Updated: Jan 8, 2021
The instruments used in the determination of humidity are known as Hygrometers or Psychrometers. There are various methods to measure the relative humidity:
1. Gravimetric method.
2. Chemical method.
3. Dew point method.
The above methods are not commonly used in testing laboratories or mills, but the following instruments are mainly used.
1. Wet and dry bulb hygrometer.
2. Hair hygrometer or Thermo hygrograph.
3. Electrolytic hygrometer.
Wet and Dry Bulb Hygrometer:
Principle: If the bulb of a thermometer is surrounded by a film of water and the air around it is not saturated, water evaporates from the bulb into the air at a rate which is proportional to the difference between the actual humidity and 100% humidity. Since the evaporation is accompanied by cooling, the temperature indicated by the thermometer will be less than the room temperature.
The wet and dry bulb hygrometer consists of a frame on which two identical thermometers are mounted. The bulb of one of them is covered by a muslin sleeve which is dipped into a reservoir of distilled water. This is known as wet bulb thermometer. The bulb of another thermometer is uncovered and from that thermometer, room temperature or dry bulb temperature can be noted. The wet bulb thermometer shows the temperature less than the dry bulb temperature and is known as wet bulb temperature.
The difference between these two temperatures is noted and the humidity table is referred for the dry bulb temperature in one column and the corresponding difference of the wet bulb indicates the percentage relative humidity.
This is also known as Whirling hygrometer. This is an instrument made up of two thermometers properly mounted on a metal frame attached to a handle. One of these thermometers is known as the dry bulb thermometer and records the dry bulb temperature. The second thermometer has its bulb covered with a wick which can be wet before use. This records the wet bulb temperature.
To make the determination of relative humidity, the clean wick of the wet bulb is moistened thoroughly. The wet bulb extends beyond the dry bulb so the dry bulb will be, and the wet bulb can be wet easily by dipping it in a small bottle of distilled water at room temperature. The instrument swings in a vertical plane by holding on to the handle and allowing the frame to swing through the air. The speed of the whirling should be such as to give a speed of 5±5 feet per second to the wet bulb.
The wet bulb should be read after swinging for about 30 seconds. Swinging should be repeated for approx. 20 seconds and a second reading is taken. This should be repeated until equilibrium of the wet bulb is reached which will be indicated by two successive similar readings. The wet bulb should be read first and promptly and then the dry bulb without contact of the hands with the bulb. Then the difference between the dry bulb and wet bulb temperatures is noted and the humidity table is referred for the perfect RH.