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Using a multimeter to measure wood moisture level

Post in 'The Wood Shed' started by precaud, Oct 25, 2009.

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  1. Skier76

    Skier76 Minister of Fire

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    I'll have to post a pic of the multimeter I have. I can't get a reading to save my life. But the thing actually has some decent sharp probes on it. I'm probably using the wrong setting. Electrics at times confuse the you know what out of me. %-P

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  2. quads

    quads Minister of Fire

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    My meter has sharp probes too (so much so that I have drawn blood more than once when reaching for it in my tool pouch). But I could not get a reading by just poking them onto/into the surface of the wood. I had to drive a couple nails in, then it worked.

    The other possibility is that your wood is ready to burn and in that case the resistance is out of the range of your meter.
  3. Skier76

    Skier76 Minister of Fire

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    I'll try the nails and see how that works. :)
  4. Valhalla

    Valhalla Minister of Fire

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    The more moisture in the wood the lower the ohmic value. The lower numbers read on your scale. It is therefore more conductive because it is "wet."

    The drier the wood the higher the resistance (ohmic value). The higher the numbers on your ohmmeter scale. It will be more of an insulator when seasoned properly.

    Wood as a resistive insulator is inversly proportional to the amount of moisture in it. More moisture, the lower the relative resistance. Drier, better seasoned wood is our goal and identified with a high resistance.
  5. precaud

    precaud Minister of Fire

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    An interesting update on the crab apple split that measured 80k Ohms and felt wet to the touch on Nov 1st: After 7 days sitting on top of my wood pile (sunny mid-60's days, upper 20's to low 30's evenings), it now measures open circuit (above 20M Ohms) in the same holes, has multiple deep cracks on both ends, feels dry, and has a sharp tone when struck. It's ready to burn. That's some quick dryout!
  6. Graf Spee

    Graf Spee New Member

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    This sounds like an interesting idea. I have a craftsman dmm, what setting should I select. My initial attempt didn't get me any reading at all.
  7. precaud

    precaud Minister of Fire

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    No reading may mean your wood is nice and dry. To get comfortable with the technique, it may be best to try it on unseasoned wood first.
  8. DBoon

    DBoon Minister of Fire

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    Hi Graf Spree, double check that you have your DMM on "ohms" or resistance mode. Put the probes together and it should read 0 ohms. That's a good check.
  9. DBoon

    DBoon Minister of Fire

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    My previous post above was from November 1st. I just went up and checked the moisture content of a split about the same size as what I checked before, and on the same side of the pile. As before, I split it in half and checked it on the inside. The split was measuring 440 kOhms. That equates to 22% moisture content per the USFS document. So, in two months, moisture content has gone from 27% to 22%. This wood is definitely going to be ready to burn next winter.
  10. roddy

    roddy Member

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    thought id add my two sense(pun intended)
    i work in the hardwood lumber industry,hence we are always strive-ing to ship lumber at the perfect moisture level(6-8 % in the the case of hardwoods for moulding,cabinets, cut to size components etc.) all this talk of moisture meters and multi meters(huh) reminds me that there is only one true way to measure accurate moisture levels in wood...the simple oven test.... green weight minus dry weight divided by dry weight multiplied by 100...moisture meters try to estimate poorly the info this technique provides...if any -one wants the low down on how to perform this test,i,ll update this post....
    rod
  11. Battenkiller

    Battenkiller Minister of Fire

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    This is all very cool stuff.

    As a woodworker for over 30 years, I am always a little bit embarrassed by:

    A. I don't actually own a copy of Bruce Hoadley's book "Understanding Wood"

    B. I don't own a moisture meter

    One of the reasons I don't own a meter is that most all of the wood I use is either many years old (musical instruments) or dead green (bowl turning). With the instruments, if I even wonder if the wood is ready to use, it isn't.

    So I go down to the shop to grab my multimeter and, low and behold, it ain't working. Powers up, but won't even measure the voltage of its own replacement battery. I'm going to have to get a new one and play around with this a bit. Thanks for posting this, precaud. I used to work at Woodcraft and I was always tempted to buy one of those $200 ones that read the wood without poking holes in it, but guys would return them all the time and say they had problems with them. All I need is the go/no-go info, and this will give me what I need.
  12. tjsquirrel

    tjsquirrel New Member

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    DBoon, any idea what resistance reading you would see with Cherry around 20%? I have a bunch of cherry to check and am not sure which species to compare it to on the FS chart.
  13. DBoon

    DBoon Minister of Fire

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    FYI, here is the link to the referenced forest service document http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf

    As you saw, Cherry was not listed there, but based on the observation from someone else, and my own reading of the table, with few exceptions you are good to go if your resistance reading is 3 Megohms or higher. Hickory (which I have a lot of) is one of those exceptions.

    In any case, my experience is that cherry seasons and dries pretty fast.
  14. pyper

    pyper New Member

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    My multi-meter only goes to 2 megaohms, so it's not especially useful for this test, and the wood I really want to measure (flowering pear) isn't in the list anyway, but I figured out an easy way to make the reading.

    Accurate results seem to depend on accurate hole placement. The author mentions that all his tests were done with 5/16 probes, 1.25" apart. Well, at first I tried drilling holes, but then I realized it's easier to just drive two 5/16" long screws into the wood, take the measurment, and back them out.
  15. gpcollen1

    gpcollen1 Minister of Fire

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    I think that the moisture meters are a waste of time in general unless you are purchasing your wood and want to verify the moisture content on delivery. We know that much of the tree species will season pretty well in one season and oak especially takes 2 seasons to get the moisture content down. For those that process your own wood, do you really need a moisture meter??

