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Testing M/C in Firewood; 2 part question

Started by firewoodguy, April 12, 2006, 08:26:07 PM

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firewoodguy

Hi, I need some help. There is one method that I just read about and I'm confused on this method and the results in checking the M/C in firewood and I'm hoping to get imput or better understanding to this method of testing the M/C in firewood. I think this testing it's called or refered to as oven dry testing ? . I guess you take a stick of processed firewood and weight it. Then place it into an over at X degrees for X amount of time and weigh it again after retreaving it from the oven. Then take the difference in weight and divide it by the weight when it was retreved from the oven. Example; orginal weight 30 Lbs ; weight retreaved from oven 25 lbs; difference 5 lbs ; divide ratio 5/25 = .20 = 20% M/C

Question 1 ;
Part A ; Does anyone know what the oven temperature (degrees)would be in performing this test ?
Part B ; Does anyone know what would the oven-time window be in preforming this type of a test?
Part C ; How accurate is this test compared to any hand held Moisture meter?

Question 2 ;
I'll give a little back ground first as to the reasons of my questions. I used to cleanup the tops after a logging company that did select clearing a while back. He would perform select clearing during the winters months in the southern NH area. I would go in the lots the following winter (12 months later) and drag out all the tops and cut them into lenghts for firewood (12", 16" 20" & 24" lenghts) therefore having about 6 to 12 pile of rounds ( between 4" to 12" in diamenter) before I left that spring. This particular wood lot would be about 150 cords. So, we are not talking a huge huge single pile, just a lot of small piles . The following September ( taking in consideration that these top has now been cut down for about 18 plus months), I would re-enter the wood lot and commence to split these rounds at the time of sale and deliver it to the comsumers and offer it as "seasoned firewood". Now as to my 2ed question;
About 3 months later ( taking in consideration the firewood has now been cut for almost 24 months), I get a visit from NH Dept Weights and Measures. I will now refer to this visit as an" Adminstration Inspection" from NHDA/W&M. I guess sometime during this inspection, The NH inspector took about 5 of 6 pieces of this firewood and took them back to his lab to perform this "oven-testing" to check the M/C of the firewood. The test results came back as somewhere( as I remember) in the mid 60 % range. Taking in consideration that this firewood has and is still being cured/seasoned via Air Drying in the wood lot for about 24 months now, but its also getting wet by precipation (e.g. rain and snow) via Air Drying. It was later explained that this firewood has the same M/C as green/ freshly cut, and therefore its not dry/seasoned as its being advertised as such. My question is, how can that be ?? and what am I missing??? is this oven-testing accurate?? I guess in performing this test by NHDA/W&M, , some pieces must of weighed 30 lbs before placing it in the oven. Then weighed 18lbs when retreaved from the oven. Difference of 12 lbs ; ( 12/18 = 66% M/C ). How can this oven testing results be  66% + - , if the trees were cut down for about 24 months. What am I missing in this oven testing method., why such a huge difference in the M/C from firewood thats been cut for about 24 months ?? Any input will be greatfull. Thanks in advance

Firewoodguy

P.S.; after I was informed the results of the oven-testing M/C in the firewood and his electronic M/C tester, I then advertised it as "wet seasoned firewood" untill it was all sold.



Firewoodguy

Minnesota_boy

The oven method is the basis for the meters.  If a meter is questioned as for its accuracy then a sample is put in the oven to determine its true moisture content and the meter is judged against that.  I don't know the temperature or how long, but you can put in a sample, dry it for a while, weigh it, dry it some more, weigh it again and keep repeating until the weight doesn't change any more.

Woods keep from losing their moisture as they grow by having a moisture resistant outer cover we call bark.  Some are more efficient than others.   To make a peice of firewood dry, you remove the bark or split the wood to make an avenue for the moisture to escape.

When you pile this wood out in the woodlot, if there are other trees remaining, the shade will keep the temperature down and the relative humidity up, perhaps too high for the moisture to escape the firewood piece.
I eat a high-fiber diet.  Lots of sawdust!

beenthere

firewoodguy
MN_boy gave you good information, from what I've learned.
Also, I don't doubt at all that the mc was high on the firewood material you described. But I'd make sure to check those findings out to know for sure.

I also suspect when the gov't took the samples back to their lab, they cut a 2" cross-section about mid-way along the length of the piece of your firewood, and then cut a piece about 2" x 2" from the center of that (i.e not near the older drying surface).   Easier to dry the small pieces that way, and a 2" cube is a good size to handle.  Drying it in the oven at about 200° F for several hours, but weighing it several times to see when it stops losing weight, then it is dry for your dry weight. It's a good exercise for anyone really interested in knowing what the mc is of the wood they have, be it for firewood or air dry, or kiln dry wood. A good scale for weighing is needed. Not a bathroom scale. It's the true test of mc. Meters are an estimate of MC, based on the oven dry test.

The kiln-drying and air-drying handbooks will also describe the mc measurement using the oven-dry procedure.  Nothing better than knowing exactly what the mc range is of your wood.

