Hygroscope – Working, Types and Application

A hygroscope, also known as a hygrometer, is a vital instrument widely used in meteorological science. One of the essential uses of the hygroscope is measuring the amount of water vapour in the air. However, it has many other commercial applications.

Humidity is the water vapour content in the atmosphere. It can be either air humidity or relative humidity. The water molecules’ content in the atmosphere relies on the air’s temperature, mass, and pressure. For instance, cold air is less humid compared to warm air. When the air temperature drops, the water content of the air decreases. 

What is a Hygroscope?

A hygroscope is a device that measures relative humidity in the air, soil or any confined space. It is a weather instrument that measures water vapour content. The instrument has advanced over time, taking various forms to suit varied needs. Some of the primary forms of hygroscope, along with their inventors, are as follows:

• In 1480, Leonardo da Vinci constructed a primitive hygroscope.
• Francesco Folli made necessary modifications and prepared a practical model during the 1600s.
• Robert Hooke also contributed substantial developments in hygroscope technology.
• In 1755, a Swiss Polymath named Johann Heinrich Lambert designed another considerable version.
• In 1783, Swiss geologist and physicist Horace Benedict de Saussure invented the world’s first hygroscope utilising human hair for calculating humidity.

Hygroscope Principle

Most of the hygroscopes mentioned above are mechanical hygroscopes. It is necessary to understand its working principle to know what a hygroscope is. The hygroscopes work on various principles depending upon their type. Their primary purpose is to measure humidity in the air or a substance to determine the maximum moisture the air or a substance can retain.

Working of Hygroscope

Hygroscope works on the phenomenon of evaporative cooling. During evaporation, water molecules tend to absorb surface heat and become cold. This effect causes the wet bulb to exhibit low temperature compared to the dry bulb consistently. A similar principle works for the immediate cooling effect when bathing.

Low humidity and dry air cause high evaporation, decreasing the temperature of the wet bulb. Conversely, when the environment gets humid, the wet-bulb temperature drops slowly because of the low evaporation stand from the wet bulb.

When there is 100 per cent humidity, the temperature of the dry and wet bulb remains the same. It results from meagre water evaporation from the wet bulb; hence the cooling effect does not occur.

Types of Hygroscope

People developed some primitive hygroscope prototypes earlier. However, the most popular one was during the Shang Dynasty for examining weather conditions. 

There are numerous Hygroscope types used for calculating humidity. The most prominent are: 

• Mechanical Hygroscope.

• Electrical Hygroscope.

• Dew-Point Hygroscope.

• Psychrometer.

• Mechanical Hygroscope

Variation in humidity causes various organic thin materials like the ox-gut hose, human hair, etc., to contract and expand. These variations cause the mechanical hygroscope’s spring to move the dial’s needle. Thus, we can calculate relative humidity through the materials’ expansion and contraction.

The mechanical hygroscope gave precise readings for a long time. However, they are analogue instruments, and hence they require periodic calibration to retain the needed accuracy.

• Electrical Hygroscope

The electrical hygroscope measures humidity from variation in the semiconductor device’s fine layer’s electrical resistance. Other hygroscopes measure moisture content from variations in weight, transparency or substances’ volume affected by humidity.

The two kinds of electrical Hygroscopes are: 

• Resistive Hygroscope.
• Capacitive Hygroscope.

The electrical hygroscope’s calculations depend on the resistance or capacitance changes. A sensor is used to assess variations in the on-chip of the semiconductor layer. 

• Dew-point Hygroscope

The dew-point hygroscope comprises a polished metal mirror, cooled at a fixed vapour content and pressure until the moisture begins to settle. The temperature at which the condensation starts in the metal is its dew point.

• Psychrometer

A dry and wet bulb hygroscope is known as Psychrometer. This type determines humidity from evaporation. The wet-bulb thermometer’s enlarged size is covered with a wet cloth. A temperature change occurs when both the thermometers are rotated thoroughly or the air is gusted over the bulbs. Thus, the air’s humidity is measured by the temperature difference between the dry and wet bulbs.

Uses of Hygroscope

The most important uses of hygroscopes are to measure air moisture in an enclosed area or substance. Therefore, it is a prominent instrument that finds application in manufacturing industries, museums, hospitals, agricultural fields, meteorology, food preservation, etc.

One of the most frequent uses of hygroscope is in manufacturing systems and pharmaceutical storage. Many pharmaceutical-based products are hygroscopic in nature; humidity fluctuations can modify their chemical compositions. Hence, a hygroscope is used to check a confined area’s humidity.

Today electronic devices such as ACs use electrical hygroscopes. Smart gyroscopes tend to function on temperature, humidity, and pressure and are vital aspects of the device.

Conclusion

Hygroscopes are measuring devices that measure humidity in various ways. The principle of their operation depends on the hygroscope type. The uses of hygroscopes are many; however, their most important benefit is in the pharmaceutical industry. Today, many home devices embed a hygroscope as an essential part. The instrument evolved into versions based on needs. However, the most prominent invention was the dry and the wet bulb thermometer. Hygroscopes are easy to use but should be calibrated every year.