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Using a multimeter to measure wood moisture level

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

  1. Detector$

    Detector$ Member

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    PGMR.... exactly! and why I didn't buy the HF meter. When you get up to the 70-80 dollar mark the more expensive moisture meters come with instruction manuals with specific instructions for each species of wood. The cheap meters are probably a waste of money.

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  2. Nonprophet

    Nonprophet Minister of Fire

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    n/t
  3. Gark

    Gark Minister of Fire

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    Excellent post, precaud. Thank you.
  4. Duetech

    Duetech Minister of Fire

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    For someone who has a HF within 25 miles, that does not stock the moisture meter and who does not want to purchase the mm without reading the pkg and handling it the multimeter approach is ideal.

    Since electricity follows any path but especially the path of least resistance readings taken by an mm will be parallel to the wood grain unless the pins are situated on opposing sides of the log in a fasion to be perpendicular to the direction of the grain.

    Thanks for posting this info!
  5. Cluttermagnet

    Cluttermagnet Minister of Fire

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    Yep. Well, I already had the cheapie moisture meter from Harbor Freight. With a sale plus a 15 percent coupon, I think it came in just under 10 bucks, well worth it to me. The meter is teaching me that my sense of touch is a pretty good moisture indicator- but not always! Depending on the wood, it may feel dry and yet hiss in the stove. But my hands have become surprisingly good at telling which wood is seasoned enough.

    For me, this is a very interesting 'academic' exercise, to check the DMM method against a moisture meter. Heck, DMM's are getting so cheap and so common that probably millions of households have one or more. I have a bunch of them. HF has some cheapie DMMs that they often sale at 2-3 bucks! Cheap, yes, but what an amazing deal. Just a pair of test leads for a domestic instrument is going to cost more than that.

    I'm with you guys, so far as laying in a wood supply for 2-3 years out. That is my goal but I'm not there yet. And yes, little doubt that properly sized and seasoned splits that old have got to be seasoned enough- no need to test.

    Yes, a cheapie moisture meter should be thought of as an 'indicator' and not relied on to be an accurate measuring instrument. Still, an indication is better than a wild a**ed guess. I believe it teaches the new wood burner to have their hands better 'calibrated' to sensing moisture content. Old pros are generally not going to need these things much if any. 'Training wheels'? ;-)
  6. Tony H

    Tony H New Member

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    Good idea and all in all not a terrible exercise to get a good result. I do have a little cheap MM and have only used if a few times to get an idea how fast some different types of wood are seasoning. Would have worked just find using my trusty Fluke instead oh well.
  7. Nonprophet

    Nonprophet Minister of Fire

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    In reading through several manuals for the higher end moisture meters, it would seem that TEMPERATURE has more of an impact than SPECIES relative to getting accurate readings.

    It seems that Doug fir at 70 degrees is the default setting for calibrating most MMs. While Teak at 70 degrees would only change the reading by 1% +/- from Doug Fir, Teak at 40 degrees would change the reading by 3%. Still fairly insignificant, but you could get as wide a variation of say 12% depending upon the temp of the wood whereas the species normally wouldn't vary by more than 1%-2% or so...........


    NP
  8. precaud

    precaud Minister of Fire

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    HP, it makes sense that temp would influence the conductivity. Just another reason to not take the absolute numbers too seriously. And to work far enough ahead to not have to worry about measuring it. :)
  9. DBoon

    DBoon Minister of Fire

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    Just checked some of my wood supply with a digital multi-meter to see if my moisture content is in the target range.

    Cherry - 18 months old, spent half the time in a garage, half the time outside covered and in the sun half the day. DMM reading: 9 to 15 megohms, even on the inside of the split. Well under 20% moisture content

    Sugar Maple - 18 months old, half in garage, half outside (like the cherry). DMM reading is 6-9 megohms, which equates to 16-17% moisture content.

    Pine - 18 months old, stacked outside in sun and wind. DMM reading is 20-25 megohms, which equates to 15-16% moisture content.

    Sugar Maple - 5 months old (fresh cut and split at that time). Stacked outside in the driveway, uncovered the first 3-1/2 months, covered last 1-1/2 months. DMM reading is 0.6 to 0.9 megohms, which equates to 24-25% moisture content.

