basic principles
Non Contact methods are of two types:
Infrared Non-contact methods
There is a method, used in industrial process, based on the absorption properties of O-H bonds in the near infrared wavelengths. This is a kind of infrared spectro-photometer, used in all kinfd of processes (food industry, tobacco, forestry products, chemical industry,minerals, waste....).
It is usually located above transport bands, and enables you to measure humidity in the surface layer of the product. It is of course non destructive, but it's a bit expensive.
(Contributed by Raphaël Peno-Mazzarino, Agronomist engineer, Agronomy and Environment dept. 21/4/2001)
Microwave Non-contact methods
Using electromagnetic waves, moisture determination can be carried out in various ways, depending on the application: by using sensors in the radio frequency range (RF), in the microwave frequency range or by applying time domain methods.
This is discussed in detail in a new book published by Wiley in January 2000
(added May 2001) See also www.sensorsupdate.com for more information on this book series, that became an electronic journal (Online ISSN: 1616-8984 and Print ISSN: 1432-2404).
advantages
The ability to measure moving product such as on a conveyor belt and so suitable for process control applications. .
Undisturbed sampling over thin films of medium that would otheriwse be very hard to measure without disturbance.
Wide humidty measuring range 0.1 to 95%
Can also measure other variables such as coating weight, film thickness, impregnations and a variety of other web processing applications ( - as in film, not www :)
Suitable for use on thin timber, cardboard, foodstuffs, grain, soaps, tobacco etc etc
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