Hygrometers are essential device for pharmaceuticals because moisture in environment causes great effect on pharmaceutical products during manufacturing. Relative humidity of atmosphere is measured by the hygrometer in percentage. A hygrometer is weather instrument used to measure the amount of humidity in the atmosphere. There are two main types of hygrometers – a dry and wet bulb psychrometer and a mechanical hygrometer. According to their names dry bulb remains dry in the air and wet bulb is surrounded by a cotton wick that is dipped in the water. Wet bulb plays main role in the measurement of the humidity.

How do Hygrometers Work?

Hygrometer works on the phenomenon called evaporative cooling. When water evaporates from any surface, it becomes cool because water molecules take heat energy from the surface during the evaporation. Due to this cooling effect wet bulb always shows low temperature than the dry bulb. You can understand this with the conditions when you are felling very hot and you take bath with water. Water evaporates from our body taking some heat and you feel cool for some time.

Evaporation of water from the surface of wet bulb is inversely proportional to the humidity in the atmosphere. In dry atmosphere when humidity is low, the evaporation of water will high resulting the more decrease in wet bulb temperature. But when atmosphere is humid having higher humidity, the evaporation of water from wet bulb remains low resulting the less decrease in the wet bulb temperature. At 100 percent humidity the temperature of dry and wet bulb will be same due to no water evaporation from the wet bulb hence no cooling effect occurs.