Author: Hans-Jürgen Schwarz
back to Air Humidity Measurement
Water has distinct absorption bands in the infrared light spectrum. The absolute humidity of gases can be detected without delay, by measuring the optical absorption. Due to the infrared gas analyzers, this process is relatively expensive. However, at moderate humidity levels it quickly delivers results at a high reproducibility.
An object is irradiated by an infrared light source, through a filter. The filter produces a light signal at wavelengths of 1.93 µm and 1.7 µm. Water has the property to absorb light at wavelengths of 1.93 μm. The light reflected by the sample is lead to a photo-resistor, which converts it into an intensity-dependent signal. The light intensity depends on the moisture content of the sample. In order to be independent of the reflectivity of the solid material, another signal is processed at a wavelength of 1.7 µm, and then the ratio of both signals is formed.