changing electrical resistance of wood?

[pybyr]

Here's a question in hopes that someone may know, or have suggestions on where to look (initial Google searches did not yield any answers).

I wanted to measure the moisture content of some of my late-cut wood, but did not want to spend $ on a moisture meter, since most other than the really expensive ones seem to be poorly made. And they're really not that accurate, as the relation of electrical resistance to percentage moisture varies a lot by species.

So I looked up the US Forest Service table for converting megohms of electrical resistance to moisture content, by species.

Then I split open a piece of White Ash, drove two nails into the wood, 5/16 deep and 1.25 inches apart, in line with the grain, as is the standard for measurement.

Then I clipped the leads of my good-quality Beckman digital multimeter onto the nails.

Measurement started out up around 8.4 megohms, but the numbers on the digital readout kept changing, and in a decreasing direction.

15-20 minutes later, it seemed to have stabilized at about 2.8 megohms.

I'm just plain curious on what would be causing that change- and also what it means for the accuracy of "quick" moisture meter tests.

 

[PNWBurner]

Good question.

My cheapy moisture meter drops over time too. It indicates drier wood if you wait 5 minutes than it does right away. I've always though maybe things were drying out around the probes.

Odd that you're seeing less resistance over time though. It seems to me that lower moisture would mean more resistance but maybe I have that backwards...

 

[CountryBoy19]

This is just a random guess, because my meter does this as well.

I think at resistance values that high, it takes the meter a while to settle into the reading. That's all I can come up with to explain it.

 

[pybyr]

This is just a random guess, because my meter does this as well.

I think at resistance values that high, it takes the meter a while to settle into the reading. That's all I can come up with to explain it.

Thanks for the try at an explanation, but the meter will immediately get a steady resistance reading with a regular resistor. So something is varying with the wood...

 

[Rockey]

This is just a random guess, because my meter does this as well.

I think at resistance values that high, it takes the meter a while to settle into the reading. That's all I can come up with to explain it.

Thanks for the try at an explanation, but the meter will immediately get a steady resistance reading with a regular resistor. So something is varying with the wood...

I think your both right. The resistor has a repeatable measurement because it designed and manufactured with a "fixed" resistance. Measuring through wet wood will be an average resistance and will vary until the measurement settles in. It may be more accurate as an integration over time. When we measure IR values in the tera and giga ohm range even the slightest humidity changes in the air will change the measured IR values.

 

[PJF1313]

What kind of nails are your using?

If you use a steel, or a galvanized type; they may change due to corrosion or to removal of galvanization.
Copper, SOLDERED to your meter leads, may be your best bet - not so much change due to oxidation (rust)
or removing the galvanization of galvanized nails (moving the leads/clips around the nails to get a good connection)

Granted, copper, is not the be-all/best all (Gold of Silver is; - yeah, right - who's gonna waist .10 on a .0001 split)
should give you better/more accurate readings.

Just my 0.0002 worth (dang - I'm worth more dead than alive! )

 

[PJF1313]

dang double post!

 

[pybyr]

What kind of nails are your using?

If you use a steel, or a galvanized type; they may change due to corrosion or to removal of galvanization.
Copper, SOLDERED to your meter leads, may be your best bet - not so much change due to oxidation (rust)
or removing the galvanization of galvanized nails (moving the leads/clips around the nails to get a good connection)

Granted, copper, is not the be-all/best all (Gold of Silver is; - yeah, right - who's gonna waist .10 on a .0001 split)
should give you better/more accurate readings.

Just my 0.0002 worth (dang - I'm worth more dead than alive! )

clean (unrusted) plain (unplated) steel nails

 

[PJF1313]

What kind of nails are your using?

If you use a steel, or a galvanized type; they may change due to corrosion or to removal of galvanization.
Copper, SOLDERED to your meter leads, may be your best bet - not so much change due to oxidation (rust)
or removing the galvanization of galvanized nails (moving the leads/clips around the nails to get a good connection)

Granted, copper, is not the be-all/best all (Gold of Silver is; - yeah, right - who's gonna waist .10 on a .0001 split)
should give you better/more accurate readings.

Just my 0.0002 worth (dang - I'm worth more dead than alive! )

clean (unrusted) plain (unplated) steel nails

Well..... Dang....


Ahh, O.K. - Open window; throw my theory out; close window QUICKLY!

Sorry that I can't help. I am also doing the same USFS M.M. on a Fluke 85 meter. The meter never did settle down for me,
I just gave up.

