Hygrometer

Ever wondered how the humidity scores of your area are measured? The answer is Hygrometer, a scientific instrument generally used in meteorological works to measure the amount of moisture present in the air, which eventually helps to understand the weather forecast of a certain area at a certain time. Here, we have covered all the essential information regarding this useful tool. 

1.0What is a Hygrometer? 

A hygrometer is a device used to measure the amount of water vapour present in the air, also known as the humidity of a certain place. There are two primary forms of humidity measurement, measured by a hygrometer, including absolute humidity, the total water vapour present in a given volume of air, and relative humidity, a measure of the amount of water vapour in the air versus the maximum amount of water vapour air can hold at a certain temperature.

2.0Types of Hygrometer

Hygrometers can be divided into different types based on their working principle and the material used to manufacture them. Below are some of the most common types of hygrometers used to measure humidity: 

1. Mechanical Hygrometers: The working of a mechanical hygrometer depends on the materials used to manufacture it and changes in response to the humidity levels of a certain area. These materials expand or shrink as per the changes in moisture in the air. These changes are reflected on the dial or pointer present in a hygrometer. The material used in mechanical hygrometers is: 

a) Hair Hygrometer: This utilises human or animal hair, which lengthens when it gains moisture and shortens when it loses moisture. These variations in length are translated into a needle on a scale, providing a humidity reading.

b) Cylindrical Hygrometer: This type of hygrometer utilises materials such as paper or metal, which respond to moisture variations. The materials expand or shrink depending on the water content of the air, and the movement is indicated on a scale or dial.

2. Digital Hygrometer: With evaluation in technology, a digital hygrometer has become one of the most advanced and widely used devices to measure humidity. It uses electronic sensors to detect the amount of water vapour in the air and then display the same on the digital screen. Digital hygrometers are highly popular due to their high precision and accuracy.

3. Thermometer Hygrometer: A Thermometer Hygrometer uses a combination of two instruments to measure the temperature and humidity. The hygrometer consists of two sensors, one for temperature and another for humidity, and the difference between the readings of both sensors is used for calculating relative humidity. 

4. Wet and Dry Bulb Hygrometer: This is a traditional and widely used hygrometer that is made up of two thermometers, one dry and one wet. The wet-bulb thermometer is fitted with a wet wick on its bulb, while the dry-bulb thermometer is not moist. When the air evaporates, the wet wick cools it and creates a difference between the two readings. This difference is used to determine the relative humidity. 

5. Electrical Hygrometers: Electrical hygrometers use sensors whose electrical properties differ according to the moisture content of the air. The sensors may record alterations in resistance or capacitance as they respond to humidity. For instance, some sensors employ capacitive hygrometers whose moisture in the air alters the dielectric constant of a substance, which in turn varies its capacitance.

6. Dew Point Hygrometers: This type of hygrometer is used to measure the dew point temperature of air, a temperature at which the air becomes saturated with moisture and condensation. A saturated state is the state of air when no more moisture can be mixed in the air. A higher dew point means that the air is more humid, while a lower dew point is the result of much drier air. 

3.0Hygrometer Working Principle

The working principle of a hygrometer can change depending on the type of hygrometer by affecting the physical properties of materials or sensors used within a hygrometer. Hygrometers detect and respond to changes in humidity by using one or more of the following principles:

1. Absorption or Release of Moisture: This principle is generally followed by mechanical hygrometers, which use materials such as human or animal hair, paper, or other fibres that react according to changes in moisture. The principle includes a change in material size or weight due to absorption (increase in size or weight) or release of moisture (decrease in size or weight). 
2. Evaporation: In the wet and dry bulb hygrometer or thermometer hygrometer, the principle of working depends on the evaporation of water. When the wick on the wet-bulb thermometer is wetted, it evaporates water from the air, cooling the thermometer. The evaporation rate depends on humidity, and the wet-bulb minus dry-bulb temperature is used to calculate relative humidity.
3. Electrical Properties: In electronic hygrometers, humidity is sensed by measuring variations in electrical properties. For instance, capacitive hygrometers measure variations in the dielectric constant of a material as the moisture content in the air influences the sensor. Likewise, resistive hygrometers measure the variation in electrical resistance as moisture content influences the conductive nature of materials.

4.0Applications of Hygrometer

Applications of a Hygrometer are many, with some of them being the most prominent uses, which include: 

• Weather Forecasting - In the prediction of rain or other weather patterns  
• Agriculture - to ensure healthy production of crops and livestock
• Industrial uses for ensuring optimal humid conditions in the manufacturing of common goods.
• Preservation of artefacts in museums
• Homes and buildings to regulate humidity for comfort and air quality