    Will have to check my hickory that is stacked in windrows in an open meadow, and now covered for the last 2 weeks. That was fresh cut and split this year, so it will be interested to see what that is right now.
    Redcloud5400 and Soundchasm like this.
  10. precaud

    precaud Minister of Fire

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    Very good results, DBoon, and very much in line with what one would hope to see.
  11. Valhalla

    Valhalla Minister of Fire

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    In summary:

    1. Can a wood moisture meter know the species or type of wood tested? No, of course not.

    2. Is a wood moisture meter probably just a single range ohmmeter? Yes, without a doubt.

    3. Can one therefore use an ohmmeter as a wood moisture meter. Yes, absolutely.

    I admit to now use my ohmmeter probes by just sticking them in the cracks of the sample splits to be tested.
    Range it, or let it autorange accordingly and smile! Priceless and very logical information, thank you.
    We can all now burn a bit more wisely.
  12. bren582

    bren582 Member

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    Great thread.. I checked some of my wood today with my Fluke. Split each split and drilled holes 1.25 inches apart with the grain on the exposed surface and stuck the probes in. nice tight fit.

    First Pic- White oak 3 years split that's been in the garage for the past year, burned some of this last year and man was it nice. 181 meg?? I guess that means its really dry.

    Sec Pic - Red oak 10 months split, stacked outside with top covered. .177 meg and i can feel the dampness on the exposed wood.

    third pic - Red oak 5 month split, stacked outside with top covered. .057 meg and its really wet on the freshly exposed surface

    I have to read that article to get an idea on what these readings represent. The temperature was 70Deg F when checked. Red Oak lives up to its rep for a long seasoning. I guess one could at least come up with range of readings that represent several degrees of seasoning and go with that as a gauge. I was going to buy a meter but now maybe not. Will just use the fluke..

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  13. precaud

    precaud Minister of Fire

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    bren, with it's autoranging MegOhm range (up to 300Meg), the 8060A is a great meter for this. The readings you're getting seem exaggerated, though. Have you checked your meter for accuracy recently? Say, measure a 10Meg resistor and see what it says? The 8060A is a great little meter but most of them are getting old enough to have some of the electrolytic capacitors in them dry up, resulting in inaccurate readings. I've rebuilt several of them for exactly that.
  14. bren582

    bren582 Member

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    That's a good point, No I haven't checked it in years.. I will have to stop by radio shack and pick up a few resistors and test it out.. Thanks..
  15. DBoon

    DBoon Minister of Fire

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    Thanks precaud. I'm going to move half of that 5 month old Maple into the garage for the winter and keep half outside covered on top. That will be my experiment to see if it dries as well inside as out.

    I was pretty surprised to see everything dry in 18 months. I think my splits are smaller (generally 3" x 4" tops) so they dry quicker.
  16. precaud

    precaud Minister of Fire

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    Yes, that will be interesting to see. What's your hunch?

    No question, diameter makes a huge difference.
  17. Jamess67

    Jamess67 Feeling the Heat

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    Tried my cheap Radio shack mm last night and couldnt get a stable reading. Guess I will be buying a moisture meter. Thats would be cheaper than a new mm.
  18. precaud

    precaud Minister of Fire

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    Jamess67, don't sell out to the dark side! A multimeter is a far more useful tool that a moisture meter, and costs no more.

    Basically only two things can cause unstable readings: a bad meter/leads or poor contact to the wood.

    To see if the leads are good, short them together and make sure the reading is down near zero.

    To see if the meter is measuring correctly, make sure the battery is good, then measure a high value resistor (something in the 1 MegOhm to 10 Megom range) and compare the measurement to the resistor value.

    Older non-working electronic devices are a good source of components for this test. Most of us have some unrepairable electronics lying around! If you know the color code, just unsolder or snip out the resistor and then measure it.

    For probes directly in the wood, add pressure to stabilize the reading. If clipping onto nails, make sure the surface is clean and rust/corrosion-free.
  19. DBoon

    DBoon Minister of Fire

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    The maple is CSD wood that I had dumped into my driveway, which I then resplit and stacked. I want to understand what it does where since I don't want to move it more times than I need to, or farther than I need to. Then, I'll know what I can get away with in the future. This is a good test case, since the summer here was so rainy that any other summer should give me much better results.

    My hunch is that the maple might dry a little faster outside, but that once I get it from 35% to 25% by having it sit in the driveway, it will still dry to <20% in the garage in the next 12 months. If it does, then I can move it just once, and only 25 feet from the driveway into the garage cribs.

