Hygrometric Measuring Methods

Author: Hans-Jürgen Schwarz

English Translation by Sandra Leithäuser
back to Air Humidity Measurement

Abstract

The hygrometric measuring method uses the properties of hygroscopic fibres for the determination of humidity in the air. If these fibers are subjected to the ambient air, their length changes as a function of relative humidity. The relative humidity can be detected from theses changes.

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

Hair Measuring Element

The functioning of the measuring element relies on the fact that hair is able to absorb humidity, i.e. in humid conditions the hair swells and expands. As the humidity rises, the hair becomes longer. At humidity fluctuations from 0% to 100%, relative humidity (RH) the relative change in length of a hair is approx. 2.5%. However, the hair only becomes slightly longer at high humidity levels.

Hair measuring elements are used inside indicating instruments for 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 -35 - +50ºC are possible. After a long period of time in low humidity conditions 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.

Plastic Measuring Element

The plastic measuring element uses a synthetic fiber instead of human hair. The fibers are treated to gain hygroscopic properties.

The synthetic 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 the regeneration is not necessary.

The synthetic measuring element is water resistant and insensitive to dry dirt, dust etc. The measuring range is (0) 30% to 100% RH, but depends on the ambient air temperature. The accuracy is at 2-3%.

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

Thermo-Hygrograph

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

The hygrometric measuring method allows measuring the relative humidity in the absence of air pressure and corrosive gases. Measurements in solvent containing and corrosive media should be avoided, because this may cause errors or the destruction of the measuring element.

Advantages:

• Simple and inexpensive
• Easy to clean

Disadvantages:

• High maintenance (regeneration of the hair)
• High imprecision
• Limited area of use 9 (15...85%RH, max. 50º C)
• Slow method
  

relative humidity