Water content in paper

Purpose and goal

The purpose with the project was to use recent technological advances in semiconductor industry, which allow us to build active circuits at frequencies as high as 400 GHz (mm-waves). While the semiconductor technologies are mature enough, the areas of applications of such circuits are still not developed and limited to space sciences, and security applications. This projects opens a new area of measurement applications and proves the potential of mm-waves to remotely measure small changes in moisture content in thin layers.

Results and expected effects

A new kind of sensor, based on a measurement of absorption of a 200 GHz signal transmitted through the layer under test, was developed. This measurement approach has certain advantages in comparison to other sensor technologies currently in use, as for example microwave and infra-red. The sensor was used to measure water content in both offset-print and in paper-production industries, it demonstrated a resolution of 0.16% for average moisture content of 5 % and 0.06% for moisture content of 40 %.

Approach and implementation

The water content is calculated from a measurement of the absorption of a signal transmitted through the paper at mm-wavelengths where water presents substantial attenuation. A compact optical system focuses the signal on the surface of the paper and, after transmission through the paper, to a receiver. The method is insensitive to vibrations in the paper and the structure of the paper. The sensor is convenient to cross scan the paper and map the water content with a spatial resolution of about 4 cm2. Further development is needed to shape the sensor as an attractive commercial product.