Wet Bulb Temperature and Its Measurement

Definition

Wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air at constant pressure. It represents the temperature of an object when it is cooled by the evaporation of water from its surface.

Main Content

1. Thermodynamic Significance

It is a measure of the cooling potential of the air, which depends on the ambient humidity level.
When the air is saturated (100% relative humidity), the wet bulb temperature is exactly equal to the dry bulb temperature because no more evaporation can occur.

2. The Role of Latent Heat

Evaporation requires energy, known as latent heat, which is extracted from the liquid water.
As the water evaporates, it absorbs heat from the surrounding air and the thermometer bulb, causing the temperature to drop below the ambient (dry bulb) reading.

3. Psychrometric Relationship

The difference between dry bulb and wet bulb temperatures is called the "wet bulb depression."
A larger depression indicates drier air (lower relative humidity), while a smaller depression indicates more humid air.

[Airflow] -> [Wet Wick] -> [Evaporation] -> [Cooling Effect]
     |              |               |               |
(Dry Air)      (Moist)       (Heat Loss)     (Lower Temp)

Working / Process

1. Preparing the Sensor

A standard mercury or electronic thermometer is covered with a clean, porous cloth wick, typically made of cotton muslin.
The wick must be kept saturated with distilled water to ensure consistent and accurate evaporation.

2. Air Circulation

Air must flow over the wet wick at a specific velocity (typically around 3–5 meters per second).
This consistent airflow ensures that the air immediately surrounding the wick remains representative of the ambient environment rather than becoming locally saturated.

3. Equilibrium Attainment

As water evaporates from the wick, the thermometer reading gradually decreases until it reaches a steady state.
Once the reading stops dropping and stabilizes, this stable value is recorded as the wet bulb temperature.

Advantages / Applications

Used in HVAC systems to determine the moisture content of air and calculate cooling capacity.
Essential in industrial cooling towers to determine the maximum cooling efficiency for circulating water.
Used in meteorological forecasting to calculate dew point, relative humidity, and heat stress indices for human safety.

Summary

Wet bulb temperature is a crucial thermodynamic property that identifies the air's cooling capacity through evaporation. By using a wetted wick and controlled airflow, we can measure how much moisture the current atmosphere can absorb. It serves as a vital indicator in weather monitoring and climate control systems.

Important terms to remember: Dry Bulb Temperature, Latent Heat, Relative Humidity, Wet Bulb Depression, and Psychrometry.