Hi All!

I am looking into making an Ohm meter which will measure the high resistance
of wood. This is to make a wood moisure meter. The resistence of wood goes
down as the moisure goes down. This meter would have to measure resistances
in the range of 1 MOhm to about 50 MOhms. If it is possible to read
higher/lower values that would also be great.

I have done some googling and other searching, but can't find any cicuits f=
or
measuring high resistance. What I have found is a paper which describes the
conductivity of wood at different moisture levels, and the different
techniuques of measuring the resistence (As in invasive, non invasive, but =
no
info on actual circuits) I will use the invasive method wich is done by
having two metal "needles" about an inch apart rammed into the wood. This
seems to be the most commin method of doing it if you look at already
available equipment. Is it possible to do it using a simple potential divid=
er
as shown below?


------o 5V
|
>
<  Wood of unknown resistance
>
|
|-----> to PIC A/D
|
>
<  Known resistance
>
|
|-----o gnd


Now, the known resistence would have to be high I guess. 100 MOhms or so? Or
would it be better to use a smaller known resistance, and use an OpAmp to
scale the signal suitably for the PIC? Can this be done this simply at all??


I know I could go out and buy one of these, but this is not a commercial
project, just something I wanted to see if I could pull off (and something I
need because I don't know if my firewood is dry enough to burn this
winter...).

Any thoughts, links, circuits, rants and flames appreciated.

Thank you,
=D8yvind Tjervaag.

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu