1780
Le Roy’s hygrometer
French scientist Jean Baptiste Le Roy, born 1720 in Paris, died 1800 in Paris.
In 1780 the physicist developed a condensation hygrometer which measured the water vapour amount in air by means of cooling. The hair hygrometers known at the time offered the advantage of displaying the humidity directly and immediately, but they were inaccurate.
Le Roy used a tin container filled with water with a thermometer attached to a holder. The temperature of the water in the container was cooled down by gradually adding small pieced of ice, thereby also cooling down the temperature of the immediate surroundings. The ambient air, which was colder now, quickly became saturated and condensed on the outer wall of the tin container. The water temperature measured now corresponds to the dew point temperature which, in turn, is dependent on the relative humidity.
1783
First hygrometer utilizing human hair developed by de Saussure
Swiss naturalist Horace Benedict de Saussure, born 17.02.1740 in Conches, died 22.01.1799 in Geneva.
In 1787 the Swiss naturalist became the second person to climb Mont Blanc, the height of which he was able to work out using a barometer, declaring it the highest mountain in Europe. His extensive meteorological observations led to the development of meteorological instruments, so he developed in 1783 the first hygrometer utilizing human hair to measure humidity.
In the past women’s hair was used changing length when moisture is absorbed. Today, synthetic substances are formed as fine threads, being able to meet today’s requirements for accuracy, long-term stability and robustness. Galltec GmbH has the experience and offers a versatile, fully developed programme of humidity measuring technology.
1792
Psychrometer developed by James Hutton (1726–1797)
James Hutton, born in Edinburgh on 3 June 1726, was a Scottish naturalist and geologist. In the context of his study of the climate, he established the meteorological conditions for precipitation.
According to John Playfair (1805) and John Frederic Daniell (1827), Hutton used a thermometer enclosed in a glass tube to estimate the dryness of the air in around 1792. Once Hutton had measured the air temperature, he wet the thermometer bulb and pointed the tube towards an air current.
It seems that Hutton himself did not publish anything in relation to this. Nevertheless, we can credit him with being one of the first to use the principle of psychrometry to determine humidity.
Psychrometric humidity measurement is based on the premise that a moist surface cools through the evaporation of the water on it. At relative humidities below 100 %, a wet-bulb thermometer indicates a lower temperature (“wet-bulb temperature”) than the air temperature. Using the air temperature and the wet-bulb temperature, it is possible to calculate the vapour pressure of the air.
People must have been aware of the cooling effect of evaporating water since very early times. In the summer months, ancient Greeks used porous clay pots that were unglazed on the inside and outside. The pots held water, which permeated through the pores and kept the outer surface of the pots permanently moist. The principle of psychrometry is still used today.
1803
Chemical hygrometer
In the early 1800s, chemical hygrometers were one method used to determine the water content in the air. We know that such measurements were being performed in 1803 using the type of instrument shown in the image. This is how it works: Suction device A with a capacity of approx. 50 litres is connected at the top to a U-shaped system with tubes 1 to 6. These tubes are filled with pumice soaked in sulphuric acid.
Once the suction device has been filled with water, the lower valve is opened. The liquid drains away and is replaced by air, which fully releases its moisture as it passes through the tubes.
1810
Dew point hygrometer invented by John Frederic Daniell
John Frederic Daniell (1790–1845), an English chemist and physicist, studied humidity measurement and other subjects.
Daniell’s publications included “Meteorological Essays” (1823) and an “Introduction to the Study of Chemical Philosophy” (1830). He is also known for his invention of the “Daniell cell”, a Cu-Zn cell, in 1836.
The measurement principle of the dew point hygrometer is still used today.
It is straightforward to follow: a mirror is cooled until the air being routed over the mirror condenses, at which point the surface temperature of the mirror is measured. An h-x diagram can then be used to extrapolate the relative humidity or other variables. The humidity measurement principles developed by our forebears still apply today, but the processing technology and materials are modern and cutting-edge. At Galltec+Mela, we have translated our extensive technical knowledge of humidity metrology into a joint range of high quality sensors.
18th century
Dew point measuring instrument created by Henri Victor Regnault
Chemist and physicist, born 1810 in Aix-La-Chapelle (Aachen), died 1878 in Paris.
Regnault’s detailed works relate to the compressibility and expansion of liquids and the specific heat of gases.
Air humidity was also considered. Using Daniell’s previous hygrometer as his basis, Regnault developed the apparatus shown in this mini diagram and described it as follows:
„A polished silver vessel contains a glass tube filled to a specific level with ether into which a thermometer is placed. Inserted through the cork stopper sealing the glass tube is an open tube with its end immersed in the ether and another tube with its end above the ether connected to a rubber hose. If air and ether vapour are removed through a suction device connected to the hose, the air entering the tube causes the ether to evaporate rapidly and cools the entire vessel to dew point so that dew is formed on the silver. The dew point and air humidity are derived from the moisture content of the air.”
18th century
Portable hygrometer developed by physicist M. Monnier
As far back as the 18th century, physicist M. Monnier (1823–1906) developed a portable hygrometer.
At that time, women’s hair was used for its hygroscopic properties. Hair lengthens as humidity rises and shortens as humidity drops. The description at the time read as follows: For some time, hair hygrometers have been undergoing modifications to make them portable. The figure provides an illustration of this apparatus, which is credited to Mr Monnier. The hair is secured to a small spindle, which is attached to the enclosure wall. From there, it is fed gradually over three rollers so that it forms a triangle, then over a central roller to which a needle is fitted, and is finally attached to a small, very low-tension spring. Thanks to this arrangement, a hair 35 cm long can be used for an enclosure with a diameter of 10 cm.
The needle moves over a scale around the edge, so that the device looks like a metal barometer. The top half of the circumference shows the hygrometer scale, while the bottom half has the thermometer markings.