The psychrometric core implements the equations from Chapter 1 of the ASHRAE Handbook — Fundamentals including real-gas correction (enhancement factor). Every quantity has been numerically compared with ASHRAE reference tables across the full HVAC temperature range. The interactive chart is available in both IP (°F, gr/lb, BTU/lb) and SI units, in the ASHRAE-style (Carrier) layout as well as the Mollier layout.
The standard formula for specific humidity assumes ideal gas behaviour: W = ε · p_ws / (p − p_ws). In a real atmosphere, water vapour mixed with dry air exhibits cross-species interactions (cross-virial coefficient Baw), which effectively increase the partial pressure of vapour. The application corrects for this physical reality with the enhancement factor f (Buck 1981 / WMO No. 8 / ASHRAE Eq. 18):
Over liquid water (t ≥ 32 °F)
f = 1.0016 + 3.15×10⁻⁶·phPa − 0.074/phPa Over ice (t < 32 °F)
f = 1.0003 + 4.18×10⁻⁶·phPa 📈 Impact of enhancement factor on accuracy (−40…+122 °F, p = 14.696 psia)
| Quantity | Without correction | With factor f | Improvement |
|---|---|---|---|
| p_ws | ±0.04 % | ±0.02 % | unchanged |
| Ws (humidity ratio) | up to −0.62 % | ±0.08 % | 8× |
| hs (enthalpy) | up to −0.40 % | ±0.11 % | 4× |
| hs abs. | up to −0.47 BTU/lb | <0.05 BTU/lb | 11× |
| vs (specific volume) | ±0.13 % | ±0.10 % | minor |
In ASHRAE mode the application calculates saturation pressure using the Hyland–Wexler correlation (equations 5 and 6) with original coefficients C1–C13. All other quantities are derived from it:
W = ε · f · p_ws / (p − f · p_ws) (eq. 20 + enhancement factor, ε = 0.621945)h = 0.240·t + W·(1061 + 0.444·t) (eq. 32, IP form, t in °F)v = 0.370486·T·(1 + 1.607858·W) / p (eq. 26, IP form, T in °R, p in psia)Reference values are ASHRAE Table 2 (properties of saturated moist air, real-gas model LibHuAirProp / RP-1485) and Table 3 (saturation pressure). Comparison performed at standard pressure 14.696 psia.
The residual deviation of humidity ratio ±0.08 % is the fundamental limit of the perfect-gas model and corresponds to the accuracy of ASHRAE equations 20 and 26 relative to a full real-gas calculation (LibHuAirProp).
Hyland–Wexler correlation. Max. deviation ±0.02 % in HVAC range.
| t [°F] | App | ASHRAE | Deviation |
|---|---|---|---|
| -4 | 0.014977 | 0.014974 | +0.020 % |
| 32 | 0.088648 | 0.088648 | +0.000 % |
| 50 | 0.178105 | 0.178135 | −0.017 % |
| 68 | 0.339215 | 0.339273 | −0.017 % |
| 86 | 0.615837 | 0.615934 | −0.016 % |
| 104 | 1.070881 | 1.071017 | −0.013 % |
| 122 | 1.791194 | 1.791361 | −0.009 % |
With enhancement factor. Max. deviation ±0.08 % — 8× improvement over uncorrected version.
| t [°F] | App | ASHRAE | Deviation |
|---|---|---|---|
| -4 | 4.459 | 4.459 | +0.008 % |
| 32 | 26.544 | 26.530 | +0.063 % |
| 50 | 53.662 | 53.641 | +0.050 % |
| 68 | 103.362 | 103.327 | +0.034 % |
| 86 | 191.359 | 191.331 | +0.013 % |
| 104 | 343.917 | 344.015 | −0.028 % |
| 122 | 607.537 | 608.041 | −0.083 % |
Absolute deviation reported (enthalpy datum is 0 °F, so relative error is undefined near the zero-crossing). Max. absolute deviation 0.041 BTU/lb up to +122 °F.
| t [°F] | App | ASHRAE | Δ [BTU/lb] |
|---|---|---|---|
| -4 | −0.287 | −0.283 | −0.004 |
| 32 | 11.766 | 11.761 | +0.005 |
| 50 | 20.318 | 20.308 | +0.010 |
| 68 | 32.451 | 32.434 | +0.017 |
| 86 | 50.712 | 50.685 | +0.027 |
| 104 | 79.388 | 79.348 | +0.040 |
| 122 | 126.104 | 126.063 | +0.041 |
Max. deviation 0.10 % — residual error of the perfect-gas model.
| t [°F] | App | ASHRAE | Deviation |
|---|---|---|---|
| -4 | 11.499 | 11.488 | +0.095 % |
| 32 | 12.471 | 12.462 | +0.067 % |
| 50 | 13.007 | 13.001 | +0.052 % |
| 68 | 13.619 | 13.612 | +0.044 % |
| 86 | 14.361 | 14.354 | +0.049 % |
| 104 | 15.333 | 15.325 | +0.052 % |
| 122 | 16.710 | 16.699 | +0.067 % |
Across the full range relevant for HVAC (−40 to +122 °F), the application results are in excellent agreement with ASHRAE: saturation pressure within 0.02 %, humidity ratio within 0.08 %, enthalpy within 0.041 BTU/lb, and specific volume within 0.10 %. These deviations are well below the accuracy of typical measuring instruments (humidity sensors ±2–5 %, thermometers ±0.2–0.9 °F).
References: ASHRAE Handbook — Fundamentals (2021), Chapter 1 · Hyland & Wexler (1983) · Buck (1981) · WMO No. 8 CIMO Guide (2018).