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Hygrometer

You may sweat a lot during some days, and some may say humidity is very high. Have you ever wondered how humidity is calculated?

A hygrometer is a specially designed apparatus that calculates relative humidity in an open or enclosed area. The term hygrometer is formulated from the Greek word ‘Hygros’, which means moist or wet.

Hygrometer

Humidity can also be explained as air humidity or relative humidity, which is stated as the quantity of water vapour in the atmosphere. The population of water molecules in the atmosphere relies on multiple factors, including the air’s temperature. Warm air usually is much more humid than cold air. When the atmosphere temperature drops, the highest amount of water the air can sustain decreases.

The video below explains the concept of temperature in detail.

Hygrometer plays an important role in manufacturing industries, hospitals, incubators, agricultural production and storage, humidors, museums, and every other instance where humidity is an integral part.

Invention of Hygrometer

In 1480, a primitive hygrometer was constructed by Leonardo da Vinci. During the 1600s, Francesco Folli made significant strides and developed a practical apparatus model. Robert Hooke had also contributed to significant improvements in the technology behind the hygrometer. Swiss polymath Johann Heinrich Lambert developed another well-rounded version in 1755.

In 1783, Horace Bénédict de Saussure (Swiss physicist and geologist) invented the world’s first hygrometer, using human hair to calculate humidity.

Hygrometer Working Principle – Dry and Wet Bulb Hygrometer

Hygrometers function by the phenomenon known as evaporative cooling. When water evaporates from a surface, it becomes cold as water molecules absorb heat from the surface during the process of evaporation. Due to this effect, the wet-bulb consistently exhibits lower temperature than the dry bulb. The same principle goes for the instant cooling effect while bathing.

A combination of dry air and low humidity can result in high evaporation, which decreases wet-bulb temperature. When the atmosphere is quite humid with very high humidity, the temperature of the wet-bulb decreases less, as evaporation from the wet-bulb stands low.

In the case of 100 percent humidity, the dry-bulb and wet-bulb will show the same temperature. This is due to negligible water evaporation from the wet-bulb; therefore, no cooling effect emerges.

Wet and dry hygrometer

The video below explains the concept of condensation in detail.

Hygrometer Types

During ancient times, there were some primitive hygrometer prototypes. The most famous one was constructed in the era of the Shang Dynasty to examine weather conditions.

Over the course of time, many versions were developed using varied techniques.

There are many types of hygrometers used to calculate humidity.

Mechanical Hygrometer:

Organic substances (thin materials such as human hair, ox gut, etc.) expand and contract according to the variation in humidity. As variations occur in the material used, the spring of the mechanical hygrometer moves the needle on the dial. Thus, relative humidity can be calculated.

Mechanical Hygrometer

These analogue devices have provided accurate readings for centuries. Since these are analogue instruments, it is crucial to calibrate them periodically to maintain the desired accuracy.

Electrical Hygrometer

An electrical hygrometer measures humidity by analysing the variation in electrical resistance of a fine layer of a semiconductor device. On the other hand, other hygrometers detect variations in transparency, weight or volume of substances that respond to humidity.


Predominantly there are two types of electrical hygrometers- Resistive Hygrometer and Capacitive Hygrometer.


The calculation of an electrical hygrometer is rooted in changes in capacitance or resistance. The device usually possesses a sensor to measure variations in an on-chip layer of semiconductor (eg-lithium chloride).

Resistive Hygrometer
Capacitive Hygrometer

Dew-point Hygrometer

Dew-point Hygrometer usually consists of a polished mirror (made of metal) that is cooled at a fixed pressure and vapour content till moisture just begins to settle on it. Dew point is the metal’s temperature at which condensation starts.

Dew-point Hygrometer

Psychrometer

The psychrometer is a hygrometer consisting of two thermometers, wet-bulb and dry-bulb. They are used to determine humidity through evaporation. The wet-bulb thermometer is covered with a wet cloth at its enlarged side. By thoroughly rotating both thermometers or gusting air over the bulbs, there will be a change in temperature. Humidity in the air can be measured by analysing the temperature difference between the wet-bulb and the dry-bulb.

Psychrometer Hygrometer

Hygrometer Uses

A hygrometer helps to calculate moisture content in the air or enclosed areas. This is essential in manufacturing industries, hospitals, museums, agricultural fields, food preservation, meteorology, etc.

Hygrometers are frequently used in pharmaceutical storage and manufacturing systems. Most pharmaceutical products are hygroscopic; therefore, variation in humidity could alter their chemical compositions.

Electronic devices like ACs make use of electrical hygrometers. Humidity sensors (smart hygrometers) are vital parts of devices that function on temperature, pressure and humidity.

Frequently Asked Questions – FAQs

What is a hygrometer?

A hygrometer is a specially designed apparatus that calculates relative humidity in an open or enclosed area.

What is another word for hygrometer?

Moisture sensor, humidity probe, psychrometer and humidity metre.

Which device is used to measure humidity?

A hygrometer is used to measure humidity.

Who invented the hygrometer?

In 1480, a primitive hygrometer was constructed by Leonardo da Vinci. During the 1600s, Francesco Folli made significant strides and developed a practical apparatus model. Robert Hooke had also contributed to significant improvements in the technology behind the hygrometer. Swiss polymath Johann Heinrich Lambert developed another well-rounded version in 1755.
In 1783, Horace Bénédict de Saussure (Swiss physicist and geologist) invented the world’s first hygrometer, using human hair to calculate humidity.

Where are hygrometers used?

Hygrometers are used in manufacturing industries, hospitals, museums, agricultural fields, food preservation, meteorology, etc.

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