Using a multimeter to measure wood moisture level

precaud Posted By precaud, Oct 25, 2009 at 9:33 PM

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  1. precaud

    precaud
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    After studying the old Forest Service article on electrical moisture meters, my curiosity got the best of me. Realizing that, all I really want is, maybe once or twice a year, a simple "go/no go" indication of wood's readiness to burn, I decided to try using a very generic digital multimeter (DMM) to measure it, using the technique and data in the USFS paper. Nothing exhaustive; I just wanted to see if it worked and if my measurements matched the theory. Guiding observations for the test:

    : The general consensus is, you want to burn wood that is 20% or less moisture content.
    : Looking at Table 1 (Electrical resistance vs Moisture content of various species) in the USFS paper, except for the few really dense hardwoods out there (ash, hickory, American Elm), the DC resistance of the majority of species used for firewood are in the range of 2.0 to 4.0 MegOhms at 20% moisture, with most in the low 3.0 range. Higher moisture levels give lower resistance readings.

    So if it measures 3 MegOhms or higher, it's ready. Well, even the cheapest of multimeters have a 10 MegOhm resistance range and so can easily measure 2-4 MegOhms. So a simple go/no go moisture test should be able to be done with a DMM. And it is much more practical to have a multimeter lying around than a single-purpose moisture meter. The DMM can be used for a multitude of other things.

    Most DMM probes are too blunt to penetrate into the wood to get a good reading. One solution is to use a nail about the same diameter as the probe tips and punch a couple holes to measure in. Space them 1-1/4" apart along the grain in the fattest part of the split. You only need to go down 3/8 to 1/2 inch. Takes a few seconds to do.

    So I took two pieces of Siberian elm - one that I cut and split in May, the other cut and split three weeks ago. I split each piece in half again to get to wood that hasn't been exposed to air, punched the 1-1/4" spaced holes in the fattest part, inserted the probe leads and watched the reading. If you just rest the probes in the holes, they won't make good contact and the resistance readings will jump around and steadily rise. But if you push on them to maintain pressure for better contact, the readings are very consistent.

    The older split measured 3.2 Megohms, right in the 20% zone. The newer one measured around 750 kOhms. Very clear difference in obviously much wetter wood.

    This impromptu test suggests this technique will easily answer the question of "is my wood moisture level at 20% or below". It may not tell you if it's 17% or 15% or lower; you might need a moisture meter to do that. But if all you need is to see if your wood is in the burnable zone, this technique should work just fine.

    So the basic guidelines are:
    If it measures 3 MegOhms or higher, it's good to burn.
    If it measures less than 3 Megs, it needs to season longer.
    If it measures open circuit (or above 10 MegOhms), then the moisture level is in the teens no matter what species it is, and it is definitely dry enough to burn.

    For the super-high-density species mentioned earlier, divide those numbers by 2 or 3.
     
  2. Dune

    Dune
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    If you really want to get nutty with your multimeter, you could calibrate it to known moisture contents, which you could determine using the weigh the sample, bake it in an oven, weigh it again method. We do something like this in the metal business, where we calibrate a multimeter hooked up to a thermocouple and using the melting temp of various metals, construct a crude yet surprisingly acurate pyrometer.
     
  3. precaud

    precaud
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    Yup, that would be a way to calibrate it.
     
  4. Jamess67

    Jamess67
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    Thanks for posting this. You just saved me $20.
     
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  5. Skier76

    Skier76
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    Very interesting! Does anyone have the USFS link for how to do that? I know I saw it here, but I can't find it.
     
  6. precaud

    precaud
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  7. Skier76

    Skier76
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    Thanks for posting!
     
  8. trafick

    trafick
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    Could you just drive the nails 1/2" into the wood 1-1/4" apart and take the reading directly off the nails? The resistence of the nails should be minimal compared to the 3.2 megs of the wood.
     
  9. Hurricane

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    That sounds like it would work fine, I agree the nails will add minimal resistance and will have better connection to the wood then the probes after the nails were pulled.
    I will try that.
     
  10. precaud

    precaud
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    Should be fine, I just have a habit of removing unneeded things from any measurement chain...
     
  11. Valhalla

    Valhalla
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    A sweet idea, thanks!
     
  12. DBoon

    DBoon
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    Yes. Electrically, it will be the same. The resistance of the metal nails will be infinitely smaller than the resistance of the wood you are measuring, regardless of moisture content.
     
  13. madrone

    madrone
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    You rock. Thanks!
     
  14. Cluttermagnet

    Cluttermagnet
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    Great thread- thanks! Probably the most important thing would be to get the electrode spacing just right. Probably the electrode diameter isn't near as important. Electrode depth, I'm not so sure about, but it's probably fairly important, too. Anyway, I'd suggest that using 3d or 4d steel finish nails would make those nails pretty easy to drive with a hammer, and pretty easy to pull out afterwards.

