Hygrometers are instruments that measure the humidity (amount of moisture content) present in air or gas. Hygrometers consists of two mercury thermometers, one of which has a wet base (to measure wet-bulb temperature) while the other one has a dry base.

Genesis of Hygrometers
The first basic hygrometer was invented in 1400s by 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. It was used with cold water and the temperature (or dew point) of the vessel was recorded to measure humidity.

How do Hygrometers work?
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 on the wet bulb thermometer, it will show a lower temperature (as compared to the dry bulb thermometer). The differential in these two readings is then translated to percent humidity, using established charts.

Calibration of Hygrometers
Precise calibration of the thermometers is critical to ensuring that the measurement of humidity is also accurate. Additionally, thermometers need to be protected from any radiant heat as well as a good airflow has to be maintained over the wet bulb. Further, for accurately measuring humidity when the air temperature is below the freezing point, temperature is actually raised by heating the air (since the vapor content remained unchanged even when the air is heated).

Different types of Hygrometers
There are several types of hygrometers used by meteorologists. The simplest and best known form of hygrometer is “psychrometer”. It is uses the principle of evaporation to determine humidity. Rapid rotation of both the thermometers and blowing air over the bulbs reduces the temperature of the wet bulb. This temperature difference between the thermometers is used to calculate the moisture content in the air.

An electronic hygrometer deploys electronic means of recording the information, using capacitors or resistors for the purpose. 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 whose conductivity changes, based on the amount of water that is present.

Other hygrometers include gravimetric hygrometers (primarily used for calibration and setting national standards), and hair tension hygrometers use human or animal hair under tension.

Application of Hygrometers
Hygrometers are used in greenhouses, industrial setups that are sensitive to vapor content (example being paints, food and beverages and pharmaceutical), saunas, humidors, museums and residences, primarily for the purpose of measuring and controlling humidity.


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