Moist Air vs. Dry Air

The behavior of moist air is different from that of dry air. The equation (P_oxygen = X_oxygen • P_air ) is valid only for dry air, and, therefore, must be re-written to account for the water vapor:

Where:

P_O2 = partial O₂ pressure in water-vapor-saturated air (kPa)

X_O2 = volume concentration of oxygen in atmospheric air (0.2095)

P_air = total atmospheric pressure (kPa)

P_water = partial water vapor pressure (kPa see article on saturated vapor pressure)

Saturated water vapor pressure varies with temperature, and, therefore, the partial oxygen pressure depends on temperature.

For example, what is the partial pressure for oxygen in water-vapor-saturated air at sea level and at an air temperature of 20°C?

This value is approximately 2.3% below the value for dry air (21.22 kPa).

With this equation, we can calculate the oxygen concentration of air-saturated water in units of the partial pressure of oxygen, i.e. in kPa. However, industry often requires the oxygen concentration to be expressed in mg/l or ppm (part per million) instead of kPa.