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'[EE:] Measuring high resistence (of wood)'
2004\08\23@045554 by

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 for
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 divider
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,
Øyvind Tjervaag.

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Oyvind Tjervaag wrote:

>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.
>

Uh, don't you mean resistance lowers when moisture increases?
David...

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Whoops... Yes, of course. The type of wood i will be intitially testing is
about 1MOhm at 25% "humidity" (If that is the correct way of saying it...),
and increases to 40 - 50 MOhms at 7 % moisture.
Thanks for the correction..

Øyvind Tjervaag

{Original Message removed}
> 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

Also:

http://www.keithley.com/servlet/Data?id=4645
http://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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>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,
>Xyvind Tjervaag.

If you get enough flames, maybe you won't need the firewood?

I have been involved in the design of a device designed to measure the
conductivity of wood, in this case to find decay pockets in wooden utility
poles (see Shigometer). This design applied a 5V AC square wave (no net DC
component) to the wood under test, and measured the current flow across a
series resistor. An opamp was used to amplify the rather tiny voltage before
converting it.

Bob Ammerman
RAm Systems

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Oyvind Tjervaag wrote:
> 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.

No, it's the other way around.

{Quote hidden}

Not as shown.  The PIC A/D has too low an input impedence.  I would use a
higher voltage going into the wood with around a 2Mohm current sense
resistor to ground.  The voltage from the current sense resistor would be
buffered and amplified by an opamp with CMOS inputs, then go into a PIC A/D.
Since this is a high impedence circuit, special attention needs to be given
to leakages, guard traces, etc.

In other words, the circuit is as you diagrammed above, except 5V is
preferably higher, the known resistance is about 2Mohm, and a high impedence
buffer with gain is inserted before the PIC A/D input.  It might also be a
good idea to divide the actual voltage and feed it into another A/D input so
that its absolute value can be factored out.  Varying that supply from the
PIC will also allow for larger dynamic range of measurement.

*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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> Is it possible to do it using a simple potential divider
> as shown below?

I'd say yes. But it has limitations.
If you use a known resistance low enough to not affect PIC A2D accuracy
excessivley then the input voltage should be >> 5v.
eg if Rknown in 10k and Rmax tested = 50 Mohm and max A2D input = 5 volts
then

Vin = 50E6/10E3 x 5v = 25,000 volts (!)
(Actually 25,005 :-) above ground).
A voltage clamp across Rknown would be "wise" :-)

At lower voltages the max voltage will be lower.

Increasing Rknown reduces Vin BUT currents reduce markedly and the A2D
starts getting inaccurate so a buffer would be needed.

At Rknown = 1M Vin = 250 volts.
Clamp still needed!
Voltage is typical of "Meggers" used for insulation testing.
(They use high voltage to induce breakdown of suspect connections).
Power in wood = 250^2/50M = 1.25 mW.

Runknown = 10M gives 25 volts (actually 30)
BUT current = 25/50M = 0.5 uA
A buffer is needed AND while 0.5 uA is measurable, it is not simple to
measure it with stability or with simple circuitry.

Some sort of compromise is going to be needed.

A bridge is probably going to help heaps.

RM

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