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Humidity Sensing

June 13, 2016

In elementary school, one of the first experiments I performed with my home chemistry set was creation of a humidity indicator. Students of my generation were able to experiment with a range of chemical compounds that are not sold to today's children for safety reasons, and one such compound was the cobalt chloride that enabled this humidity sensor. Cobalt chloride is a suspected carcinogen. While it seems as if everything tested is found to cause cancer, this chemical is also a common allergen, so I agree that it's best to keep this chemical away from children.

A 1940s era Gilbert chemistry setA 1940s era Gilbert chemistry set.

Cobalt chloride was likely included in this set, as well as other experimental staples as sodium ferrocyanide and phenolphthalein.

(Wikimedia Commons image by Jmabel.)

My chemical humidity sensor was just a blot of cobalt chloride solution on paper. Its principle was the difference in color between the hydrated form of cobalt chloride, cobalt chloride hexahydrate, CoCl2·6H2O, and the anhydrous form without the waters of hydration. The hexahydrate has a deep purple color, while the anhydrous form is sky blue. The water in the air was in equilibrium with the water in the hydrate, so the chemical blot was blue when the air was dry, and generally pink when the air was wet. As I remember, I wasn't really impressed by the shallow change in color that I saw.

Humidity indicators using cobalt chloride were made commercially from the late 1940s onwards, principally as a quality control measure for military supplies. One such humidity indicator was patented in 1952.[1] In that indicator, a porous silica gel was impregnated with about 2-5% anhydrous cobalt chloride by dry weight. This gel was then dried for placement in shipping containers. Less hazardous humidity indicators are based on copper chloride, CuCl2, which is yellow in its anhydrous state, and blue in its di-hydrate form (CuCl2·2H2O).

Anhydrous and hydrated Cobalt(II) chlorideAnhydrous (left) and hydrated (right) cobalt(II) chloride.

(Left image, and right image, by Wilco Oelen, via Wikimedia Commons.)

There are quite a few other techniques, aside from the color change of a chemical, to make a hygrometer; that is, a humidity gauge. The simplest sort was invented by Leonardo da Vinci in 1480. His hygrometer was a pan balance with a wad of cotton on one pan, and a counterweight on the other. As the cotton absorbs water from the atmosphere, its weight increases, so the humidity can be measured by a change in weight.

Another mechanical method uses the change in length of fibers, such as horsehair and human hair, as they absorb water from the air. Since the change in length is small, a better technique is to observe the twist angle of a twisted fiber such as wool thread. In an action similar to that of a bimetallic thermometer, a salt-impregnated paper glued to a coil of metal or another elastic material that doesn't absorb water will enable a low cost and reasonably accurate gauge device for humidity.

Fiber hygrometer (1688)Rapunzel might have donated a strand of her hair to make this hygrometer, but it's more likely that cotton twine was used.

The mechanism is easy to understand.

(A 1688 copper engraving on paper by Joachim d' Alencé, Accession number df_tg_0004700 of the Deutsche Fotothek, via Wikimedia Commons.)

Modern electronics has facilitated many precise methods of humidity measurement, such as chilled-mirror dew point hygrometers. As everyone who has needed to defog an automobile windshield on a cold summer's morning has noted, lowering the temperature will condense moisture from the air. If we cool a surface, the temperature at which water starts to condense is called the dew point. As shown in the following graph, the dew point will give the relative humidity at a particular air temperature.

Relative humidity and dew point relationshipRelative humidity and dew point relationship.

The curves are calculated using the Magnus-Tetens approximation.

(Modified Wikimedia Commons image by Easchiff.)

Electronic chilled mirror dew point hygrometers function by monitoring the reflection of light from a mirror as it's chilled. These hygrometers are easy to build using available electronic components, as shown in the figure. Unfortunately, pollutants in the air will build up on the mirror after repeated cycling through the dew point, so this type of hygrometer requires continued maintenance.

A chilled-mirror dew point hygrometerA chilled-mirror dew point hygrometer.

The components shown are quite inexpensive, although the thermoelectric cooler requires a fanned heat sink to extract heat from the Peltier hot junctions, and some microcontroller circuitry is needed for temperature control, data acquisition, and data analysis.

(Created using Inkscape.)

Some material properties are sensitive to humidity. For example, the dielectric constant of some polymers and metal oxides will change with humidity, enabling capacitive hygrometers. The electrical conductivity of some salts and conductive polymers changes with humidity, but it changes also with temperature. The thermal conductivity of air increases with humidity; but, as can be imagined, hygrometers based on thermal conductivity are difficult to build.

While we congratulate ourselves on the various ways we've devised to measure humidity, the lowly fruit fly (Drosophila melanogaster) also measures humidity. A team of scientists from Northwestern University (Evanston, Illinois), Lund University in Sweden, and the New York University School of Medicine, have found that fruit flies, which prefer the distinct humidity range of their native habitat, sense relative humidity through neurons in an small organ structure, a sac in their antennae known as the sacculus. Relative humidity and temperature are processed by different cells in the Drosophila antenna.[2-3]

The research team investigated the genes and neurons necessary for hygrosensation in the common fruit fly, Drosophila melanogaster. It's quite logical, from an evolutionary standpoint, that insects should possess a "sixth sense" for detecting water vapor in the air, since such a sense would direct them to the most favorable environment.[2] Says Marcus C. Stensmyr, an associate professor at Lund University and a co-author of the study,
"That insects are able to detect humidity levels has been known since the beginning of the 20th century, but how they do it has remained enigmatic... Our study reveals for the first time the genes and neurons that underlie this ability, which is very exciting."[3]

It appears that fruit flies sense humidity by the same principle as hygrometers utilizing tension on strands of hair. The mechanical deformation of the sacculus is likely the means of fruit fly humidity sensing.[3] This study may help in the design of strategies for mosquito population control, such as a means to prevent the insects from finding water in which to lay their eggs.[3]

Says Marco Gallio, an assistant professor of neurobiology at Northwestern University and a co-author of the study, "Our discovery is very important for sensory biology and offers a possible tool for fighting mosquitoes and the disease they can carry."[3] This work received funding from the National Institutes of Health.[3]

References:

  1. Paul Bell Davis, "Cobalt chloride humidity indicator," US Patent No. 2,580,737, January 1, 1952 (via Google Patents).
  2. Anders Enjin, Emanuela E. Zaharieva, Dominic D. Frank, Suzan Mansourian, Greg S.B. Suh, Marco Gallio, and Marcus C. Stensmyr, "Humidity Sensing in Drosophila," Cell (In Press, May 5, 2016), DOI: http://dx.doi.org/10.1016/j.cub.2016.03.049.
  3. Megan Fellman, "Scientists Are First to Discover Sensory System That Detects Air Humidity," Northwestern University Press Release, May 5, 2016.

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