Humidity Ratio Air

Overview

Engineering reference data for Humidity Ratio Air 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

Example Calculation

Consider saturated moist air at $20^\circ\text{C}$ with a water vapor partial pressure $p_w = 2333\,\text{Pa}$ and atmospheric pressure $p_a = 101325\,\text{Pa}$. Using the humidity ratio formula by partial pressure:

$x = 0.62198 \times \frac{p_w}{p_a - p_w}$

Substituting the values:

$x = 0.62198 \times \frac{2333}{101325 - 2333} = 0.0147\,\text{kg/kg} = 14.7\,\text{g/kg}$

This result aligns with the saturation humidity ratio from the table, confirming the relationship between vapor pressure and humidity ratio.

Practical Considerations

• The humidity ratio equations assume ideal gas behavior, which is accurate at moderate conditions. Be cautious when applying these equations at higher temperatures, as deviations may occur—refer to resources on temperature and moisture holding capacity for details.
• Saturation pressure of water vapor increases exponentially with temperature, leading to a rapid rise in the maximum humidity ratio. This property is critical in drying processes, where higher temperatures significantly enhance moisture capacity.
• For most atmospheric conditions, the water vapor pressure is small relative to total pressure, making the humidity ratio nearly linear with saturation pressure. However, this approximation simplifies at extreme values.

Interactive Charts

References

• Original Source