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Weighing Wood

Discussion in 'Getting Started' started by Mark Jundanian, Nov 11, 2018.

  1. Mark Jundanian

    Mark Jundanian

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    I am looking for some suggestions for scales to weigh green wood to evaluate drying. Block sizes for me are typically going range from 4 to 16 inches so I am looking for something with a reasonable range.
     
  2. Zach LaPerriere

    Zach LaPerriere

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    Hi Mark,

    If you're weighing rough turned bowls (not blanks), a kitchen scale is usually good enough. Kitchen scales generally go to 10 or 11 pounds. I have two scales, but always reach for the lighted one because it's easy to see the numbers when lighting isn't perfect. The second consideration is batteries. Get something that doesn't use tiny expensive watch batteries! Those don't last long, even in the non-lighted scales.

    Oh yeah...I'd also recommend making sure your scale can read in grams. I write weights right on the blanks I'm watching, and over time I've learned about how much percentage drop I can expect in a given species.

    I also use my scale for mixing small amounts of paint after wrapping it in cling wrap. This one has served admirably for almost a year and a half:
    https://www.amazon.com/gp/product/B0113UZJE2
     
    Mark Jundanian likes this.
  3. Damon McLaughlin

    Damon McLaughlin

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    I had started out using a kitchen scale that went to 8 pounds but as my blanks got larger they also got heavier than the scale would read. And if the size was too large I couldn't see the display. I have since gone to a postal scale with a remote display. The scale goes to 110 pounds and handles all of my rough turned blanks now. I really do like the remote display.
     
    Zach LaPerriere likes this.
  4. Zach LaPerriere

    Zach LaPerriere

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    Wow! That's a beast of a scale at 110 pound capacity. The remote display is genius. I sometimes zero out my scale with a cardboard tube riser, but that's a lot better. Never seen anything quite like that.
     
  5. Damon McLaughlin

    Damon McLaughlin

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    I've only had it a few months and never really tested to see if it would actually read 110 pounds. I too used to use risers, this is much better. It comes with an ac power adapter but it also works on batteries which is really convenient. My rough turned blanks are stored in my wood shed so instead of having to carry the blanks to the shop, weigh, then take them back to the wood shed I can now just take my scale with me and weigh everything while in the shed. Really nice! and for $25 its really not hard on the budget. My kitchen scale has found its way back into the kitchen.
     
  6. Dean Center

    Dean Center

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    Some kitchen scales weigh to 10 kg (22 pounds). Office supply stores are a good source for choices in postal scales, which also come in variable maximum weights. The Cheapskates Association recommends kitchen scales from discount stores--how many blanks over 11 pounds are you going to have?
     
  7. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Good point. I have a small digital scale that will weigh up to 12 pounds and that is all that I have needed for all but a few pieces. I would recommend one that can also weigh in grams
     
  8. Mike Bayer

    Mike Bayer

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    check with bass pro or cabelas in the meat processing area. i got mine at cabelas. it has a remote digital read. works great!
     
  9. Greg Thomas

    Greg Thomas

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    How about some feedback on how much weight needs to be lost percentage wise. I normally wait un-till a rough turned bowl stops losing weight before I consider it stable. Occasionally I’d like to speed things up, any suggestions.
     
  10. hockenbery

    hockenbery AAW Advisor Staff Member

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    Air drying with weight only makes sense if you know the RH.
    The forestry service hasn’t several on line publications to help.
    https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr117.pdf

    Heat and relative humid determine what the final MC will be in your wood drying room.
    The table below shows the MC of wood that does not loose weight.
    You can see that in humid areas wood will be 11-12% MC
    A dehumidifier will give you control of the RH and speed drying and moderate heat will speed drying.
    These are elements of a kiln. There are lots of plans for home made kilns to speed drying.
    9A670BBC-802B-4714-9E17-B301D28C8E8E.jpeg
     
  11. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Al, you make some good points, but I take exception to the first paragraph. I contend that we don't really need to know the relative humidity. The thing that we really want to know is when the wood has stopped warping so that we can we finish turning it. The chart that you show is for wood that is already dry and in equilibrium with the stated temperature and humidity in a stable environment and shows the moisture percent content of the wood. Basically, the chart is a conversion from relative humidity to absolute humidity.

    When it comes to the local weather we use relative humidity because we want to know how humid it is for the current temperature and whether it might rain or become foggy. But, relative humidity is a day to day variable and only influences the rate that a piece of wood is drying. If your weather is like mine, it might be humid one day and dry the next, but wood is only concerned about the absolute humidity in the external environment which doesn't vary as widely as the relative humidity.

    We can gather all that we really need to know by just weighing the wood and don't really need to know the moisture content of the wood. There is a trade-off ... we save the expense of buying a moisture meter, but we need to take at least two weight measurements of the wood over a period of a week or two or three. Also, weighing the wood tells us a lot about the state of the drying process. I think that I've posted the following chart before, but it's probably worth repeating.

    There is initially a very rapid weight loss as the "free water" evaporates and then a slower rate of weight loss as the "bound water" evaporates. The definition of these two terms can be found in the glossary of the FPL paper that Al referenced. When plotted on a graph, the combination of these two components of moisture loss looks like what an engineer would call a "decaying exponential" curve ... initial very rapid weight loss and then slowly converging on a final steady state value which is called the Equilibrium Moisture Content (EMC) ... when the wood is in equilibrium with its environment.

    elm_bowl_drying_a.jpg


    The above chart is from an elm bowl about 16 inches in diameter IIRC. I didn't start taking weight measurements until about a week after the bowl was roughed out. Most of the moisture loss during the first month or so is "free water". Wood doesn't shrink or warp as free water is lost, but does when bound water is lost since bound water is part of the cellulose and lignin molecules that make up the structure of wood.

