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Wood MC/Math problem.

Post in 'The Wood Shed' started by JustWood, May 4, 2009.

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  1. JustWood

    JustWood Minister of Fire

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    A 10 lb piece of wood measures 50% MC. How many lbs will this piece of wood weigh when the MC measures 10% ?

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  2. mayhem

    mayhem Minister of Fire

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    Dunno. Is it on the same track as the train that left Chicago at 7:15?

    I keep looking at that question and keep thinking there isn't enough information to solve the problem, but I don't know what info thats missing.
  3. JustWood

    JustWood Minister of Fire

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    I don't know all the info either, that's why I'm a askin'. :bug:
  4. fossil

    fossil Accidental Moderator Staff Member

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    Assuming your percentages are by weight, then your 10 lb piece at 50% is 5 lbs of wood and 5 lbs of water. After the moisture has decreased from 50% to 10%, the piece should weigh 5.556 lbs. Rick
  5. johnn

    johnn New Member

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    A 10lb. piece of wood,,,is going to weigh 10lb.~s
  6. pastera

    pastera Feeling the Heat

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    If the 50% is by the total weight, then the wood weight is 5 Lbs.
    At 10%, the weight would be

    5Lbs x (1 + 10%) = 5*1.1=5.5 Lbs

    If the 50% is by moisture weight compared to wood weight:
    Total = wood weight + (wood weight * moisture)
    10 = ww + (ww * 0.5)
    10 = 1.5*ww
    Wood weight = 10/1.5 = 6.67 Lbs

    Total = 6.67+(6.67*10%) = 7.33 Lbs

    Aaron
    Someone please double check - writing on an iPod is not conducive to any clear thought process
  7. JustWood

    JustWood Minister of Fire

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    Does this mean that .634 gallons of water were removed from the wood? Not being a smart azz. Trully curious how this works out.
  8. pastera

    pastera Feeling the Heat

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    How is moisture content measured??
    Is it a percentance of total weight or a comparison to the dry weight of the wood?

    Sorry if I am being to lazy to look it up
    Aaron
  9. fossil

    fossil Accidental Moderator Staff Member

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    Oh c'mon. Starts out at 10 lbs total weight, half of which is water. So there's 5 lbs of wood there. If you got it to 0% MC, it would weigh 5 lbs. At 10% MC, the 5lbs of wood plus it's 10% moiture content brings the total weight to 5.556 lbs. Going from 50% MC to 10% MC, you evaporated out 4.444 lbs of water, or about half a gallon (2 quarts). Rick
  10. pastera

    pastera Feeling the Heat

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    My calcs show either 4.5 Lbs or 2.67 Lbs
    Quite a lot of water!!

    Aaron
  11. jdinspector

    jdinspector Feeling the Heat

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    I have to weigh in here (pun intended). Actually, wood can't get moisture readings of 50%. The saturation point of wood is around 35%-40%. depending on the species and the temperature. If you are registering 50% moisture readings with a digital moisture meter, it is giving you erroneous results. Most meters will interpolate readings above 40%. Sophisticated meters (costing many hundreds of dollars) will frequently cut off at 40%. I have one that I consider my most accurate meter (A Protimeter SurveyMaster for those keeping score) that reads 99% readings. Since I use it for work related things most of the time, I don't report wood moisture readings over 40%.

    So, perhaps the question is more appropriate to ask -A 10 lb piece of wood measures 40% MC. How many lbs will this piece of wood weigh when the MC measures 10% ?

    I can't figure that one out.
  12. fossil

    fossil Accidental Moderator Staff Member

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    6.667 lbs
  13. pastera

    pastera Feeling the Heat

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    I looked it up and while I did not find a definitive answer, I did find several references to >100% moisture content.
    Therefore, only the second definition in my calculations is valid:

    Total weight = wood weight * (1+ moisture percentage)

    Solve as needed for the unknown

    Aaron
  14. fossil

    fossil Accidental Moderator Staff Member

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  15. jdinspector

    jdinspector Feeling the Heat

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    I think you're right. If there is a 100% moisture reading, then it's water!. It can't be more than 100%. However, I do recall that some of the meters out there display readings that are displayed as a percentage of the dry wood weight. Quick research (google) has a an article from the University of Tennessee stating that "Moisture content of greater than 100 percent means that there is more water in the wood than there is dry wood substance." http://www.utextension.utk.edu/publications/wfiles/W179.pdf

    Now I'm really confused!
  16. fossil

    fossil Accidental Moderator Staff Member

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    In that case, then they're talking about some sort of "relative" percentage, rather than an "absolute" value. I think that most commonly, wood MC is expressed as an absolute value...the percentage of the total weight of the wood in question that's attributable to water. It will always be less than 100%, although it can be more than 50% in some species of living trees. In humans it's anywhere from ~55% to ~ 70% or so. Rick
  17. jdinspector

    jdinspector Feeling the Heat

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    Interesting article in that Wiki. They state 25%-30% max. is the point of wood fiber saturation. I may have to move my moisture reporting levels down from 40%. You see, much of my work relates to moisture testing of materials. In particular, I frequently measure wood moisture behind wall claddings (stucco, EIFS, wood siding, brick, etc.) My reports are often used to determine who is responsible for making repairs to residential or commercial property (it boils down to who pays for what, and how much). Oftentimes, 1 or 2 % can make a big difference.

    In the case of firewood, I think the 20% rule that I so often read about is probably good enough. Getting firewood much below that is difficult and verges on pointless. I simply look for wood that burns. I don't test moisture in my firewood too often.
  18. jdinspector

    jdinspector Feeling the Heat

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    I couldn't resist. I just ran out to my work truck and stuck my meter in my arm. You're right! I was 65%. My wife is so pissed about the blood in the kitchen...
  19. LLigetfa

    LLigetfa Minister of Fire

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    This may fall into the "too much information" category but I drank an awful lot of water today doing a bowel prep for day surgery tomorrow, so I think my MC might exceed 70%.
  20. hensonconst1

    hensonconst1 Member

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  21. btwncentres

    btwncentres New Member

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    Hi....The ultimate way...truly accurate... is the Oven dry Method.....cut a cross section of wood....no longer .. with the grain...than an inch or so... weigh it...this is the Wi weight .....dry it in an oven until it no longer loses weight....weigh it...this is the Wod weight.....eeeek.. don't know how to write it out on computer....I'll verbalize.......Moisture Content equals.. bracket....Wi times Wod over Wod ... bracket...times one hundred........50 % Moisture content is not 50 % of the weight...you can have over 100 % Moisture Content.
    ............. MC = ( Wi X Wod ) X 100
    .......................... _______
    .......................... Wod ...........Have fun..................ugh..can't get that line and Wod to move over...g'night......... ..............actually............cut your wood way ahead of time...the ends will all check...the drier it gets the more musical the wood sounds when you hit it......at furniture grade it sings....................................wood's the most amazing thing on earth.................................I know..I'll put dots to move them over............oops..too many.......ahh..much better.........wood's amazing but I love the Edit feature.........................again g'night.......................
  22. pastera

    pastera Feeling the Heat

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    From what I read, the moisture content is measured by the water weight as a percentage of the dry wood weight not the total weight.
    If you have a split the fully kiln dried would weigh 1 Lb and add 1 Lb of water you get 100% moisture content.

    Aaron
  23. jebatty

    jebatty Minister of Fire

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    In the lumber industry, MC% = (wet weight - ovendry weight) / ovendry weight x 100%. Aaron had it right in his calculations.

    BTW, using this formula, many green woods have more than 100% MC; a few are: aspen, basswood, and cottonwood. Your favored oak is about 80% and sugar maple 70%.
  24. wendell

    wendell Minister of Fire

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    May fall into? You definitely crossed the line there! :)
  25. Jags

    Jags Moderate Moderator Staff Member

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    You OBVIOUSLY have never seen the commercials for "Sham-wow". ;-)
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