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Dry Bulb Temperature (DBT)

What is the dry bulb temperature (T)?

The dry bulb temperature is the temperature measurement taken with a mercury or conventional thermometer that has a dry bulb. This is precisely where its name comes from, although it is also known as dry temperature. Together with what is known as the wet bulb temperature, it is a temperature that is generally used as a standard for thermal comfort. That is, for measurements where the amount of humidity is related to the temperature. These concepts and their importance are explained in more detail below.

What is the dry bulb temperature (DBT)?

The dry bulb temperature is the temperature that is considered to be the actual temperature in humid air. This is why it is generally referred to simply as air temperature. It is, in fact, the temperature usually displayed by an ordinary thermometer. A dry bulb thermometer does therefore reflect the ambient temperature. A wet bulb thermometer, on the other hand, will indicate the temperature resulting from evaporation. In general, this temperature is more related to wind chill.

The wind chill can be determined when the temperature drop is stabilised by the evaporation of water in a wet cloth surrounding the wet bulb thermometer. If relative humidity is to be calculated, a handle near the instrument is generally used, which is raised or lowered according to the dry bulb temperature. This type of apparatus can be adjusted to indicate the wet bulb temperature, the dry bulb temperature and the dew point.

It should be noted that the dry bulb temperature is necessary in order to know the relative humidity and the dew point.

The above values are used to generate psychrometric diagrams or charts, which usually have two values. One of them corresponds to the dry temperature. They have numerous applications, ranging from mining and sustainable architecture to air conditioning. The unit of measurement used is the Kelvin (K) Standard International Unit, although Celsius is also widely used.

Differences between dry bulb temperature and wet bulb temperature

First of all, and as mentioned above, a device called a psychrometer is used at weather stations to measure the temperature of a given environment. These devices have two thermometers, one of which is usually wrapped in a cloth that is moistened. This mechanism is called the wet bulb. The other – the dry bulb – is in thermal equilibrium with the air in the surrounding atmosphere. With this arrangement the wet bulb will receive a constant air flow through a ventilation system. It can also be supplied of course by a natural ventilation source.

The process that takes place is as follows: the water surrounding the wet bulb evaporates with the heat flow and eventually consumes heat. This heat is obtained from the wet bulb itself. The temperature of a wet-bulb thermometer thereby falls until it reaches a point of equilibrium where it is finally stable.

This is the point at which the temperature known as the psychrometric wet bulb temperature (or WBT) is recorded.

All of the above makes it possible to determine the difference between the dry bulb temperature and the wet bulb temperature. They will therefore be defined separately below.

 

Dry bulb temperature

The dry bulb temperature is considered to be the actual temperature of the humid air. This temperature is also known as air temperature, and is in fact the temperature indicated by a common thermometer.

Wet bulb temperature

The wet bulb temperature corresponds to a type of temperature measurement in which the properties of a system are shown physically in terms of the evaporation of the water contained in the air. It should be noted that heat must be consumed in order for evaporation to occur, and, when it occurs, a cooling process is generated. The reason for this is that, as evaporation takes place, the system has less kinetic energy on average. This is what causes the temperature of the liquid to also decrease.

This phenomenon is called evaporative cooling, and it is precisely what happens when you sweat. When evaporation occurs, cooling takes place as sensible heat is consumed and converted into latent heat. Thus, the wet bulb temperature can also be defined as the lowest temperature that can be reached by water evaporation alone. Thus, unlike the dry bulb temperature, the wet bulb temperature establishes a record of the amount of moisture in the air. So the lower the humidity in the air, the greater the cooling.

Another difference that has been mentioned is that the wet bulb temperature is measured with a specific thermometer; it includes a bulb surrounded by a porous type mesh that comes into contact with distilled water. The mesh is kept at a constant humidity as a result of the capillary principle, which causes it to absorb liquid. The evaporation process that takes place in the screen removes heat from the bulb, causing the thermometer to register a lower temperature than the surrounding ambient air. The latter temperature is the dry bulb temperature.

So the drier the air in the atmosphere, the greater the reduction in wet bulb temperature, and it is zero when the atmosphere is saturated with water vapour (in conditions known as 100% relative humidity).

Finally, it should be noted that the wet bulb temperature can be used as an indicator of how much the human body itself can be cooled by sweating.

Other important concepts

To better clarify the concepts of dry bulb and wet bulb temperature, let us explain some related concepts.

Vapour pressure

Vapour pressure is the partial pressure generated by water vapour molecules in humid air. If the air is completely saturated with water vapour, the pressure is called saturated vapour pressure.

Mixing ratio

This corresponds to the ratio of the mass of water vapour in a given volume compared to the mass of dry air. It differs, therefore, from absolute humidity, which compares the volume occupied by the mixture of dry air and vapour with the mass of water vapour.

Relative humidity

This the ratio of the water vapour pressure in moist saturated air to the water vapour pressure at any given time, and at the same temperature in both cases.

Dew point temperature

Finally, the dew point temperature is the temperature at which moist air that was not saturated becomes saturated. This is the temperature at which water vapour condenses.

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