Hygrometer

Hygrometer

Hygrometer is an instrument used for measuring humidity

A hygrometer is a meteorological instrument that is used to measure the humidity of the air. A common way these devices work is by using a material that attracts moisture. This material changes depending on how hydrated it is.

There are different  type of hygrometer;

1. Wet  and dry bulb hygrometer
2. The dew point hygrometer

1. WET AND DRY BULB HYGROMETER

The wet and dry bulb hygrometer consists of two thermometers. The wet bulb thermometer and the dry bulb thermometer.

The bulb of the wet bulb thermometer is wrapped with moist cloth inside a water pot . The dry bulb thermometer indicates the temperature of dry air , while the wet bulb records a lower temperature because evaporation of water from the cloth cools it.

The lower the humidity , the lower the temperature of the wet bulb thermometer relative to the dry bulb thermometer. This is because when there is much water in the atmosphere water from the wet cloth evaporates slowly. The difference in the temperature of the two thermometers will be small. When the humidity is low water evaporates faster in the wet cloth. The wet bulb will then record a low temperature making the difference between the two temperature high.

The difference between the reading of wet and dry bulb thermometer is a measure of relative humidity.

1. DEW POINT HYGROMETER

The figure above shows the dew point  hygrometer. It is also called the Regnault’s dew hygrometer.

The hygrometer is made up of a large test-tube containing ether, through which air is blown by means of either a bulb or a small pump. The outside of the tube is tight fitting with silver thimble and as the ether cools by forced evaporation , dew forms on the silver surface. A thermometer in the ether is used to read the temperature at which this happen and it is the dew point.

This hygrometer is also used to work out the humidity of air using standard tables.

Read more about PHYSICS for High Schools