On Thu, 6 Sep 2001, Also-Antal Csaba wrote:

>
> How many is the very high impedance? Many opa have 1-10Gohm input
> impedance. Is it enough?
>
  Depends in which range you consider the wood as dry.
Usual is not enough. I think 100Gohm will be more appropiate.
There are some modulation-demodulation operational amplifiers tehniques
which solve this problem.
Your fortune is you don't need to measure exactly the moisture, only to
make reproducible measurements and probably to modify a technological
process. Probable using a smaller impedance meter like usual, will alter
the same all measurements and the error will be a constant.

I forgot to tell you, there are also non-invasive moisture
measuring methods using RF or microwaves, there are two basic principle:
reflection and transmision methods. We built here in 90' the first
romanian microwave moisture meters using a 10 GHz 10mW transmision method.
It's not simple. You need a stabilized emiter and a sensitive receiver.
The atenuation of microwave or RF spot will be proportional with wood
moisture contents but also with wood temperature, wood density, distance
between emiter and receiver etc.
Anyway is better than invasive method with electrodes, and works on
technological flux.

Vasile

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