Moisture Content

[rschoensta]

I tested some wood this morning for moisture content.
I cut about a 1" slice out of the center of a piece of recently cut and split hardwood and put it in the oven at around 225 degrees.
It weighed 10 ounces (wet weight) when I put it in and now about 8 hours later it weights 6.9 ounces (dry weight).
The difference, 3.1 ounces is the weight of water removed.


What I don't understand clearly is what is the proper method for determining the moisture content using this information.


A google search indicates there are two different procedures.
The wet method and the dry method - as I call them.

Depending on which method you use you get a big difference in the answer.

Using the dry method (weight of water removed/dry weight) the moisture content is 45%.

Using the wet weight method (weight of water removed/wet weight), the moisture content is 31%.

Anyone have an idea of what the proper method is?

 

[jdew1920]

Seems to me the logical method would be the weight of water / total weight.

Think of it this way. What if you removed 5 onces of water? Using the "Dry Method" as you show, that would result in 100% moisture content 5 oz water / 5 oz dry wood. That would make no sense.

I say you have wood with a 31% moisture content.

 

[hkobus]

You always devide by the total weight before drying. So if you devide the 3.1 oz by 10 you get 31% moisture content. When we test feed ( corn etc.) and the lab reports back they report dry matter. So they devide the 6.9 oz by 10 and give 69% dry matter (DM).

The challenge is always, do we know if we removed all moisture.....

 

[rschoensta]

First re is it really dry.
After being in the oven for about 12 hours it was down to 6.8 oz.
After another 8 hours it was still at 6.8.
I assume that would be a reasonable base point.

There are clearly two different methods.
Here is a site which explicitly addresses this.
(broken link removed)
Here is another.
(broken link removed to http://www.scionresearch.com/BKC/explanation_dry_and_wet_basis.html)

The problem is it's not clear to me which one people are using when they say wood should be at a certain percentage of moisture.

Here are some sites which say use the oven dry method.
http://extension.oregonstate.edu/news/story.php?S_No=829&storyType=garde
(broken link removed to http://www.firewood.asn.au/Moisture) Content A4 070614.pdf
http://paforeststewards.cas.psu.edu/NewsArchive/2005/05Nov22.html
http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2523/NREM-5021web.pdf
(broken link removed to http://forest.wisc.edu/extension/Publications/89.pdf)
(broken link removed to http://www.nfs.unl.edu/documents/ruralforestry/TT) November 2007.pdf
http://www.rain.org/~mkummel/stumpers/18oct02a.html

Note that several of these mention that the moisture content can be greater than 100%.
And one notes that the standard method for measuring is not intuitive. (The wet weight method would be the more intuitive method.)

There are a few sites which say use the wet weight method.
(broken link removed to http://www.coford.ie/iopen24/pub/pub/firewood.pdf) (Ireland)


Also the final answers absolute difference varies with the amount of moisture.
So for example using the 2 methods, I would get the following answers depending on the initial starting weight.
9%,10%
15%,17.5%
20%,25%
and so on to
32%,47% in my case.

So if your wood is properly cured it probably doesn't matter which method you use since both will give an answer within or close to acceptable limits.

The problem occurs in a situation like mine.
32% is high but possibly manageable if I can get some lower moisture content wood to mix in.

I would think that 47% is too high under any circumstances.

After a more thorough review it looks to me like the dry weight method is the method used in this and most countries.
It looks like Ireland is one exception.

 

[DaveBP]

The whole thing is arbitrary as long as everybody knows which method you're using. The trouble with the "wet" method is it takes as its basis the starting weight of the moist wood. That is going to vary by species, time of year, what part of the tree( sapwood/heartwood),how long it's been down before you got it onto the scale, phase of the moon, height of tide, who wins the pennant and probably other factors. Too many variables. Yes, some species can have more water than wood, especially conifer sapwood.
What you did with the oven is correct. Lab standard is 210 to 220F until it stops losing weight. No higher than 220 or you can boil off some of the volatiles like turpentines that are good fuel.
Water removed divided by oven dry wood is the U.S. Forest Products standard. That would make your wood's moisture content 47%.

I took an armload of beech and sugar maple out of a few cords that I dropped and split in June for next year and weighed and labeled the individual splits. Some time soon I'm going to bake and weigh a couple splits to get an idea what one season split, stacked and covered can do for firewood here in Northern New England. I want to do a few more next spring to get an idea of how much it can dry over the winter. Too small a sample to be defintive but I just want to put some real numbers to what is a very contentious subject. I've heard a few local 'experts' say that letting the wood get rained on helps dry it out faster!

 

[jdew1920]

The way I see it, you have all the same information either way you calculate it. Neither way offers more information. But I will say that mass mositure / total moisture is more intuitive when saying something has a certain moisture content.