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Hygrometer And Its Importance in Pharmaceutical Manufacturing


Hygrometers are devices that measure the relative humidity of the atmosphere. There are several forms that exist, however the wet-dry bulb thermometer hygrometer is one commonly used variety. As the name suggests, it is composed of two thermometers, one that is kept dry and another that wet.

How it Works

The hydrometer works on the principle of evaporative cooling. The ‘wet’ thermometer is kept moist by distilled water on a wick or sock that covers the bulb of the thermometer. As the water evaporates from the surface, it lowers the temperature of the thermometer, and therefore the ‘wet’ thermometer always shows a lower temperature than the ‘dry’ thermometer. This is the same principle as when we sweat, where the water on our skin takes heat away from our bodies. Distilled water is most often used in hygrometers, as attempts to use alcohol failed due to the fact that alcohol causes too much of a cooling effect to be effective.

The rate of evaporation depends on the humidity of the air, with water evaporating faster in lower humidities. At very high humidity there is very little evaporation from the ‘wet’ thermometer and very little reduction in temperature, When humidity reaches 100%, there is no loss of water from the thermometers and therefore no temperature change. The differences in temperatures shown between the ‘wet’ and ‘dry’ thermometers is the basis for the wet-dry bulb thermometer hygrometer.

Relative Humidity

The relative humidity is measured using the ambient temperature of the atmosphere, as shown by the ‘dry’ thermometer and the difference in temperature as shown by the ‘wet’ thermometer.

The Importance of Monitoring Humidity

Many of the ingredients used in pharmaceutical manufacturing are hygroscopic, meaning they can absorb water, even from the air. A high humidity can therefore affect the characteristics of the product and impact the quality of your product. The manufacture of capsules is one situation where a maintaining a low humidity, ideally below 40%, is important due to the hygroscopic nature of gelatin. Hard gelatin capsules and other hygroscopic capsules must be stored in low humidity storage.