    So for me, this is a pretty good trick to show b/e there is no way I am buying a moisture meter. If I every get that curious, I can just do it this way - but I doubt it.
  16. Battenkiller

    Battenkiller Minister of Fire

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    Me, too.

    Wish I had a resistance number for the cherry.

    I went out and bought an inexpensive DMM to replaced the one I fried somehow. $20 at Sears, goes up to 20 MOhm.

    I've had a split of freshly cut cherry sitting on an old produce scale I have in my wood shop where the stove is. It weighed 7 lbs 12 oz two weeks ago, and now it weighs 5 lbs 11 oz - a loss of over 2 pounds of water, or about 25%. Green cherry is supposed to have about 40-42% MC when first cut, so that means it is well below 20% MC by now.

    When I sank two finishing nails into it about 1/2", it didn't even register, but as I drove them in all the way (about 1 1/2 ") I got a reading of only .75 MOhm in the center. Obviously still a bit damp way inside. RH in the shop is < 10% as measured accurately with a sling psychrometer.
  17. DBoon

    DBoon Minister of Fire

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    In July 2010, I moved a cord of the meadow drying hickory into my garage wood crib. I didn't check it at that time, but I checked it today. I was averaging 4 MOhms when I split the wood in half and checked inside. This equates to about 16-17% moisture content. Bark is peeling off, rounds are deeply cracked on both ends - all evidence of good seasoning.

    I tested some other Hickory that I left outside uncovered - it averaged 1 MOhm, or 20% moisture content. I moved this into the garage today. Good enough to burn, but not as good as the other batch that I moved into the garage earlier in the summer.

    From my previous pseudo-experiment with Sugar Maple, I would conclude that drying Sugar Maple or Hickory outside in sun and wind for a full year is plenty of time when the wood is 4-6" on a side and 16" long. Putting it in a covered enclosure (garage, barn) after half this time seems to make no difference, and may actually help if the garage or barn is well-ventilated. Obviously, a wetter climate or a wetter than normal summer would probably effect these results, but you definitely don't need two full years for these woods in these sizes IF you have a good sun/wind location for the first six months of the seasoning.
  18. Redcloud5400

    Redcloud5400 New Member

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    Great work DBoon! if more people posted their readings, species of wood, and confirmed moisture readings, and type of moisture meter, we would have a good data set to draw from. I have an idea for some test pins--will have to play with this and post up some numbers for the wood I have layin around!
  19. Ralphie Boy

    Ralphie Boy Minister of Fire

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    It has been a while since I've said this; the best way to check the moisture content of your firewood is with a multi-year calendar.;)
  20. NateH

    NateH New Member

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    Okay, first time wood burner (patiently waiting to install). I was about to buy a moisture detector but after reading through these posts I'm wondering. I thought this was something I should have prior to purchasing wood... then there's the matter of wood - where to get it, refutable suppliers, and can I even find anything this late ready to burn this winter?
  21. YkDave

    YkDave New Member

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    So for kicks, i gave this a shot.

    Last saturday i cut down some big ol green jack pine as i am starting to stockpile for next winter (or maybe this spring if need be...)

    These trees were green as can be, split and stacked them on skids sunday, and threw them up on the pallet racking in our shop just because their was some free real estate up there.

    Its hotter than the gates of hell up there during the winter as the 25-30ft height mark gets all the heat while the lower half of the shop freezes, so i figured it would dry this up pretty quickly.

    Well, ill be damned if i didnt check it today with the multimeter and im getting 10-12ohms. According to the chart that would be in the 18-19% range already...

    Id like to have an actual moisture meter to verify this... Maybe i should get into the wood drying buiseness
  22. MrNoBuddySpecial

    MrNoBuddySpecial New Member

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    I know this is an old thread... but, would this work?
    image.jpg
    this is roughly 1.25" apart at outer edges.
    image.jpg
    measured at inside and get a stupid low #... this was clearly wet wood after splitting the split. this is pin oak felled in Feb, left on ground chunked and split 2mo ago.

    maybe will get the MM from amazon. not sure I could trust myself to do it correctly.
  23. YkDave

    YkDave New Member

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    Not sure what your issue is?

    You know it's wet, and your dmm numbers reflect that.

    Resistance increases as the wood dries, and the number will increase
  24. MrNoBuddySpecial

    MrNoBuddySpecial New Member

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    don't think you understood my post... I was asking if using a HAMMER to get the "holes" would be an effective alternative it finding nail, measuring, ensuring grain, pounding in, removing, inserting probes etc, etc.

    I like the idea of using what I already have, but not sure I trust myself/meter/myself to get an accurate reading.

    does it HAVE TO BE 1.25" apart EXACTLY?
    does it HAVE TO BE the same grain line?
    do i HAVE to get the nail depth EXACTLY right.
  25. YkDave

    YkDave New Member

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    Now i got ya... LOL

    The answer is yes, and no!

    If you are going to be using someone elses resistance-Moisture% chart, then yes you have to be as exact as possible to their testing method of you skew the results.

    That being said, you could do one "exact test", then on the same piece of wood, do your hammer method and see what your result is. In a sense, calibrating your results to the known resistance-moisture% charts. But, the biggest problem with your method is that its pretty hard to get repeatable results, which is pretty important here. Your method could be out by a billion OHMs compared to the charts, which is fine, but your testing method has to be repeatable or you dont know what the correction % should be
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