    Here, I have a pretty good quality Hewlett Packard DMM with a set of test leads that have micro hooks on them. They will clip onto the small nails, making it a hands- free measurement. It's a pain trying to hold two regular test prod tips against 2 nails. The clips make it much easier. BTW garden variety 'alligator clips' would also be great here.

    Maybe this thread should be made a 'sticky'- or it might be added to that 2000+ question FAQ on site.

    I'll compare this DMM method against my cheapie Harbor Freight moisture meter and see if I get good agreement.
    FWIW the prong spacing on my HF 96472 meter is almost exactly 1 inch, close to the 1-1/4 inch used for the DMM method. You could calibrate for other electrode spacings as well. Changing the spacing should change every entry in the table by a proportionate amount. I think that driving the nails using a template of cardboard, plastic, Masonite, etc. would make it quick and easy to get the spacing just right every time.
     
  15. DBoon

    DBoon
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    Most certainly, the spacing of the electrodes is the most important thing. A nail ("electrode") with a diameter big enough to hammer into a piece of wood is all you need to make this measurement accurately. To confirm this, just measure the resistance of the length of the nail - it is a conductor, of course, and will have minimal resistance. Varying the nail/electrode depth will allow you to check the moisture at different depths - just make sure both nails are at the same depth - otherwise, your distance between the nails will be more than 1-1/4 inches.
     
  16. Nonprophet

    Nonprophet
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    Or, buy a moisture meter for $29 and be done with it.........no calibrating, no pounding nails, no varying the depth of the nail penetration, no cardboard templates to maintain proper electrode spacing......

    Geesh guys, I'm pretty frugal and I have a nice Fluke DMM, but even I spent the $29 bucks.........lol


    NP
     
  17. Lumber-Jack

    Lumber-Jack
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    What I need to know is how to make a multimeter out of my moisture level meter. ;-)
     
  18. precaud

    precaud
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    Yes, those are all terrible inconveniences...

    You're framing it as a $$$ and convenience issue. To each their own.

    For me, for something I'll do maybe twice a year, I don't want yet another special-purpose tool laying around when a general-purpose one will do just fine.

    After all, a moisture meter is just a 2-digit multimeter with all of the most useful functions removed. :)
     
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  19. quads

    quads
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    I think this is a very cool idea.

    I have never, and will never, own a moisture meter. No reason for it for me, even if they were only 59 cents. I know how long to wait before I can burn my firewood. But, I do have a multimeter and I think it would be interesting to play with it and see what I come up with. Not because it's useful for me to know the moisture content of my firewood, but because of the curious experimenter in me and the fact that the multimeter is something I already have laying around.
     
  20. Hurricane

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    +1

    I do not care about the $29 but why spend it if not necessary? To support the chinese ?

    I forgot who said it I think it was Backwoods Savage but get your wood C/S/S and let time do it's thing. I was one of those people who always cut n the spring split in the fall and burned in the winter. I have now switched so I am 2+ years ahead and shooting for 3+. I burn what I have regardless of what the meter says.
     
  21. pgmr

    pgmr
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    From the pdf:

    "Effect of Grain Angle
    The conductance of wood parallel to the grain is about
    double that of conductance perpendicular to the grain.
    Ratios of conductance perpendicular to the grain, in
    relation to the longitudinal value, are about 0.055 for
    radial and 0.50 for tangential."

    The table of resistance levels in the pdf is for measurements WITH the grain. To use it with at least a modicum of accuracy, you'll need to make sure your nails/DMM probes are parallel to the grain. If not, you'll be doubling the resistance and think your wood is dryer than it really is.
     
  22. DBoon

    DBoon
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    I don't buy a moisture meter even though that would be easier than a multimeter for the same reason I don't buy oil even though that would be easier than cutting, splitting, stacking, and loading wood. :) Sometimes its more fun to "rough it".
     
  23. 'bert

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    I am an electrician and have a formal education in electronics eng. but when reading this thread the only thing I could think of was "why do it the easy way, when this works?"
     
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  24. pgmr

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    After looking at the resistance table, the rhetorical question that comes to my mind is this: How does a cheap moisture meter know what species one is testing?

    A 20% value for American Elm is .48 MO and 20% for Black Spruce is 6.3 MO - greater than 12x the resistance. Might have to chuck that HF meter in the trash.
     
  25. Nonprophet

    Nonprophet
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    If you read the original Forest Service article carefully, you'll see that for 99% of wood species in the US the variation is less than 1% between species. If you're making/milling thousands of dollars in wood products it might make a difference (which is why they buy $400 moisture meters and they don't fool with DMMs........) but for the average wood burner the HF $14.99 model will work just fine.........!


    NP
     
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