The best/most consistent readings I did get, where from copper nails - but it did fluctuate +/- 500k ohms - better than S.S./ Iron / Gal (better to worst) Maybe my (our) meter is just too sensitive? I do have an old Simpson 260 that I have to dig out, but may be better - being analog = slower to react?

Good luck to you. I'll see if the 260 is better or not (gotta a mission tomorrow! )

 

[Battenkiller]

It may be that over time the nails have developed a thin oxide coating through the process of passivation. After they are in the wood for some time, this coating may react with the damp wood and dissolve, decreasing the electrical resistance.

 

[BurnIt13]

Why not compare apples to apples....so to speak? When using a moisture meter it doesn't penetrate the wood a half inch, right?. Maybe you should, if you havn't already, just use the two probes from the meter and manually apply pressure to the wood 1.25" apart.

This would rule out any galvanic corrosion, passivisation, humidity, lunar cycle, etc. You'd be testing it the same way the moisture tester would. If the readings still vary, try another meter to back it up. If another multimeter does the same thing......go and buy a moisture meter

 

[CountryBoy19]

Why not compare apples to apples....so to speak? When using a moisture meter it doesn't penetrate the wood a half inch, right?. Maybe you should, if you havn't already, just use the two probes from the meter and manually apply pressure to the wood 1.25" apart.

This would rule out any galvanic corrosion, passivisation, humidity, lunar cycle, etc. You'd be testing it the same way the moisture tester would. If the readings still vary, try another meter to back it up. If another multimeter does the same thing......go and buy a moisture meter

I'm not so sure you follow what is going on here.

BTW, doing as you say would cause any readings that you do get to vary more.

 

[BurnIt13]

I'm not so sure you follow what is going on here.

BTW, doing as you say would cause any readings that you do get to vary more.

I'm on board with whats going on.... He's trying to get a moisture reading using his multimeter...perfectly understandable. What I'm trying to say is to try and measure the wood using a multimeter the same way a wood moisture meter would and see if you get similar results.

To the OP...what size nails are you using??? I wonder if the act of hammering a nail into the wood changes the characteristics of the test? By inserting the nail into the wood you would be compacting the material directly surrounding the nail. I wonder if the denser would surrounding the nail has anything to do with it.

 

[Jags]

clean (unrusted) plain (unplated) steel nails

Your just never going to know until you use gold plated nails. ;-)

Here it goes - my guess...everything around you is in a state of flux. The pounding of the nails compresses the wood - changes the "value", slamming it on a table could also. Maybe you are under a florescent light fixture that was just turned on...etc. Basically what I am trying to say is that "things have to settle down" when you are talking about high resistance readings. Low resistance is much easier (like shorting the leads together = 0 - immediately). This is just a swag, but if you don't think it can happen, just think about how a person can "witch" for an underground power line or tile (like a farmers field). Same kinda idea - a disruption.

 

[CountryBoy19]

I'm not so sure you follow what is going on here.

BTW, doing as you say would cause any readings that you do get to vary more.

I'm on board with whats going on.... He's trying to get a moisture reading using his multimeter...perfectly understandable. What I'm trying to say is to try and measure the wood using a multimeter the same way a wood moisture meter would and see if you get similar results.

To the OP...what size nails are you using??? I wonder if the act of hammering a nail into the wood changes the characteristics of the test? By inserting the nail into the wood you would be compacting the material directly surrounding the nail. I wonder if the denser would surrounding the nail has anything to do with it.

A moisture meter is calibrated to penetrate the wood a certain depth; your manual should specify this.

On the same note, if he is using the chart that I have, it was made using controlled laboratory samples of wood using nails spaced a very specific distance apart, driven to a very specific depth. Varying from those parameters will only set you further away from the results reflected on the chart. But that isn't even the problem, his problem isn't that he is getting inconsistent results using his multimeter. The problem is that it takes his multimeter several minutes to "seek out" the actual resistance. This has nothing to do with the consistency of the reading or "compacted wood around the nail" or the nail size, those things wouldn't cause the meter to seek out the resistance in such a slow manner.

 

[Jags]

This has nothing to do with the consistency of the reading or "compacted wood around the nail" or the nail size, those things wouldn't cause the meter to seek out the resistance in such a slow manner.

If it was still in a state of change it could. Just as capacitance can change.

For the sake of argument - pounding a nail in the wood could be in a state of change for hours or days until a true equilibrium is reached. This is a silly argument, but somewhat true never the less.

