Relative Humidity Calculation: How does a Hygrometer Work

The relative humidity in the air is measured using the hygrometer. The humidity in the air is always less than the saturation point (absolute humidity) of the water vapor. The 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 that proved to be important in meteorological department. The hygrometer is also known as phychrometer.

Types of Hygrometer

Wet and Dry Hygrometer
The wet and dry hygrometer consists of two thermometers. The bulb of one of the thermometers is covered with an absorbent cloth and it is dipped in water. When the water evaporates, the temperature of one of the thermometers is decreased so as 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 of air. The wet bulb temperature and the dry bulb temperature is 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. The wet and dry bulb depression is used to calculate the relative humidity from the relative humidity table.

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

Hair Hygrometer
The hair hygrometer was invented by Benedict Saussure and is also known as the mechanical hygrometer. The working of the mechanical hygrometer is a bit complex than the wet and dry hygrometer. The hair hygrometer is made from human hair available in abundance. The human hair is made up of flattened layers of the cuticle cells. The cuticle cells are made up of lipids that are a type of oils. The hair repels water due to these lipids. The hair is cleaned with alcohol and water to make the hair hygrometer. The hair is stretched by applying slight tension and fitted in a needle gauge. The hygrometer is marked form 1 to 100 percent along the length of the hair. The hair expands and contracts as the humidity in the air changes.

Electronic Hygrometer
The electronic hygrometer is used to calculate the relative humidity of the air. A sample of air is passed through the electronic hygrometer. The electronic hygrometer measures either the capacitance or the resistance. As the humidity of the air increases, the static charge 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 of the air. As the humidity of the air 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 ceramic sensor gives the accurate measurement of relative humidity.

The hygrometer must be calibrated from time to time. There are different ways to calibrate the hygrometer. Nowadays, digital hygrometers are available. They have the facility off measuring the temperature along with the relative humidity. The hygrometer is widely used for greenhouse monitoring and inspection. Many industries use a hygrometer to monitor the relative humidity of air. The hygrometer is a very useful device in weather forecasting for calculating the relative humidity of air.