Genesis of Hygrometers

• The first basic hygrometer was invented in 1400s by an Italian polymath named Leonardo da Vinci. However, a more practical hygrometer was built by Francesco Folli in 1664. British chemist and meteorologist Sir John Frederic Daniell made the dew point hygrometer in 1820. Dew point hygrometer initially used cold water and the temperature (or dew point) of the vessel (i.e., hygrometer's body) was recorded to measure humidity.
  
  

Working Principle

• At a particular temperature and velocity, the cooling effect of air is directly related to humidity. This phenomenon is used in hygrometers. When air is circulated within the wet bulb thermometer, it will show a lower temperature reading, compared with the dry bulb thermometer. The differential in these two readings is then translated to percent humidity with the help of conversion charts.
  
  

Calibration

• Precise calibration of hygrometers ensures accurate readings of humidity. Additionally, thermometers need to be protected from any radiant heat. Also, a good airflow has to be maintained over the wet bulb. Furthermore, when the air temperature is below the freezing point, the surrounding temperature of the hygrometer must be raised by heating the outside air to get precise readings. The vapor content will remain unchanged even when the air is heated.
  
  

Types

• Today, meteorologists use different types of hygrometers. The simplest and best known form of hygrometer is psychrometer. It is uses the principle of evaporation to determine humidity. It consists of two similar thermometers: wet bulb and dry bulb. Rapid rotation of both the thermometers and blowing the air over the bulbs reduces the temperature of the wet bulb. This temperature difference between these thermometers is used to calculate the moisture content in the air.
  
  An electronic hygrometer deploys electronic means of recording the information with the help of capacitors or resistors. The capacitive sensors can detect the presence (and quantum) of water by passing an electric current and measuring the change in capacitance caused by the presence of water. Alternatively, resistive sensors use a polymer membrane in which the conductivity changes based on the amount of water that is present.
  
  Other types of hygrometers include gravimetric hygrometers (primarily used for calibration and setting national standards) and hair tension hygrometers (which use human or animal hair under tension).
  
  

Applications

• Hygrometers are used primarily for the purpose of measuring and controlling humidity in greenhouses, saunas, humidors, museums and residences, as well as industrial setups that are sensitive to vapor content such as paints, food and beverages, and pharmaceuticals.