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Wet vs Dry Moisture Content?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Scottydont, Jan 10, 2011.

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  1. madison

    madison Minister of Fire

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    I posted this in another thread yesterday, but it is more appropriate in this thread:
    Here are a couple of links regarding moisture meter accuracy, tips for use etc: The first pdf is the best for the science buffs (Battenkiller - you’ll be digging the first link) . Jump to pg 72 for the conclusions. Note that temperature of the wood is rather important in using the both types of MM and especially the resistance type of moisture meters

    http://www.vtt.fi/inf/pdf/publications/2000/P420.pdf

    http://www.woodweb.com/knowledge_base/Electric_Moisture_Meters.html

    http://www.ces.ncsu.edu/nreos/wood/wpn/methods_moisture.htm

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  2. Battenkiller

    Battenkiller Minister of Fire

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    Crap! I tried it and it didn't work. Not only do I have inferior wood, now my lips are full of black locust splinters. :shut:
  3. gyrfalcon

    gyrfalcon Minister of Fire

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    Picture!!
  4. Martin Strand III

    Martin Strand III New Member

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    "Burning better" is a subjective judgement.

    "Burning hot and clean" is objective as can been observed from lack of chimney smoke.

    Arguing numbers applicable to WC of wood that burns hot and clean makes as much sense as arguing stove A (industry rated at 75% 'efficiency') burns better or is more efficient than stove B (industry rated at 72% 'efficiency').

    Heat from burning wood comes from solids (30%) in the wood and gases (70%) from combustion. To get the most heat and a clean burn, firebox temperatures need to approach 1100* F. Firebox temperatures less than this result in more "wasted heat" going up the chimney, more unburned VOC's and pollutants going into everyone's air and increases chances of chimney fires from creosote build up. Most metal stoves are over fired and not designed to burn much over 900* F.

    So why do people burn wood in metal stoves?

    Aye,
    Marty
  5. Battenkiller

    Battenkiller Minister of Fire

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    Very cool, Madison. Can't wait to sink my teeth into this one. But first, on to the sad news...


    That means that by the time you can even begin to trust your meter, the wood is already good to burn. And that's with a pro meter, with species and temperature correction algorithms on board (or using the cumbersome tables instead)... if you are even half sure of the species you have (at least 1/2 of those new burners posting here seem clueless about the exact species of much of their wood).

    At best, a conventional resistance MC meter can be used as a "go/no-go" device when it comes to firewood. The higher the MC reading, the less you should trust it. If you really want to know how much water is left in your firewood, you must do what the scientists do and use the oven-dry method of MC determination. But even that won't help you to see if there is a moisture gradient in the wood (drier on the outside than on the inside) unless you take numerous samples across the thickness of the split. Is anyone (even me) really up for that?

    With the obsessiveness of some folks who insist their wood has to be be below 20% water dry-basis (17% water by weight) through and through in order to burn... good luck with that one. If you buy into that fallacy, you have absolutely no choice but to leave your wood in a windy and covered shelter, well-spaced between rows for several years.

    Thankfully, this isn't so, or we'd all have to be holding 15-25 cord under shelter for many, many years. The real truth is that once you've driven out the free water, the wood should burn acceptable well. In most locales, it will continue to dry beyond that point until it averages about 15% water by weight, at which point it will hold all of its magic basically forever, as long as it is kept dry. Allowing it to get wet over and over during numerous summers will only allow it to become deteriorated in time, losing any possible potential heat gain gotten by getting it there in the first place. In very dry parts of the country (much of the West), it will eventually become too dry, although you probably have at least three years before than happens.

    I have too much time on my hands during slow months, so I enjoy calling up major stove manufactures and bugging them with silly questions. During one such conversation (I was asked not to quote the maker, so I won't, but it was a real biggie) the engineer I spoke with said this:

    "All we really want is to get people to realize that the easiest thing to do is to get their wood cut, split, and seasoned under cover for one full year before they intend to use it. As long as they do that, they can just forget about the moisture content of the wood. If we could get them to burn wood at 30% MC or below, we'd get half the calls we do. The fact is, the vast majority of people using our stoves are buying their wood and trying to burn it right away, and that won't work in any stove. Get it down to 30% and it should work OK."

    Humorously, even the engineer seemed confused about the wet-basis/dry-basis distinction, but I'm fairly certain he meant 30% dry-basis (as read on a MC meter, which is 23% water by weight). Interestingly, the fiber saturation point (FSP) of almost all domestic wood, the point where no free water is left and only bound water (in the cells) remains is between 28% and 30% water dry-basis (22%-23% water by weight). H-mmmm.....

    Peace out.
  6. Renovation

    Renovation New Member

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    Absolutely! And I'm not suggesting it, but pointing out that we've already been doing that for a long time, and merely suggesting that we should be clear and consistent.

    Please correct me if I am wrong, but isn't it true that this forum recommends, over and over, when someone has burning problems, that as a rule of thumb their wood should be cut, split, and stacked for at least a year, and measure 20% or less on a moisture meter? (the resistance-type that is common, that measures dry-basis moisture)? Is this in any way new or controversial, or different from what the forum has been doing for a long time?

    In the OP's case, by suddenly switching to EPA test fuel moisture value, you're telling him that wood that reads 25% on his moisture meter (dry-basis) is fine, since it translates to 20% wet basis, which is in the range of the EPA test fuel specifications:

    I hope folks will correct me if I'm wrong, but in the past, wouldn't someone with wood reading 25% moisture on a standard meter be told his wood is probably too wet?

    That is not a rhetorical question--I really want answers, for from my thread searches, 20% maximum moisture (dry-weight) has always been the rule of thumb.

    I realize that every situation is different, but I'm talking about the general advice given here.

