Hygrometric Measuring Methods

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Author: Hans-Jürgen Schwarz

English Translation by Sandra Leithäuser
back to Air Humidity Measurement


The hygrometric measuring method uses hygroscopic fibers to determine the humidity present in the air as these fibers expand and contract as a function of relative humidity. These changes allow determining the relative humidity of the air.

The fibers used for hygrometric measuring elements are usually human hair or specially prepared polymer fibers.

Note: These sensors are not recommended by the European standard EN16242:2012. This standard recommends instruments based on measuring the dew-point, electronic-fan psychrometers and thin film capacitive or resistive hygrometers [Camuffo:2014]Author: Camuffo, D.
Note: Camuffo, D. 2014. Microclimate for Cultural Heritage- Conservation, Restoration, and Maintenance of Indoor and Outdoor Monuments. 2nd ed. Waltham, Mass. & San Diego, Calif.: Elsevier. 528p. ISBN-13: 9780444632968 Abstract: Presents a theoretical, and practical handbook concerning microclimate, environmental factors, and conservation of cultural heritage. This book includes the information on experimental research on environmental factors and their impact on materials, such as the behavior of water and its interactions with cultural heritage materials.
Publisher: 2nd ed., Elsevier LTD, Oxford
Title: Microclimate for Cultural Heritage- Conservation, Restoration, and Maintenance of Indoor and Outdoor Monuments
Year: 2014
Link to Google Scholar

Hair Measuring Element

The measuring element relies on the ability of hair to absorb humidity, i.e., in humid conditions the hair swells and expands. As the humidity rises, the hair becomes longer. For humidity fluctuations between 0% and 100% relative humidity (RH) the relative change in length of a hair is about 2.5%; only at high humidity levels, such as above 50% RH, does hair expand.

Hair measuring elements are used in instruments for use in interior climate control. The hair’s change in length is translated to a needle gauge or pen by a precise mechanism. In order to provide mechanical stability, the hair is bundled or arranged into a hair harp.

The measuring method ensures an accuracy of 3% in a measuring range of 0-90% RH. Ambient temperature ranging from -35º- +50ºC are possible. After a long period of time in low humidity conditions, i.e., below 40% RH, the hair element has to be regenerated. For this purpose, the hair hygrometer is placed in nearly moisture-saturated air for 60 minutes. The gauge settings may be adjusted afterwards, using a setscrew.

Hair hygrometers are sensitive to hygroscopic dust and are to be protected, as well as cleaned periodically.

Polymeric-fiber Measuring Element

The polymeric-fiber measuring element uses a man-made fiber instead of human hair. The fibers are specially treated so as to have hygroscopic properties.

The polymeric-fiber measuring elements can be used at higher temperatures (up to 110°C) than the hair measuring elements. It is also possible to use them at low relative humidity levels, for a longer period of time, and regeneration is not necessary. Also, they are water resistant and insensitive to dry dirt, dust etc. The measuring range is usually between 30% to 100% RH (in some cases, it might range from near 0% RH), but this depends on the ambient air temperature. Their accuracy is about 2-3%.

On an industrial scale, hygrometric sensors with polymeric-fiber elements are used for the continuous measuring of climatic conditions, because they are less sensitive and have a higher temperature tolerance.


The thermo-hygrograph is a chart recorder that detects the relative humidity and the temperature using hair or synthetic measuring elements.

A thermo-hygrometer allows measuring the relative humidity in the absence of air pressure and corrosive gases. Measurements in solvent and corrosive media containing environments should be avoided, because this may cause errors and/or the destruction of the measuring element.


  • Simple and inexpensive
  • Easy to clean


  • High maintenance (regeneration of the hair)
  • Low precision
  • Limited use range (15...85%RH, max. 50º C)
  • Slow method


[Camuffo:2014]%editor% Camuffo, D. (2014): Microclimate for Cultural Heritage- Conservation, Restoration, and Maintenance of Indoor and Outdoor Monuments, %edition%, 2nd ed., Elsevier LTD, Oxford, %address%, %pages%, %url%, %doi%Link to Google Scholar