I split my firewood and stack it for air drying for at least two years before I consider it dry enough to burn for heat. Anything less, I figure I am just burning it to get rid of the water in the wood.  :)
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

Ianab

Using an oven and scales is the accurate method, meters take a guess based on the conductivity of the wood. So the measurement varies with the species and even from tree to tree. They are accurate enough because we really just want to know if the wood is around 8% ( or 15%, or more than 30%)
Drying in an oven is accurate because you weigh the sample, then you bake it in the oven untill there is zero water in it (12 - 24 hours). Weigh it again, as long as your scales are accurate, a bit of quick maths will tell you what the original M/C was.

If you are in a hurry you can use a microwave oven, zap it on low for a few minutes, then weigh it again. Then zap it / weigh it again. Repeat untill it stops loosing weight for 3 weighs in a row. It's now oven dry (in 1/2 an hour instead of overnight) and you can work out the starting M/C. Dont Nuke it too fast or it will start to char (or even catch fire). This will stuff up the calculations / make a mess / set off fire alarms etc. Keep the power low, and if the wood gets too hot to touch give longer breaks and shorter zaps. A hint... dont use the wifes microwave  ;) , get an old one for the workshop.  ;D The oven will smell 'woody' afterward.

Logs left laying in the forest will dry very slowly, the bark seems to keep moisture in and there is very little exposed fibre for the water to escape. It just dries so much faster once it's bucked to shorter lengths and split.

Cheers

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Minnesota_boy

The change in moisture content may have more to do with how it was measured than an increase in the actual moisture.  Your moisture tester probably has pins that are poked into the wood.  These pins then measure the resistance of the wood and calculate the assumed moisture based on the resistance found.  The pins only project a short ways into the wood and measure at this distance, but most firewood I've seen have a fairly large cross section.  Farther into the wood, the moisture would be higher as it takes a differential moisture content to move the water out of the wood.   If the inspector used a tester, he may have driven the pins in farther than you did which gave a higher reading.  When the inspector used the oven test, he likely took a sample from near the middle of the piece where the moisture content would have been the highest.  That would explain why the moisture content appeared to increase.
I eat a high-fiber diet.  Lots of sawdust!

beenthere

You give some pretty scary testimony of what the NHDA is all about. Didn't think (other than the activities of the BATF-Bureau of Alcohol Tobacco Firearms) that kind of activity was happening in the States.

The only thing that I can think of is that your moisture test that showed low mc was taken near the surface, and possibly the samples the NHDA measured were internal to the firewood piece, thus not yet dry. Other than that, have no explanation because there are too many variables that may enter in. If treated the way you were, I wouldn't deliver air-dry firewood either. Sorry to hear you being treated that way too.

I see that MN_boy hit on the same possibility. I don't believe any of the resistance meters will accurately measure mc above about 30% mc, so the higher readings would likely come from the oven drying test.
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

jimF

the short pieces should have been much lower.  Did he take the longer ones?  I would have split them the same time as I cut them to length.  This opening of the log would have given them greater surface area and exposed it to the air instead of being "sealed' by the bark.

sawguy21

I am wondering why the customer filed a complaint. Does the NHDA encourage consumers to have their firewood deliveries inspected?
old age and treachery will always overcome youth and enthusiasm

beenthere

I was curious what the rules or law states, and did a search. Only found a Newspaper Article
of 2003 that indicated that the moisture content of purchased firewood was the buyers responsibility. Says "no legal definitions for these terms" referring to Green, seasoned, or dry firewood.  That sure is different from how you were treated.
I looked further at the NH Weights and Measures Division, and the rules section on 'Wood' was repealed in 1994. Couldn't find anything else on the subject.
Do you have a reference or Dept. in the state where the rules you have to follow are listed?  Again, just curious.

I know of no meter that can measure mc above 30 %. Maybe others do.  There is free water in the wood and that foils any attempt to measure above fiber saturation point (which is the maximum of bound water just in the cell walls).
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

beenthere

That is the range of the meters, but they don't claim accuracy above fiber saturation point.  But a reading above 30% means the wood is above 30%.

I don't have a FAX capability set up. Surely the NH dept. has a site to see what is in effect now. Hope we can help you out, or learn more ourselves.
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

kilndry

If you think there is a discrepancy between his results and the truth, I would ask for another independent test. Take a few pieces yourself over to Fernald Lumber or HHP or somewhere local, and ask them to measure the MC for you.

An oven-dry test is absolute, provided they use the "oven-dry" method (weight is measured differently for lumber/firewood than for pulpwood or chips). And no meter will give you an accurate reading much over fiber saturation point, so the oven test would be the best one.

Unless this stuff (the actual test pieces) was sitting in contact with wet ground for the whole 24 months, I find it hard to believe it was 66% moisture content. You don't find much green hardwood in that range around here.


FYI, the oven dry test  formula is:


green weight - oven dry weight/oven dry weight

Play with this formula and you will understand why it is possible to have wood over 100% moisture content.


beenthere

Quotegreen weight - oven dry weight/oven dry weight

Add parens to that formula to be sure to do the subtraction first, then divide that difference by the oven dry weight.

(green weight - oven dry weight)/oven dry weight

then multiply by 100 to get the answer in moisture content as a percent (%)

ps.  my math teach made me do this.  :)
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

kilndry

Always a math teacher looking over my shoulder!      ;D

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