    Comments please?
  7. gyrfalcon

    gyrfalcon Minister of Fire

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    No, not really. You're translating a general target into a hard-and-fast rule, which isn't reasonable for something as variable as wood-burning, nor cheapo moisture meters from Hong Kong. "Somewhere in the range of 20 percent" would be a more accurate representation of the best advice given here. It's been said that 20 percent is ideal, but how often in life do you get to work with the ideal? A Range Rover would be an ideal vehicle for where I live, but I do just fine with my Jeep.
  8. Jags

    Jags Moderate Moderator Staff Member

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    I can only speak for myself, but I usually state: less than 25% = burnable, 20% and less = ideal.
  9. Renovation

    Renovation New Member

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    I'm not translating it into a hard and fast rule. I've been very careful to say that it is the rule of thumb, and all situations vary. I'm simply trying to be consistent about the general recommendation here.
  10. gyrfalcon

    gyrfalcon Minister of Fire

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    I find myself at a loss to understand what your problem is then. A target range in the vicinity of 20 percent is generally thought to be ideal. The farther above that you are, the more trouble you're going to have-- depending on your stove set-up, species of wood, etc., etc., blah, blah, blah. Did you actually read BK's post about how the wood burns? That's the acid test, of course. Everything else is just noise, as you will find out when you actually get your own unique set-up and wood supply and start to work with it.
  11. Renovation

    Renovation New Member

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    My concern is suddenly giving different advice to newbies, because of BK's unique situation.

    In fact, you're making my point, by equating this thread with BK's wood-drying one--the advice here is to a newbie wondering what rule of thumb to use regarding moisture, and he's received advice different from the usual.

    I've read all of BK's posts with great interest, and do not dispute the results in his unique situation. I'm trying to be consistent about the rule of thumb.
  12. Battenkiller

    Battenkiller Minister of Fire

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    George, the idea of having wood in the 20% range came about before most folks even had moisture meters. It has been on stove forums for years now, and it comes from the recommendations set forth by the hearth industry itself. It stems from the EPA test loads, which are 20% wet-basis at a maximum. There is absolutely no disputing that. Before the EPA, some folks were putting any old thing in their stoves, without a clue about the actual water contained within. Anybody who was the least bit hip knew they had to season the wood for a full year, they just didn't have any numbers to put to it.

    Then along come the meters, and now every newbie is out there claiming they have to get their wood to the right 20% MC, but they're using the wrong scale and they don't even know it. It's like a world of people living in a world where it's always -40ºF living with a bunch of folks living beside them that live at -40ºC. They don't seem to think it makes much difference which scale they use. And it doesn't, as long as they stay living at -40º. Well, not exactly, but I couldn't resit the metaphor.

    Cripes, we even have guys who never even owned a meter and have no intention of ever buying one proclaiming that wood is crap until it gets below 20% on the meter. How could they possibly know? Does their opinion carry any weight when it is just rote repetition on an invalid consensus mentality and doesn't even stem from their own personal experience?

    Bottom line is that you should burn the wood that the stove was designed to burn, at the moisture content it got the best emissions and efficiency numbers with during the test. That just makes perfect sense to me, while listening to hearsay doesn't make any sense. You don't need to hit the nail on the head here, but if you hit it at 25% MC on a resistance meter, you'll be closer than anyone ever needs to be. And all of this has nothing to do with the way my black birch is burning right now.

    You say you want the truth, but what you seem to be seeking is agreement regarding you claim to be true.
  13. Renovation

    Renovation New Member

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    Not at all. I was asking if it were not true that the rule-of-thumb advice generally given newbies is that wood should generally be split and stacked for one year prior to burning, and/or read no more than about 20% moisture content on a standard gauge, which reads dry-basis moisture.

    I have said that explicitly and consistently, no more, no less. I have made no claims as to whether that advice is valid, and as a matter of fact, if folks want to debate it here, share their own experiences and see if they want to modify it, I'm all for it. I'm sure I would learn from that.

    What I want is useful, clear advice for newbies searching for guidance.

    That may be of no import to experienced burners like you and many forum regulars, who may never read the Wiki about measuring moisture or search threads about it. But, as a newbie, I am very aware of how important that information is, and how important this site's guidance is, so I'm trying to make it as clear as possible.

    I do not doubt that experienced burners like you can effectively burn 33% MC wood in a non-EPA stove, but I don't want an inexperienced burner with an EPA stove think that that's going to be easy to do.

    I retract my request for confirmation of the 20% MC rule of thumb, for it seems surprising divisive, and that's not my intent. I'm content to point out that it seems to be the general advice given to newbies here--if people want to debate, dispute, or modify that, I'm all for it.

    I really have no more to add on the topic, unless I have been unclear in some way. I wish all well, and particularly hope that noob burners like me will find my comments useful. :)
  14. Lumber-Jack

    Lumber-Jack Minister of Fire

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    George I hear what you are saying, and I think shooting for 20% on a moisture meter is about the best "rule-of-thumb" as you're gona get. Split and stacked for a year doesn't mean much because it isn't always gona get you well seasoned wood, especially when you are talking hardwood. Man, I've heard of some people with 3 year cut and split oak that is still up around 30% MC.
    On the other end of the spectrum, the trees I cut (beetle killed standing dead Lodgepole pine) are pretty much all already below 20% on my moisture meter still standing, so there are fine to burn as soon as I cut them, no seasoning time required. However it was using my moisture meter that I learned which trees to cut and which ones to leave.
    As many people have already pointed out you can burn wood that is higher than 20% MC, it's not a magic number, it's just something to shoot for, and as far as I'm concerned though, it's the most reliable rule-of-thumb you are going to find.
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