 

[billb3]

can you change the scale of reference on that meter so it appears stable ?

 

[CountryBoy19]

This has nothing to do with the consistency of the reading or "compacted wood around the nail" or the nail size, those things wouldn't cause the meter to seek out the resistance in such a slow manner.

If it was still in a state of change it could. Just as capacitance can change.

For the sake of argument - pounding a nail in the wood could be in a state of change for hours or days until a true equilibrium is reached. This is a silly argument, but somewhat true never the less.

True, but I don't think the change would be large enough to see the effects that the op and myself are seeing.

Just to note, in my experience, the resistance changed by an entire order of magnitude in 20 minutes. Example, I connected the leads to the wires and it said 10 Mohm, 20 mins later it said 1 Mohm. Those numbers are not real, it's been so long that I don't remember the actual numbers, nor do I remember if it increased or decreased to the ending value, I just remember that it changed an entire order of magnitude, which is significant, and tells me that something more than miniscule things like the nail depth, or compression of the wood around the nail are happening.

 

[Cowboy Billy]

This is probably way off base but just a wild thought I had.

Could it be channeling? When you put the ohm meter across the nails it induces a current between them and a magnetic field. Could it be the longer you leave the current on the better it finds a path between the nails? Like when water runs across loose soil and creates a channel where resistance to flow is lessened. Either by finding a easier channel or aligning the magnetic field between the nails.

Did I mention I am nuts?

Billy

 

[Danno77]

This is probably way off base but just a wild thought I had.

Could it be channeling? When you put the ohm meter across the nails it induces a current between them and a magnetic field. Could it be the longer you leave the current on the better it finds a path between the nails? Like when water runs across loose soil and creates a channel where resistance to flow is lessened. Either by finding a easier channel or aligning the magnetic field between the nails.

Did I mention I am nuts?

Billy

I thought something similar whilst laying in bed last night. It also seems to me that finding resistance means that the meter is sending a small current through the wood. is it possible that this small current dries out its path the longer it tries to read it? I guess what I'm saying is that this could be like trying to read a thermometer by putting it up to a light bulb; you change the thing you are measuring, because you are measuring it---ala, "Observer Effect"

 

[Jags]

is it possible that this small current dries out its path the longer it tries to read it?

Less moisture in wood equals higher resistance. Usually the resistance reading will decrease.

 

[Danno77]

is it possible that this small current dries out its path the longer it tries to read it?

Less moisture in wood equals higher resistance. Usually the resistance reading will decrease.

thanks, I was thinking (since I haven't read it since yesterday) that the OP was saying MC was decreasing, and as you point out he was saying resistance was decreasing.

 

[DiscoInferno]

Have you checked if the resistance decreases whether or not the meter is left attached and on during the 20 minutes? What if the meter is attached for 20 minutes, then removed and discharged, then reconnected? What if it's reconnected swapping positive and negative terminals? These will narrow the possible effects down between the mechanical, chemical, electro-chemical and the electrical. So if it happens without the meter, it's probably something mechanical or chemical. If it only happens with the meter attached, then it's electrical or electro-chemical. If it repeats each time the meter is attached the same way, it's probably electrical not electrochemical. If swapping terminals changes anything, then electrochemical. I would have guessed something electrical with a long time constant, but the resistor test rules that out (a log w/ nails isn't much of a capacitor or inductor).

 

[CountryBoy19]

Have you checked if the resistance decreases whether or not the meter is left attached and on during the 20 minutes? What if the meter is attached for 20 minutes, then removed and discharged, then reconnected? What if it's reconnected swapping positive and negative terminals? These will narrow the possible effects down between the mechanical, chemical, electro-chemical and the electrical. So if it happens without the meter, it's probably something mechanical or chemical. If it only happens with the meter attached, then it's electrical or electro-chemical. If it repeats each time the meter is attached the same way, it's probably electrical not electrochemical. If swapping terminals changes anything, then electrochemical. I would have guessed something electrical with a long time constant, but the resistor test rules that out (a log w/ nails isn't much of a capacitor or inductor).

VERY GOOD, why didn't I think of that.

Essentially, what he is saying is, "If the meter does the process all over again after disconnecting and reconnecting it, that's just how long the meter takes to seek out the proper resistance value".

 

[Cascade Failure]

While we are trying things...

Someone please use a multi-meter and a moisture meter on the same piece of wood at the same time. Do the readings decrease in both, during the same time period, by the same percentage?