DMM-based moisture meter comment

[jophysx]

The thread about using the ohm-meter capability of a digital multimeter (DMM) to measure wood moisture content was an interesting one. I have one comment to add, but the thread is closed.

Straight from the bored nerd department . . .
A few folks posted that the separation between the electrodes was important for an accurate reading. That seems like common sense, but I think this is a case where common-sense will lead you astray. As long as the separation between electrodes is large relative to the electrode dimensions (i.e. diameter of pins or nails) and the separation between electrodes is small relative to the dimensions of the wood being measured, then in theory the measurement should not be sensitive to the electrode separation. i.e. it's not critical and you should get basically the same reading regardless of the separation within reason.

The reason is that as you separate the electrodes further, the distance the current has to flow goes up, which would tend to make the resistance increase. But at the same time the effective cross-sectional area through which the current can flow increases, because the current can "spread out" more. This causes the resistance to decrease. For two pins stuck into the planar surface of a semi-infinite volume, these two effects exactly balance and the result is the resistance you measure will be independent of the separation.

Now a chunk of wood is not quite the same thing as a ideal semi-infinite volume, but it's reasonably close on the scale of the electrodes. Wood is also not an isotropic medium. The conductivity along the wood fibers is probably different than across fibers. So you will probably see some small dependence on separation. My point is that it's probably not critical to be precise, especially since moisture measurements are inherently not that accurate.

Now if you had an infinite two-dimensional plane of wood, then the resistance you measured would increase with the logarithm of the electrode separation. And if you had a one-dimensional filament of wood it would increase linearly. So keep that in mind if you come across any wood like that.

-Jim

 

[blades]

You really are bored arn't you? Best you be gettin out side and splittin and stackin firewood - saves on gym fees as well. Generally if you can get your self 2-3 years ahead on a rotational basis of firewood use, you can by pass the MM altogether. If your buying from a dealer well then it helps to prove how long their nose is, as we all know they sell nothing but season wood ( I keep asking what kind of seasonings they use- ain't got an answer yet)

 

[DBoon]

A DMM-based moisture measurement is based on a simple resistance calculation (the DMM) and a lookup table (the referenced document). The resistance readings provided in the lookup table are based on a specific distance between electrodes, so any other distance appreciably different will not work with that lookup table.

You are correct in that a different lookup table calibrated to a 1/2", 1-1/2" or 2" distance could also work, but that is not what is available for use.

 

[jophysx]

My point was that, at least in theory, the calibration tables for 1", 1.5", 2" separation should all be the same, at least to within the uncertainty of the moisture reading itself. Of course there's a great quote about theory vs. practice. I think it's from Einstein

"In theory, theory and practice are the same. In practice, they are not."

-Jim

 

[CountryBoy19]

You've proposed the theory, now lets see some data from your experiments

IE, get out there, split some wood, and start taking measurements so we can get some real data to see if your "theory" matches up to the real-world.

 

[DBoon]

My point was that, at least in theory, the calibration tables for 1", 1.5", 2" separation should all be the same, at least to within the uncertainty of the moisture reading itself.

In theory, the resistance should increase or decrease linearly with the distance, so using 1" instead of the 1-1/4" should result in 80% of the resistance and therefore a moisture reading that is 25% higher than what it otherwise should be if you used the lookup table based on 1-1/4" distance.

 

[jophysx]

@DBoon: You are missing my point. Resistance of a wire increases linearly with length. Resistance of a large solid object probed a relatively small distance apart (relative to the overall size of the object) does NOT depend on distance at all (in theory). This is counterintuitive, but I tried to explain why in my original post.

I have been challenged to take some data to see if the theory matches the real world, which is of course the right thing to do. I will do this and post back.

-Jim

 

[CountryBoy19]

In theory, the resistance should increase or decrease linearly with the distance, so using 1" instead of the 1-1/4" should result in 80% of the resistance and therefore a moisture reading that is 25% higher than what it otherwise should be if you used the lookup table based on 1-1/4" distance.

His explanation from an electrical resistance perspective does make some sense. For a fix, small cross-section piece of wood (think 1/4" diameter dowel rod) the resistance would increase proportional to the distance, but on a large cross-sectional piece (piece of firewood) the resistance does not increase proportional to distance. The paths of conductivity in this case would "spread" similar to the way magnetic flux does as you vary the spacing between 2 magnets.

That being said, the point at which I think I may disagree with him is that the distance doesn't matter. Intuition tells me that the resistance change as the probes for moved further apart will not be insignificant. I think it will land somewhere in the middle of "proportional change" and "no significant change". That is why I encouraged the OP to take some actual measurements. I think his theory warrants further research/discovery and his explanation has merit. I'm curious to know for sure...

 

[jophysx]

I can tell you that the "theory" is 100% correct for an ideal isotropic medium. The only question is how well wood approximates this ideal. I will take the data and report back. I agree with your intuition. The resistance will not be proportional to the distance, but it won't be perfectly constant either. It will be somewhere in between. I predict that the variation with distance will be small enough that you can ignore it in the range of 0.5-3.0" relative to the uncertainty of the moisture measurement itself.

-Jim

 

[jophysx]

OK. I said I would do it and I did it. Took me awhile because my multimeter died and I had to get a new one. I found one that is supposed to be able to measure up to 60Meg. But I wasn't able to get stable measurements with it. The probes were pretty blunt and the the meter seems to have stability issues. So I improvised with a couple of stick pins (steel) for contact and used my moisture meter with some copper wire to contact the pins. Then I got a big split and split it fresh. I started with the pins far apart and moved them closer and closer, recording the moisture reading at each step. I repeated this on two different splits. Split #1 was 3" wide x 3" thick x 15" long. Split #2 was slightly larger. Both were Douglas Fir. The results are shown below.

The reading has almost no dependence on the distance between the pins. Trial #1 shows a slight trend from 6" to 2" separation - about a 0.7% difference (15.5% to 14.8%). But this should be compared to the uncertainty in the measurement. This is a basic moisture meter with no calibration for different wood species. According to the tables, different wood species can easily result in corrections of over 1%. So I think this backs up my claim that you don't have to worry too much about the exact separation of the contacts when you use an electrical resistance measurement to estimate wood moisture content. It just doesn't make much difference.

One thing that is not quite as I would have predicted is that I would have expected the reading to start to be sensitive to the separation distance once the distance started getting greater than about 3", since that was the width and thickness of the split.

This post is my entry for the "nerd of the year" competition.

 

[CountryBoy19]

Thanks for following through! Intriguing results, not quite what I expected but not too far off either.