooling: use the bubble point at liquid pressure.
HVAC Charge Pro applies this automatically: superheat uses the dew curve and subcooling uses the bubble curve.
HVAC Charge Pro and this guide are calculation and reference aids. A superheat or subcooling value alone does not decide whether to add or remove refrigerant. Follow the equipment manufacturer’s instructions, approved service procedures and applicable safety requirements.
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