A wet bulb thermometer measures the extent of cooling as moisture dries from a surface (evaporative cooling). The wet bulb temperature is always lower than the dry bulb temperature except when there is 100% relative humidity, making the wet bulb temperature a more accurate measurement of product temperature.
Beside this, why would you use wet bulb temperature?
Wet Bulb Temperature (WBT in short) is a measure of how much moisture or water vapour is present in the air. The difference between the dry bulb temperature (AKA normal temperature, DBT in short) and this determines how much dry the air is. If DBT-WBT is large, then the air has lower relative humidity.
At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature) and is lower at lower humidity. It is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat supplied by the parcel.
The Wet Bulb temperature is always between the Dry Bulb temperature and the Dew Point. For the wet bulb, there is a dynamic equilibrium between heat gained because the wet bulb is cooler than the surrounding air and heat lost because of evaporation.
The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation and moisture. DBT is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature.
A relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the moisture content remains constant and temperature increases, relative humidity decreases, but the dew point remains constant.
Relative humidity also measures water vapor but RELATIVE to the temperature of the air. Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.
Most people are comfortable with a dew-point temperature of 60 degrees Fahrenheit (16 degrees Celsius) or lower. At a higher dew point of, for example, 70 F (21 C), most people feel hot or "sticky" because the amount of water vapor in the air slows the evaporation of perspiration and keeps the body from cooling.
an instrument used to measure the relative humidity of the atmosphere. It consists of a thermometer with a bulb that is wet or moist and one that is kept dry. The relative humidity is calculated from the difference in readings of the thermometers when water evaporates from the wet bulb, decreasing its temperature.
The depression in Wet-Bulb temperature allows the humidity to be calculated. If the air is fully saturated (100% relative humidity) the water cannot evaporate, so both the wet and dry bulb temperatures are the same.
The dewpoint temperature is the temperature at which the air can no longer "hold" all of the water vapor which is mixed with it, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.
The WetBulb Globe Temperature (WBGT) is a measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation). This differs from the heat index, which takes into consideration temperature and humidity and is calculated for shady areas.
The reduction in the measured reflected light indicates the mirror is at the dew point temperature. A psychrometer is an alternative instrument for determining the dew point. A psychrometer consists of two thermometers, one of which has a wet wick around its bulb and is called the wet-bulb thermometer.
Dry-bulb temperature - Tdb, can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture. The temperature is usually given in degrees Celsius (oC) or degrees Fahrenheit (oF). The SI unit is Kelvin (K).
CALCULATING RELATIVE HUMIDITY. Calculating the RH requires the correct equation(s). The RH is the amount of moisture in the air (via moisture mass or vapor pressure) divided by the maximum amount of moisture that could exist in the air at a specific temperature (via max moisture mass or saturation vapor pressure).
A vintage psychrometer. These devices have been used for more than 100 years to measure the moisture content, or relative humidity, in the air. You may have noticed that humidity (the amount of water vapor in the air) makes a difference in how comfortable you feel outside.
The humidity ratio is sometimes referred to as moisture content or the mixing ratio. It is the mass of water vapor per unit mass of dry air. The humidity ratio (W) can be calculated if the % moisture by volume (%MV) is known.
Relative humidity can be measured by an instrument called a hygrometer. The simplest hygrometer - a sling psychrometer - consists of two thermometers mounted together with a handle attached on a chain. One thermometer is ordinary. The other has a cloth wick over its bulb and is called a wet-bulb thermometer.
Adiabatic Saturation And Wet-build Temperatures. Adiabatic saturation temperature refers to a temperature at which water converts into air by the process of evaporation adiabatically. The device used for this type of process is known as adiabatic saturator. The adiabatic saturator device is shown below in Figure (1).