Humidity Measurement

Overview

Engineering reference data for Humidity Measurement in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Symbol	Description	Unit
$\omega$	Humidity ratio	kg/kg
$\phi$	Relative humidity	%
$P_v$	Vapor pressure	Pa
$P_{vs}$	Saturation vapor pressure	Pa
$T_{db}$	Dry bulb temperature	°C
$T_{wb}$	Wet bulb temperature	°C

Humidity Lookup Tables

Measurement Method

The dry bulb temperature ($T_{db}$) is measured directly with a standard thermometer. The wet bulb temperature ($T_{wb}$) is measured with a thermometer whose bulb is kept wet using a wick or cotton sleeve. Continuous airflow over the wet bulb is critical for accurate evaporation and reading; this is often achieved using a sling psychrometer or aspiration device.

Interactive Charts

References

• Original Source