TILAK BHUSAL / ENGINEERING TOOLS

Understand the air.

Precision psychrometrics. From two known properties to the complete picture.

02 / EXPLORE THE AIR STATE

Psychrometric chart

h & wet-bulb lines

│ Dry-bulb─ Humidity ratio⌒ Relative humidity● Your state

Click the chart or drag the state point. Arrow keys move the focused point.

03 / COMPLETE AIR STATE

Your results

Tdb + RH → Tdb, W → complete state

Dry-bulb temperature Tdb

25.00°C

Wet-bulb temperature Twb

17.89°C

Dew-point temperature Tdp

13.86°C

Relative humidity RH

50.00%

Humidity ratio W

0.009881kg/kg dry air

Specific humidity SH

0.009784kg/kg moist air

Vapor pressure Pv

1.58kPa

Saturation pressure Pws

3.17kPa

Enthalpy h

50.32kJ/kg dry air

04 / THE SCIENCE, MADE SIMPLE

Meet the properties.

Nine ways to describe the same air. Formulas below use SI units; W is per kg of dry air.

Tdb

Dry-bulb temperature

What a regular thermometer reads. It tells you how hot the air is, without directly measuring its moisture. Think of the temperature on a weather app.

Tdb = (h − 2501W) / (1.006 + 1.86W)

Twb

Wet-bulb temperature

The temperature a wetted thermometer approaches as water evaporates. Like sweat cooling your skin, evaporation lowers the reading. Drier air usually creates more cooling.

W = [(2501 − 2.326Twb)Ws − 1.006(Tdb − Twb)] / [2501 + 1.86Tdb − 4.186Twb] (Twb ≥ 0 °C)

Tdp

Dew-point temperature

The temperature where cooling air first reaches saturation. Think of droplets forming on a cold glass. Below freezing, this calculator uses saturation over ice, so the value is a frost point.

Pws(Tdp) = Pv; invert the saturation curve

Relative humidity

How close water vapor is to saturation at the current temperature. Like a fullness gauge, 50% means half the saturation vapor pressure. Heating air without adding water usually lowers RH.

RH = 100 × Pv / Pws(Tdb)

Humidity ratio

The mass of water vapor mixed with each unit mass of dry air. Imagine weighing the water vapor and dry air in separate containers.

W = 0.621945 × Pv / (P − Pv)

Specific humidity

The fraction of the total moist-air mass that is water vapor. Unlike humidity ratio, the total includes both the dry air and its water vapor.

SH = W / (1 + W)

Enthalpy

An energy account for the air and its water vapor. Both warming the air and adding vapor increase this account. Its zero point is a chosen reference, like the starting balance of an account.

h = 1.006Tdb + W(2501 + 1.86Tdb)

Vapor pressure

The part of air pressure caused by water-vapor molecules. Picture water vapor and dry air each contributing to the total push against a container.

Pv = P × W / (0.621945 + W)

Pws

Saturation pressure

Water-vapor pressure at saturation for a given temperature. It rises sharply as temperature rises. It is the reference line used by the RH fullness gauge.

Pws = exp(ASHRAE polynomial in absolute temperature) / 1000, in kPa

Equation details, operating limits & sources

Saturation pressure uses ASHRAE equations 5 and 6 with absolute temperature K = Tdb + 273.15. The ice branch applies at or below 0.01 °C; the water branch applies above it.

ln(Pws [Pa]) = −5674.5359/K + 6.3925247 − 0.009677843K + 6.2215701×10⁻⁷K² + 2.0747825×10⁻⁹K³ − 9.484024×10⁻¹³K⁴ + 4.1635019 ln(K) [ice]

ln(Pws [Pa]) = −5800.2206/K + 1.3914993 − 0.048640239K + 4.1764768×10⁻⁵K² − 1.4452093×10⁻⁸K³ + 6.5459673 ln(K) [water]

Below 0 °C wet-bulb: W = [(2830 − 0.24Twb)Ws − 1.006(Tdb − Twb)] / [2830 + 1.86Tdb − 2.1Twb]. Ws is the saturation humidity ratio at Twb. Wet-bulb inversion uses bracketed bisection, split at freezing.

Altitude: P = 101.325(1 − Lz/T₀)^(gM/RL), where L = 0.0065 K/m, T₀ = 288.15 K, g = 9.80665 m/s², M = 0.0289644 kg/mol, and R = 8.3144598 J/(mol·K). Valid altitude: −500 to 11,000 m. Actual weather pressure can differ.

Saturation curve: −100 to 200 °C. Complete states must be below the local boiling point, within the wet-bulb curve range, and unsaturated or saturated. Dependent inputs (for example W + SH) cannot determine temperature. Inconsistent measurements require correction; the solver tries alternative pairs but never silently replaces a measurement.

Enthalpy in both displays retains the SI reference of zero dry-air enthalpy at 0 °C. BTU/lb = kJ/kg ÷ 2.326; this differs from charts using the conventional IP enthalpy datum. Specific humidity uses mass per mass of moist air, never per mass of dry air.

Reference-table humidity ratios include effects beyond the ideal-gas approximations; the published test tolerances reflect this difference. PsychroLib uses a small moisture floor, which can affect near-dry states. Independent numerical checks account for those differences.

Sources: PsychroLib equation references · ASHRAE table checkpoints · ASHRAE Handbook. Independent checker: PsychroLib 2.5.0, MIT licensed.