Hair hygrometer
A hair hygrometer is a measuring device for measuring the relative humidity . The physicist Horace-Bénédict de Saussure was the first to use the property of human hair to expand when exposed to humidity to invent a measuring device. In 5 years of work (until 1783) he developed a reproducible and comparable measuring device, which was used to measure the humidity in the network of the “ Societas Meteorologica Palatina ”.
Layout and function
A bundle of mostly human hair is used as the measuring element , the extent of which is implemented using a lever mechanism and displayed on a scale. Hair expands when it absorbs moisture, and contracts when it loses moisture. Blonde women's hair is particularly suitable for use in hygrometers , as it is particularly fine. When the humidity changes by 10%, these react with a change in length of 0.25% on average. Because the change in length of hair is almost independent of temperature and can also be recorded with simple means, they are well suited for measuring humidity. The length expansion and thus the scale division is not linear and decreases with increasing humidity. This allows real hair hygrometers to be easily distinguished from synthetic fiber hygrometers (linear scale division).
properties
When used indoors, where the air humidity is constantly and relatively low, hair hygrometers must be regenerated every 2 to 3 weeks, otherwise they will soon show values that are too high. To do this, the measuring element is moistened with distilled water or the entire device is wrapped in a damp cloth for about an hour. Then a humidity of 95% should be displayed. If this value is not reached or exceeded, the pointer must be corrected using an adjustment screw attached to the device. After regeneration, the hair hygrometer shows correctly in all moisture ranges. Hair hygrometers are therefore well suited for measurements outdoors, because high humidity values are often reached here at night.
The measuring accuracy of good hair hygrometers is around +/- 3% in the medium and high humidity range, and decreases to 5% in the low humidity range. If the device is used for a long time in low humidity areas, the measurement error increases significantly, which makes regeneration necessary. In addition, the hair must be shielded from direct sunlight, as it is sensitive to radiation.
literature
Helmut Kraus: The atmosphere of the earth - An introduction to meteorology, 3rd expanded and updated edition, 2004, Springer-Verlag Berlin Heidelberg New York, ISBN 3-540-20656-6