Hygrometer - Measuring Humidity
Humidity, or the relative water vapor in a given air sample compared to the maximum amount of water vapor air could possibly hold in a given temperature, can be measured by a device called hygrometer. The first crude version of hygrometer was invented by Leonardo da Vinci in 1400, while a more convenient form of the device was developed sometime in 1664 by Francesco Folli.

Psychrometer
There are several varieties of hygrometer available in the market, usually distinguished by the way they measure air humidity. The most common of them all is the “wet-bulb and dry-bulb psychrometer,” which is, in reality, a set of two thermometers with one thermometer having wet cotton or linen spun around its base. The water absorbs heat from the thermometer, causing it to drop. The difference between two readings are then recorded and compared to a psychrometric chart, which, in turn, gives a reading of the relative air humidity.

Mechanical Hygrometer
Mechanical hygrometers are also used to measure air humidity. Mechanical hygrometers revolve around the principle that organic matter like human hair expands and contracts in response to varying air humidity. Such expansion and contraction moves a needle gauge that indicates the air humidity. The first mechanical hygrometer using human hair was developed by Horace Bénédict de Saussure, a Swiss geologist and physicist in 1783.

Electric Hygrometer
Electric hygrometers are also used and are considered one of the most accurate means to measure humidity. The principle behind this device is the electric resistance of a particular material and the humidity of the air it is in. Electric currents are passed through wires and the materials, and it is recorded using a sensor.Another form of electric hygrometer use moisture-absorbing materials to measure air humidity. To do so, a certain amount of gas is passed on a moisture-absorbing material like phosphorous pentoxide. The material is weighed before and after the experiment to measure how much moisture it took from the air.

Dew-Point Hygrometer
Dew-point hygrometers are also used for precise humidity readings. It operates by placing volatile liquids like ether into the hygrometer. The ether cools down the fluids inside the tubes, which, in turn, cools water vapor outside the hygrometer, causing the latter to condense on the outside of the hygrometer. This condensation, along with the temperature readings inside and outside the apparatus, is compared to a U metal, and the results are read using a standard table.

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