Bacharach type sling hygrometer for indication of Relative Humidity percentage according to the wet-bulb-dry-bulb thermometer principle. The plastic housing of the instrument is equipped with a conversion scale that calculates the Relative Humidity percentage.
Relative Humidity can be measured by an instrument called a hygrometer. A simple hygrometer - a sling dry hygrometer - consists of two thermometers mounted together with a handle attached to a chain. A thermometer is ordinary. The other has a cloth core on the bulb and is known as a wet-bulb thermometer.
When it comes to reading, first immerse the wick in water and then rotate the instrument. During rotation, water evaporates from the wick, cooling the wet-bulb thermometer. The temperature of the two thermometers is then read.
If the surrounding air is dry, more moisture evaporates from the wick, cooling the wet-bulb thermometer, making the temperature difference between the two thermometers even greater. If the surrounding air retains as much moisture as possible – if the Relative Humidity is 100% – there is no difference between the two temperatures. Meteorologists have developed graphs of these differences for each degree of temperature so that observers can easily find the Relative Humidity.
