Green wood to seasoned in one hour!

Discussion in 'Woodturning Discussion Forum' started by odie, Mar 8, 2010.

  1. Vaughn

    Vaughn

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    Huh? Robert's chart clearly shows that the boiling point of water at sea level drops as the vacuum increases. Negative air pressure. Your link discusses the boiling point of water at various positive atmospheric air pressures as a function of elevation above sea level.
     
  2. Steve Worcester

    Steve Worcester Admin Emeritus

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    The vacuum kiln principle works that way. I know of some real large vacuum kilns that are used by furniture makers and they get the wood dry much quicker and more stable than a conventional kiln.
     
  3. Robert Manning

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    It makes me want to take my vacuum pump up the mountain for a test. I wonder how that will affect the maximum vacuum of my machine. Last time I checked here at 1,000 feet, it pulled between 28 and 29 which worked great on my former pickup truck.
     
    Last edited: Mar 10, 2010
  4. steelguy

    steelguy

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    Vacuum Drying of Wood

    For those of you who loved H.S. or college chemistry or for those of you who wish you had paid more attention, look at the following link. It is a dissertaion for a student at Virginia Tech dealing with the impact of vacuum pressures on wood drying. No, you don't have to read the whole thing. The introduction gives you the Cliff notes version. Note the interesting things like the very low pressures required (18mm, Hg) and the fact that wood dries principally in the longitudinal direction. And with a good vacuum, temperature is not that important.

    A practical aspect of maintaining vacuums, is the effect of off gasing of materials under a vacuum. These off gassed materials have to be handled by the pumping action also. If the volume gets too large, the pumping efficiency is affected and the attainable pressures rise. Wet wood has a very high moisture content, hence a lot of vapor that must be handled by the pump.

    I doubt that the pumps which woodturners use for chucking have the capacity to be much good in bowl drying.
    http://scholar.lib.vt.edu/theses/public/etd-02198-185538/materials/DISS.PDF

    I still think that my wood stove process is probably the most effective.:)

    Jerry
     
  5. Bill Boehme

    Bill Boehme Administrator Staff Member

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    I think all of these tables are confusing us. All air pressures are positive since it is conventional to state atmospheric pressure as an absolute value (meaning that the zero reference is a perfect vacuum). Vacuum, on the other hand is normally stated as a gauge pressure, meaning that it is a relative pressure given as the difference between atmospheric pressure and the pressure within the vacuum container. It is normal to state this as a positive number if the term vacuum is used to indicate that it is less than the ambient pressure (but still a positive pressure in absolute terms). The pressure in a pressure vessel such as a shop air compressor tank is also a gauge pressure, so it is a relative term with respect to the ambient pressure.

    The thing to keep in mind when comparing relative vacuum values to absolute atmospheric pressure values is to subtract the vacuum value from the ambient absolute pressure to obtain the absolute pressure within the vacuum vessel. In the case of a pressure vessel, these terms are added together. Conversion of units always seem to be involved because it is typical to have vacuum stated as inches of mercury while pressure is typically stated as pounds per square inch. Atmospheric pressure seems to use an almost unlimited number of measurement scales including inches of water, centimeters of mercury, inches of mercury, bars, atmospheres, pascals, PSI, PSF, and my favorite, pennyweight per hectare.

    I will go ahead and state the obvious: in absolute terms, there is no such thing as a negative pressure since a pure vacuum is the absolute zero reference.
     
  6. Vaughn

    Vaughn

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    Thanks for the correction, Bill. You're right...I was incorrectly using positive and negative to compare atmospheric pressure and vacuum.
     
  7. MichaelMouse

    MichaelMouse

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    Read what's there, gentlemen. The first clearly states that it is a vacuum of X being drawn. The second indicates what the boiling point will be at a specific pressure (or altitude, on that "standard" day), not vacuum.
     
    Last edited: Mar 11, 2010
  8. Joe Greiner

    Joe Greiner

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    The Virginia Tech apparatus is described in US patent #6634118.

