Types of Hygrometers

The humidity in the air is always less than the saturation point (absolute humidity) of the water vapor. Relative humidity is the percentage of actual water vapor in the air, as compared to the saturation point of the humidity in air.

Robert Hooke invented the hygrometer in addition to many other devices, which play an extremely important role in meteorological departments. This device is also known as a psychrometer.

When the water evaporates, the temperature of one of the thermometers is decreased, to have a temperature difference between the two thermometers. The thermometer with a wet bulb is used to measure the absolute humidity of air. The dry bulb of the thermometer measures the current temperature of air.

Both the readings are used to calculate the relative humidity. The wet bulb and the dry bulb temperatures are noted, and the difference between them is called the wet bulb depression. The dry bulb depression is the same as that of the dry bulb temperature. Both these depressions are used to calculate the relative humidity from the relative humidity table.

These thermometer types are used in textile processing industry for measuring the relative humidity. The fumes and condensable oils in these industries often contaminate the wet wick. This leads to frequent replacement of the absorbent cloth used for the wet bulb thermometer.

It was invented by Benedict Saussure, and it's also known as the mechanical hygrometer. Its working is a bit complex than the wet and dry hygrometers. It is made from human hair available in abundance. The human hair is made up of flattened layers of the cuticle cells, which are further are made up of lipids; hence it can repel water.

The hair is cleaned with alcohol and water to make this measuring instrument. It is stretched by applying slight tension, and is fitted in a needle gauge. The device is marked from 1% - 100% along the length of the hair, which expands and contracts as the humidity changes.

It is used to calculate the relative humidity of the air, when an air sample passes through it. This device measures either the capacitance or the resistance. As the humidity increases, the static charge that is stored in the capacitor rises. This is in the case of the capacitive electronic hygrometer.

The resistive hygrometer uses a ceramic material to measure the humidity. As it increases, the ceramic material absorbs the water from the air, and this results in a change in the resistance of the ceramic sensor. The amount of current flowing through this sensor gives the accurate measurement of relative humidity.

The hygrometer must be calibrated from time to time, and there are different ways for this process. Nowadays, digital varieties of this instrument are available in the market. They have the facility of measuring the temperature along with the relative humidity.

It is widely used for greenhouse monitoring and inspection. Many industries use this device to monitor the relative humidity of air. It is very useful in weather forecasting for calculating the relative humidity of air.