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general moisture checker

Post in 'The Wood Shed' started by Jack Fate, Jan 28, 2013.

  1. Jack Fate

    Jack Fate Feeling the Heat

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    Just got one , I always question accuracy on any measurement device, so any kind of calibration test ? I put probes in water reads 50%:confused:??? So far my wood is 11%-16% with the ash at the low end & oak at higher.

    just wondering

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  2. fabsroman

    fabsroman Minister of Fire

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    Yeah, I would be skeptical of those readings unless your wood has been seasoned for many years. I just split open a largish piece of oak that has been seasoning for 16 months and I got a 31% reading. Split another piece that was pretty thin to begin with and got 17% or 16%, cannot remember which it was.

    Also, remember that you have to take the moisture reading from a fresh split (i.e., you need to split a piece and measure the moisture content on the inside). I apologize in advance if you already know this. It just seems as though those readings you took are super low.

    As far as calibration is concerned, I have no idea how that is accomplished. Did they mention anything about it in the owner's manual? I have not read mine yet (i.e., just got the moisture meter last week). Just tried to google how to calibrate it and the first two hits sucked.
  3. Jack Fate

    Jack Fate Feeling the Heat

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    Ok ,no i didn't know the fresh split thing ,thanks & makes sense .
    Found out scale only goes to 50% on top & 5% bottom .

    The ash has been dead standing for 2-5 yrs. & split 9 months to 2 yrs so I can almost believe its that dry

    thanks for the input
  4. Mr A

    Mr A Minister of Fire

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    I am more satisfied with the $12 moisture meter from Harbor Freight. I had the General and the prongs broke because they are too long. The HF one has short sturdy prongs
  5. Jon1270

    Jon1270 Minister of Fire

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    Even if the meter goes to 50%, it's going to be very inaccurate for anything over 30% or so. If you check 2 pieces of the same species and one reads 38% while the other reads 45%, all it really tells you is that the second piece is wetter than the first; the 38 and 45 are likely way off.

    The meter is calibrated for a particular temperature and species -- usually fir at 70F (perfect for checking a 2x4 on a nice summer day). Taking readings off colder wood will make it read lower than it should. Some wood species have unusual electrical properties that can throw it off. Hopefully your meter's manual has tables of temperature and species corrections. If you're trying to measure the MC of a species not listed in the tables then you're kind of stuck unless you want to start weighing and oven drying samples to make your own correction tables.

    The pins need to be inserted along/parallel to the grain, preferably in the same annual ring; not in the end of a split, and not perpendicular to the grain.
  6. Jack Fate

    Jack Fate Feeling the Heat

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    Thanks, instructions are not that detailed. Will try checking at different temps. I get what you saying about readings on or near the end of the scale as being not to accurate ,as touqre wrenches are the same way

    Will pick up that $12 meter as second opinion .

    All good stuff ,can now sort data better

    Thanks all
  7. Woody Stover

    Woody Stover Minister of Fire

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    You also should push the prongs in firmly, not just poke the surface. They say to measure along one growth ring but I've not found that to make a lot of difference. Also, 11% is low because equilibrium (the driest wood can get outside) is around 13% in our area, and most of the country. I've found the General to give pretty useful readings on White Ash and Oak; When it says 16%, the wood burns well. I think the species adjustments only vary like 3%. So if you're getting 22% it might really be 19% and could still benefit from additional drying...
  8. Lumber-Jack

    Lumber-Jack Minister of Fire

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    Often people talk about the inaccuracies of these meters, but what seldom gets mentioned is the actual degree of the inaccuracies. For instance, most of the meter mention in their specs that they are accurate to something like +/- 1 or 2 %. That small inaccuracy accounts for the difference in temperature and wood species, and really isn't going to make much of a difference in how burnable your firewood is going to be. Also I have never personally found even any negligible difference measuring with or against the grain, but it's an easy thing to test that out for yourself when taking your measurements.
    However, the big inaccuracies occur when measuring the outside of splits compared to the inside of freshly split pieces of wood The outside can be a lot drier (or wetter) than the inside by 30% or more depending on the condition of the wood to begin with, how long it has been sitting, and what sort of weather it has recently been sitting in. For instance, if the wood was cut green, but has been sitting for a few months in the summer sun it could measure a very low moisture content on the outside that is exposed to the sun and wind, say as low as 15% or less, but if your were to split it open you'll likely find the inside could still be over 40% moisture content. On the other hand, if you have some firewood that has been sitting for a few years and take an outside reading right after a recent prolonged period of rain, the outside could easily read 30% MC or more, but when in fact the inside is actually less than 15% MC. As you can see, these differences are hugely significant compared to the slight +/- 1 or 2% inaccuracies involved in temperature or wood species.
    Also, as mentioned, making sure the prongs fully penetrate the wood is very important. In softer wood it's easy to stick them in far enough, but on harder wood you may have to work at it a bit.
    The trick really is to experiment and take lots of readings, focusing mostly on the inside of the wood, not the outside, then you'll get a better overall idea of what the true moisture content of the wood is.
  9. Paulywalnut

    Paulywalnut Minister of Fire

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    I don't know how long it takes to get a 16 from red oak. It would be 4 or 5 years probably.
    Something interesting. Put the moisture meter lightly on your arm and see what you get.
    The body is mostly water.
  10. Lumber-Jack

    Lumber-Jack Minister of Fire

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    Thing is, these meters are calibrated to measure electrical resistance of wood, they'll be totally inaccurate taking reading in water, or human skin, or marshmallows, etc... ;)
  11. Mr A

    Mr A Minister of Fire

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    It would be interesting for you to compare readings between the two if you're going to own them both. Please share your results.
  12. Jack Fate

    Jack Fate Feeling the Heat

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    I'd be happy to do that but ,2 things ,1) I was testing today found 3-4% variance in readings same fresh split 17-21% ash 2-5 yrs dead standiing 9 months-2 years split 2) It will be awhile before I get to Harbor Freight

    Thanks all again

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