    A freeze-drying process is described in US patent #5852880, from Alaska.
     
  9. Robert Manning

    Robert Manning member

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    Steelguy, I glanced over Mr. Chens report and 18mm Hg is clearly a typo. It's supposed to be 18cm Hg which isn't that much vacuum, either. He only tests Red oak so the study has limited usefulness for drying the other hundreds of species. I'm also left wondering how this qualifies for a doctorate in Philosophy.

    Is anyone picking up a reading on their BS meter?
     
    Last edited: Mar 11, 2010
  10. MichaelMouse

    MichaelMouse

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    Are you thinking in reverse again? He means the pressure was 16 or 18mmHg depending on what process or test he's discussing. It is natural, as indicated above, to use the term pressure not the differential which would, in a STP case be 760-18 or 742 mmHg in your mind, equal to a "gage reading" of 29.21 in.Hg.

    Pressure is the only meaningful measure scientifically, since it remains the same regardless of external atmospherics. Denver, at a mile high, would show a standard pressure of around 24 in Hg, which would start the differential considerably below the sea level 29.92.

    Yes, I remember the difference between absolute and corrected altimeter readings. ;)
     
  11. n7bsn

    n7bsn

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    Em, no

    I suggest you spend just a little more time looking at the report. He repeats the 18 mm hg, over and over again (at least for times by my quick count). Plus most of his charts terminate around 18 mm Hg.

    See Wikipedia

    760 mm of Hg is (roughly) sea-level
    so 18 mm of Hg is listed as "medium" vacuum (above link). It's about has hard as the pumps many of us use for chuck can, maybe, reach. It's about the same vacuum as is in a light bulb.

    Further, actually glancing through his report, it looks solid enough that I can see how his advisor approved it for submital. It appears to be enough of his own work, with lots of references to others.

    I was interested to see the differences between red and white oak's drying, an interesting read.

    TTFN
    Ralph
     
  12. Robert Manning

    Robert Manning member

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    Thanks for your civilized persistence, Michael. Now I see what you're saying. You're speaking in terms of atmospheric pressure, which I have always thought of as 14.7 PSI, but can surely be measured as inches of mercury. I was thinking only in terms of the pressure differential on the gauge. This may answer my question on whether my pump will pull the same pressure up on the mountain. Will it be easier for the pump at higher elevation? It is starting with a lower pressure, but the air is less dense at a given temperature. I'm going to guess that this pump's maximum differential will always be the same gauge reading at any elevation, although temperature/air density, might make a difference.
     
  13. Bill Boehme

    Bill Boehme Administrator Staff Member

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    The dissertation was for a Ph.D. in Wood Science and Forest Products and not a Ph.D. in philosophy.

    As far as covering every known species of wood known to man in his dissertation is concerned, I think that the familiar anecdote about the connection between advanced degrees and specialization is applicable, "the more advanced the degree, the more that you know about less until you reach the pinnacle of academic achievement by knowing everything about nothing".

    More seriously, red oak is probably a good representative of commercially viable domestic hardwoods. Dissertations in science are generally not written in an academic vacuum (groan), but are often driven by real-world issues.
     
  14. steelguy

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    Thesis Topics and Turning

    You guys shouldn't be so hard on advanced degree people and their thesis topics. :)

    In the sciences there are normally two different doctoral degrees floating around . Ph.D. and Sc.D. Doctor of Philosophy and Science respectively. There generally is a specialization attachment to the degree. So there are PhDs in Physics, Chemistry, Engineering, and yes even philosophy..... Just like BA and BS degrees are granted for many different specializations.

    The programs tend to be 4-6 years in length and generally involve a Masters degree, also. The thesis is the independent work portion of the degree which also includes about 3 years of course work and several qualifying exams. The purpose of the thesis is to demonstrate that at least once you have done something really well, have done it independently, and that it has passed peer review. In the engineering sciences, they generally are on topics funded by some organization or company and as a result have practical relevance.

