Dry Wet Bulb Dew Point Air

Overview

Engineering reference data for Dry Wet Bulb Dew Point 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

Graphical Representation

On a psychrometric chart, the Dry Bulb Temperature (Tdb) is represented by vertical lines along the bottom x-axis. The Wet Bulb Temperature (Twb) lines run diagonally from the upper left to the lower right. The Dew Point Temperature (Tdp) is located along the saturation curve (100% relative humidity line).

In a Mollier diagram (enthalpy-humidity chart), constant dry bulb temperatures appear as horizontal lines.

Measurement Methods

• Dry Bulb Temperature (Tdb): Measured with a standard thermometer shielded from direct radiation and moisture.
• Wet Bulb Temperature (Twb): Measured using a thermometer with its bulb wrapped in a wet wick, exposed to air flow. The temperature drops due to evaporative cooling.
• Dew Point Temperature (Tdp): Can be measured directly by cooling a surface (e.g., a polished metal can with an ice-water mixture) until condensation first appears.

Psychrometric Relationships

The three temperatures are interrelated. Key properties include:

• The Wet Bulb Temperature is always between the Dry Bulb and Dew Point temperatures: Tdp ≤ Twb ≤ Tdb.
• The Dew Point Temperature equals the Dry Bulb Temperature when the relative humidity is 100% (i.e., the air is saturated).
• The Dew Point Temperature remains constant if no moisture is added to or removed from the air parcel, even as the dry bulb temperature changes.

Formulas

While complex relationships exist, a common empirical formula for the Dew Point Temperature (in °C) for moderate humidity ranges is derived from the Magnus formula. Given the Dry Bulb Temperature (Tdb) and Relative Humidity (RH), it can be approximated as:

$$T_{dp} \approx \frac{243.5 \cdot \ln(RH/100) + \frac{17.67 \cdot T_{db}}{243.5 + T_{db}}}{17.67 - \ln(RH/100) - \frac{17.67 \cdot T_{db}}{243.5 + T_{db}}}$$

The calculation for Wet Bulb Temperature is iterative and depends on psychrometric data, but a fundamental principle is the adiabatic saturation process where enthalpy is approximately conserved between the air and the added water vapor.

References

• Original Source