Humidity Ratio

Humidity ratio ($x$; ASHRAE symbol $W$) is the mass of water vapor per 1 kg of dry air. It is expressed in g/kg or kg/kg of dry air.

Do not confuse it with absolute humidity (mass of vapor per 1 m³ of air, kg/m³) or with specific humidity (mass of vapor per 1 kg of moist air).

Formula

$$x = 0.622 \cdot \frac{p_w}{p - p_w}$$

where $p_w$ is the partial pressure of water vapor [Pa] and $p$ is the total barometric pressure [Pa]. The factor 0.622 is the ratio of the molar masses of water and dry air.

Key property: it is conserved

The humidity ratio does not change during:

• heating or cooling of the air (until the dew point is reached)
• passage through a heat exchanger (as long as no condensation occurs)

That is why $x$ is the horizontal axis of the Mollier h–x diagram (in the ASHRAE/Carrier psychrometric chart the axes are transposed): sensible heating and cooling are vertical processes that do not change $x$.

Typical values

State	$x$ [g/kg]
Winter outdoor air (−10 °C, φ 80 %)	≈ 1.3
Comfortable indoor air (22 °C, φ 50 %)	≈ 8.3
Summer outdoor air (30 °C, φ 60 %)	≈ 16.2

In practice

The humidity ratio is used to calculate:

• humidifier and dehumidifier duty (Δ$x$ × mass flow)
• the moisture balance in an air-handling unit
• dew-point checks on exchanger surfaces