psychrometrics-si

Moist-air (psychrometric) property calculations in SI units, following the ASHRAE Handbook of Fundamentals. No dependencies.

Everything you normally read off a psychrometric chart — saturation pressure, humidity ratio, relative humidity, dew point, enthalpy, specific volume — as plain functions.

Conventions

• Temperature: degrees Celsius
• Pressure: pascals (default total pressure is sea-level, 101325 Pa)
• Humidity ratio W: kg water vapour per kg dry air
• Relative humidity: fraction in [0, 1]

Install

npm install psychrometrics-si

Usage

import {
  saturationPressure,
  humidityRatio,
  dewPoint,
  enthalpy,
} from "psychrometrics-si";

saturationPressure(25);      // ~3169.7 Pa
humidityRatio(25, 0.5);      // ~0.00988 kg/kg
dewPoint(25, 0.5);           // ~13.9 C
enthalpy(25, 0.010);         // ~50.6 kJ/kg dry air

At a non-standard altitude, pass the local barometric pressure as the last argument:

humidityRatio(25, 0.5, 90000); // Pa

Functions

• saturationPressure(tC) — Hyland–Wexler, over ice below 0 C.
• humidityRatio(tC, rh, p?) / relativeHumidity(tC, W, p?)
• humidityRatioFromVaporPressure(pv, p?) / vaporPressureFromHumidityRatio(W, p?)
• dewPoint(tC, rh, p?) / dewPointFromVaporPressure(pv)
• enthalpy(tC, W) — kJ per kg dry air
• specificVolume(tC, W, p?) — m³ per kg dry air

Accuracy

The saturation-pressure correlation is valid roughly from −100 to +200 C. Dew point is recovered by bisecting that curve, so it inherits the same accuracy.

License

MIT