Psychrometric Wet Bulb⚠ unverified
Physics / Thermodynamics · Compute the humidity ratio from dry-bulb and wet-bulb temperatures
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| Tdb | Tdb | — | 1.0 | Dry-bulb temperature, in degrees Celsius |
| Twb | Twb | — | 1.0 | Wet-bulb temperature, in degrees Celsius |
| P | P | Pa | 101325.0 | Total (atmospheric) pressure, in pascals (Pa). Default is 101325 |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | Pws | — | Humidity ratio w, in kilograms of water per kilogram of dry air |
The science & history
Understanding the Parameters
- $T_{wb}$ — wet-bulb temperature in Celsius for Magnus coefficients 17.27 and 237.3.
-
$T_{db}$, $P$ — would be needed for relative humidity, humidity ratio, or psychrometric wet-bulb iteration; currently ignored.
-
$P_{ws}$ — vapour pressure of pure water in equilibrium at $T_{wb}$ (approx 610.8 Pa at 0 degC).
Derivation (Approaching a Proof)
The Magnus–Tetens family fits the Clausius–Clapeyron integrated vapour-pressure curve of water with an empirical exponential. Coefficients (610.8, 17.27, 237.3) are a common SI choice for $T$ in degC and $P$ in Pa over typical meteorological ranges — empirical, not exact thermodynamics.
A true psychrometric wet-bulb calculation couples energy and mass transfer at the wet wick and solves for $T_{wb}$ given $T_{db}$ and humidity (or vice versa) — not implemented here.
History
Wet-bulb thermometry and psychrometric charts are classical HVAC/meteorology tools; Magnus-type formulae are standard digital approximations for $P_{sat}(T)$.
Related Concepts: Enthalpy Moist Air, First Law DeltaU
Notes: Registry calculator psychrometric-wet-bulb (unverified). Misnamed / incomplete:
outputs $P_{ws}(T_{wb})$ only; $T_{db}$ and $P$ unused. $T$ labelled dimensionless (use degC).
Result may be labelled dimensionless — physically Pa.