    The hickory I have (now) covered and drying in the open meadow was split quickly into roughly 6"x6" pieces. That gets a lot of wind and sun. I'm expecting that two summers + one winter (18 months) will dry that out to <20%. Then, I can bring that off the hill and straight into the garage, without resplitting until necessary.
  20. quads

    quads Minister of Fire

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    My oak results:

    My recently processed firewood; trees were standing dead with no bark and gray surface; cut, split, and stacked within the last few months: lowest 268k, highest 1655k

    Today's supply next to the stove, 3+years old: infinite/open circuit

    After I got the infinite reading in the house, I thought I'd try some from the row in the woodpile that I will be burning within the next month, 3+ years old. I still got an infinite/open circuit reading with every split. I tested about a dozen from varying locations in the row.

    I was a little surprised. I never give my firewood any special treatment; stack it and forget about it for years. It's not covered, except with snow, leaves, and pine needles. The bottom row of splits is stacked directly on the ground (I did not unstack enough to test any of those). It has been raining often lately. Almost 7 inches in the last 4 weeks.
  21. precaud

    precaud Minister of Fire

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    DBoon: You'll have to report back next year and tell us which batch dried more.

    Quads: That's interesting results on the stuff you cut recently. I would have thought greyed standing dead would be lower moisture. But then, you're in a wetter climate than I.

    Here are a couple more of mine.

    A 7" dia pinon, cut as standing dead two years ago and stacked as a log, cut and split in half yesterday, measures open circuit (greater than 20 Megs). There's no reference data on pinon pine in the USFS paper, but this is consistent with my experience with the standing dead pinon I gather - I can pretty much cut, split, and burn it right away.

    On the other hand, taken from the same stack, a piece of crab apple, the 20" live tree was cut down last fall, stored as a log for one year, cut and split from the bottom trunk section today, measures 80k Ohms. It felt wet to touch after splitting it.

    What I find interesting about this is that the resinous species, like the pinon, reject water even after being felled, while the others (like the apple and Siberian Elm) absorb and hold it even when dead, which causes them to start rotting quickly. I have pinon logs in my stacks that sat on the ground for years and were still fine, as long as they were in a place where water didn't pool and they had some sun exposure.
  22. Valhalla

    Valhalla Minister of Fire

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    I second this! Here we have a great solution to a common wood burner question.
  23. DBoon

    DBoon Minister of Fire

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    I plan on it. I just got everything moved and stacked - 50% in the garage and 50% outside. We'll see what happens in the next few months.

    I checked my hickory in the open meadow. Wood that I had split from rounds yesterday was 16-20 kOhms of resistance, which is off the charts on the USFS PDF. I'm guessing that this is about 40% moisture (or more).

    Then I checked wood that I had cut and split into ~4-6" pieces (bigger than I would burn, but small enough to dry) in the late spring, early summer. I just covered this wood 2 weeks ago, and we had a rainy summer so it got a lot of moisture. A split was measuring about 350 kOhms. That would equate to ~27% moisture content (it's off the chart, so I extrapolated). Then, I checked a 5-1/2" round - I split it in half and checked the moisture on the inside - 20 kOhms. This is essentially as moist as wood that was split a day before. Lesson learned - split the rounds as soon as possible. Luckily, I only need half this wood for next winter, so I'll resplit rounds in the spring and restack.

    My father-in-law is burning some of this hickory this year. He has stacked it in his barn to dry. Then, we bring in about 2-3 weeks supply and drop it in his basement where his woodstove is. His belief is that the woodstove dries out the wood to "bone dry" in 2 weeks or so. So I checked some in the barn ~200 to 300 kOhms, or nearly 30% moisture content. And the wood in the basement next to the woodstove for 2 weeks? Between 400 and 600 kOhms (25-20% moisture content). So there definitely is some drying out going on there. FYI, these splits are no bigger than 3" on a side. His woodstove is already burning better this year than last. I look forward to seeing how it operates a little later in the winter when his wood has had more time to dry out.
  24. Jamess67

    Jamess67 Feeling the Heat

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    Thanks guys. I have checked the probes and they are fine. The batteries my be an issue. If I complain enough to management (the wife) maybe I can get the Fluke of my dreams.
  25. quads

    quads Minister of Fire

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    Quite often it's still so wet when cut that the water practically runs out of the trunks of the standing dead oak, even after standing dead for 10 years. The limbs are usually fairly dry. I'm guessing the wettest readings were splits from the trunks and the driest (highest resistance) were splits from the limbs. I did get a reading on every piece of the recently cut stuff, as opposed to not even one piece of the seasoned wood.

    One thing I learned; if you are using nails to make holes in the wood, or as probes, put a little piece of pink duct tape or similar on them. Those nails sure like to disappear in a hurry when pulling them out of the split! As long as they don't reappear someday in my woodhauler tire. Ha ha!

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