    My experience has been that those who shoot at those with advanced degrees wish they had one:) It is similar to the banter in turning groups about "flat boarders".

    I was reminded last evening about the "instant gratification" aspect of wood turning. I have taken a hiatus from turning for the past 2 months while I have been doing flat boarding. I am building a very large (102"L x 40"H x 20" D) cherry side board for our church with six inset doors. It is frame and panel construction with attendant stock preparation, mortise and tenon joints, resawn panels, staining, finishing, etc. It has about 100 bd ft in it and I still have about 6 weeks of work left.

    What has that got to do with woodturning? Well, two of my friends were making sausage yesterday (don't laugh - they are retired) and broke the chintzy plastic pusher that had been provided to pack the sausage mix into the auger that fills the casings. They dropped off the broken piece and the machinery and asked that I turn them a new one out of wood, and gee could I increase the length of the handle, put a knob on it, etc.... So I took a 1 hour break from flatboarding, and made a wonderful, very nice oak part which has style, finish, utility and could be a piece of art. I felt a wonderful sense of accomplishment and gratification. I even got suitable oohs and ahs from my wife when I showed it to her.

    This morning - back to flat boarding - Spent all morning flattening boards for the 1.5" thick sideboard top!!

    Jerry
     
  15. Robert Manning

    Robert Manning member

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    Getting back to drying the wood in one hour, and just to show you that I'm not just whistling dixie, I'm posting a photo of my pump and gauge. I'm very close to making this work.

    I have some of my own ideas for a pressure/vacuum vessel, but I would appreciate some input on ideas for a vessel. I would also like to see some information or a photo of the gauge that measures 18mm Hg. and how much this costs.
     

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  16. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Deferred gratification also has its benefits. I am on a sabbatical from turning since my wife says that I have deferred some projects too long. I know that any hope of getting a nice fancy lathe some day in the future means deferring instant gratification for a while.

    Dixie is spelled with a capital "D". ;)

    It will take something fancier than a Sears engine vacuum gauge to measure 18 mm-Hg. A round about way is to use a surplus aircraft barometric altimeter with the Kollsman window set to read pressure altitude. Next, you would need to convert from the altimiter reading of feet to the desired units such as mm-Hg. This is not a linear relationship so it would need to be computed from a standard day database with corrections for temperature and humidity (OK, so that's two more instruments that you will need).
     
  17. Richard Baker

    Richard Baker RIP

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    Robert,
    Just use whatchagot. If you can't pull 29.2" Hg vacuum (or whatever), or 100 cfm at that vacuum (or whatever), it will just take a little longer. The challenge may be finding/constructing a large enough vessel (to hold a reasonable size turning) that will withstand the vacuum. Of much greater interest to me and perhaps others is whether the method will dry a wet turning without cracking. Am inclined to be skeptical while awaiting the test results.
     
  18. steelguy

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    Vacuum Measurement

    Robert - I don't think that Fisher Scientific is in Dixie, however they carry all manner of vacuum pumps, vacuum gauges, vacuum desiccators....... (www.fishersci.com)

    An "inexpensive" vacuum gauge capable of measuring in the 0-60 mm Hg range is $177.

    Jerry
     
  19. Martin Kircher

    Martin Kircher

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    I guess no one wants to spend $14 to find out the secret and share with the rest of us.
     
  20. Robert Manning

    Robert Manning member

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    I should have known about the "D" in Dixie, Bill, given my ancestry. I'm not sure if my ancestors would consider Texas, Dixie, though, since you have flown seven different flags. Make up your mind.:cool2:

    And that particular Sears gauge is Made in USA so it could work miracles.

    Steelguy, $177 puts the project way beyond the $20 budget promised on Ebay. Cool link, though.

    I think I'm going to take Texians advice and 'run what'cha brung'. I've got my eye on a 5 gallon propane tank for the vessel.
     

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