Dry bulb temperature
The dry bulb temperature, or just dry temperature, air temperature measured without considering environmental factors such as radiation, humidity and air movement, which have the potential to significantly affect thermal sensation.
One of the most used instruments for measuring the dry bulb temperature is the mercury thermometer, which consists of a thin glass tube whose base, bulb-shaped, containing a metal deposit of the slurry.The assembly is hermetically sealed to maintain a partial vacuum therein. Thanks to its large capacity expansion mercury rises in the tube as the temperature increases, enabling the measurement of the latter by a graduated scale. The mercury thermometer is exposed directly to the air but protected from moisture and sunlight. Because mercury is bright white on the other hand, is considered reflective enough to avoid almost completely absorbing heat radiated by the elements of the environment. When all these conditions are met, the mercury thermometer accurately indicates relatively dry air temperature.
Due to the toxicity of mercury, the European Union recently banned the manufacture of thermometers with this metal. Instead the use of liquids with properties similar expansion, such as alcohol or ether is encouraged. On the other hand it is important to mention that there are other types of thermometers to measure the dry bulb temperature:
- Resistance thermometer. Uses materials whose electrical resistance changes with temperature, generally transient metal oxides such as magnesium, cobalt, copper or nickel.
- Gas thermometer. It is based on the pressure variations of certain gases, according to its temperature.
- Bimetallic strip thermometer. Rely on differential expansion due to temperature, wrapped with two sheets of very different expansion coefficients.
- Thermocouple. Use the electromotive force generated by increasing the temperature of the welding of two different metals.
- Digital thermometer. It includes a built-in electric circuit, which is sensitive to temperature changes microchip.
