> 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.
/>

Quick thought.

- Consider using a constant current source and measuring the voltage drop
across the sample with a VERY high impedance buffer. The 3 amplifier
instrumentation amplifier discussed here recently would be a good starting
point.

- Many multimemters have a 200 or 400 megohm range. Looking at how they do
it may be useful.

- It's usual to use higher than somewhat voltages for such high resistances
unless there is a reason not to.
I suspect that DC may change the wood's resistance due to moisture, and high
voltage more so.

- A bridge circuit would allow much more usable values in the actual
measurement input.
At 50 MOhm and 10v you have 0.2 uA flowing. Many many opportunities for
errors at such low currents.
An AC bridge may be needed.

- 4 wire "kelvin sensing" measurement may be needed at such high
resistances.

- Depending on other properties of the material, other methods such as thos
which rely on the dielectric properties (eg capacitive measurement)  may be
more useful.

- Banging in pins may require standardisation of pin size and depth and
material for consistent results.

For starters, why not try an existing high ohm range ohm meter and see
where/if  it is inadequate.

Also:

    www.keithley.com/servlet/Data?id=4645
    www.keithley.com/servlet/Data?id=6584
    http://builder.itpapers.com/abstract.aspx?&scid=106&x=40&docid=637



        Russell McMahon

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