    If I had waited long enough it looks like the weight would have converged on a final value of about 2300 grams (slightly over 5 pounds), but there is no need to wait for the wood to completely stop drying when you can see that it is "close enough" (which is the difference between an engineer and a mathematician). I stopped drying at day 148 and did the final turning.. I don't know the final moisture content of the bowl, but I would guess that it is somewhere around ten percent.
     
  12. hockenbery

    hockenbery AAW Advisor Staff Member

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    If your humidity averages out around 55% or lower you will be fine as long as you don’t take final weights in higher humidity months.

    My goal is an end product that will stay as a round looking bowl or a smooth rimmed platter in a normal home environment. I need Under 9% MC to achieve that.

    A bowl dried in an RH of 70% will be stable in the environment it dried in and can be turned to finish and look good in that environment it will dry more and warp if it spends months in a home with a humidity of 30% and depending on the shape it might crack.
    Most production turners use a kiln for speed of drying but also for MC consistency. They know the kiln dried bowl if quickly turned and finished with be stable in any home.


    RH and Temperature define the MC the wood can reach as well as the rate of drying.
    I have a dehumidifier in the drying room set at 45.
    This provides a stable environment. So my bowls dry to under 9% MC
     
  13. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Not always faster drying, but definitely consistent results. Trent Bosch devised a drying chamber to control the rate of drying that actually slowed down the drying time for his environment because his location in Colorado is so dry that a lot of bowls cracked from drying too fast using typical drying techniques (bagging and Anchorseal). Wood in Hawaii would never dry unless a kiln were used according to Kelly Dunn. Fortunately conditions here are just right. I can't remember the last time that I've had a turning crack while drying.
     
  14. Gerald Lawrence

    Gerald Lawrence

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    It seems to me that Bill's charts and Al's RH are only extra worry and math when for turning most risk is alleviated when EMC is reached , that is relatively no weight change for 3to 7 days. Not to slight the work these tow have done on moisture as it is enlightening it is just another step when I could be turning more wood.
     
  15. Bill Boehme

    Bill Boehme Administrator Staff Member

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    You are correct, but after I plotted the data for a couple dozen bowls of various sizes and species I now feel like I can just use the calendar for most pieces. Sometimes I will put a Post-It note in the bowl with the weight and date.
     
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  16. hockenbery

    hockenbery AAW Advisor Staff Member

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    I agree on less non turning work.
    I stopped weighing many years ago and just use time in a drying area with known humidity supported by a moisture meter reading.

    All you have to know is that the humidity of your drying areas has to be 45% or less to get a bowl to dry under 9% MC.

    In my area bowls in an open air drying shed would stop loosing weight at about 12% MC
    Jackson MS is a little more humid than Tampa FL. A bowl dried in Jackson will have a higher MC than one dried in Tampa.
    I run a dehumidifier in my drying room at 45% so that wood will dry to under 9%.
     
  17. Mike Johnson

    Mike Johnson

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    Bill Boehme likes this.
  18. odie

    odie

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    Now there is something new! If using desiccant in this manner produces results that are repeatable, and with a high success rate, then I'm sure we'll hear more about it. This is one more method where woodturners come up with creative ways to speed up the drying process......if it does it with consistency.....and, that seems to be the problem with those who've experimented with micro-wave ovens.....o_O

    He is right about one thing though.....the only real sure-fire method of knowing if your bowl has reached equilibrium MC, is by weight.

    -----odie-----
     
    Last edited: Nov 21, 2018
  19. odie

    odie

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    Sounds like a pretty good system of drying you've developed, Al.

    If you are reaching 9% MC in your roughed bowls, I'm wondering if that's better than the 12% MC in roughed bowls that are dried without a humidity controlled room.....? It does seem like a 9% MC finished bowl will actually increase in MC when it's at the end-destination.....where the MC of wood will eventually stabilize to be a bit higher......like 12%.....?

    Do you see any measurable difference in how a 9% MC bowl turns, as opposed to a bowl that is 12% MC?

    I purchased some dried roughed bowls from a turner in TN, who was using a drying room like you do. Some of his roughed bowls metered to 6% !!!! I saw no drawbacks to that at all, and those bowls turned out very nice......but.....I'm wondering if there is any advantage (or disadvantage) to drying bowls to a lower MC than they will eventually be in the location they end up at.....?

    -----odie-----
     
  20. hockenbery

    hockenbery AAW Advisor Staff Member

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    Most of my bowls will be used in homes that are climate controlled in the summer and winter and perhaps all year round. Climate controlled space is rarely above 50% humidity so my bowls will be stable.
    A 12% bowl with a rim to the center of the tree is going to get slight peaks at the rim if it dries to 8% MC inside a home.
    Size matters here
    The MC for small bowls does not matter much since they do not move as much as larger bowls and the peaks won’t be as obvious to the eye. They will rock a little if put rim down on a flat surface but look ok visually.
    Large bowls will move more and the rim peaks will be obvious.

    Movement in 14” diameter platters are especially noticeable. The thin rims will get a slight ripple as they dry more.

    Most everything gets magnified as the size of the object increases.
    The flaws in curves and wood movement become more obvious in larger bowls and less noticeable in smaller bowls. Slightly off turning techniques that we can get away with turning small bowls won’t work on large bowls.
     
    Last edited: Nov 21, 2018

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