1 ¸ c worth:  

Dual band Infra Red transducers would also work and can be calibrated very accurately.
The absorption and reflection of Infra Red by a substance is greatly affected by the 
substances water content. Using 2 bands of Infra Red improves the effect.

MikeW


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The normal method of measuring effective humidity in wood, concrete, sand,
etc., is by conductivity.
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