Making Sense of Wood Moisture??

[XmasTreefarmer]

OK - this is a curiosity question - read no further if you are looking for something practical!

So we all know that if I have an oak split that tests at 20% moisture on my trusty meter, I'm good to go to burn that piece.

Let's say my split weighs 8#. If 20% of that weight is water, I have 1.6# of water in that split. Using the, "A pint's a pound the world round", that means that in that split is 1.6 pints of water. Another way to look at it is that in the split there is 6.4# of actual wood that will give me my BTU's - the 1.6 pints of water needs to be converted to steam and sent up the pipe.

My question is, Am I looking at this correctly? Something just doesn't seem right to me. That seems like it's way, way too much water to have to drive off. If anyone can shed some light on this, I'd sure appreciate it.

 

[hickoryhoarder]

8 pounds is a pretty big split, I'd say. A pint is a pound. And an 8 pound split would have 1.6 pounds of water, I'd think, at 20%.

 

[SpaceBus]

This is why manufacturers suggest 20% for the maximum water content. Less water is better, to a point, but you probably couldn't get firewood that dry.

 

[CincyBurner]

A few years ago I harvested small diameter branch wood (up to 2" diam.), cut to length, stacked in cardboard box, and recorded
weights.
1) shingle/ sawtooth oak mix
11/5/16 (C/S/S) - 30.5#
11/25/17 - 21.0#
3/11/18 - 21.0#

2) Metasequoia
Cut 11/24/17 - 29.0#
3/11/18 - 20.0#

Lots of water in wood.

 

[johneh]

Here you go kiln operators manual
(broken link removed to https://www.esf.edu/wus/documents/DryKilnOperatorsManual.pdf)
have fun chapter one everthing you don't need to know about wet and dry wood
Have not looked at this in 20 years now I remember why

 

[Ashful]

8 pounds is a pretty big split, I'd say. A pint is a pound. And an 8 pound split would have 1.6 pounds of water, I'd think, at 20%.

You must be one of them thar softwood guys. An 8 lb piece of oak 20” long is only 4” x 4”, based on 45 +/- 2 lb/ft3 at 20% MC. That’s a mighty small split.

 

[Poindexter]

Imagine a a four pound piece of wood oven dried to 0.00% MC. It is absolutely dry.

Put it in a big zip lock bag with one pound (one pint ) of water and zip the top closed.

Come back in a few days, the wood has absorbed all the water. There is no water left in the bag. You weigh the wood, it weighs five pounds now.

Wet basis: 1 pound of water of water compared to 5 # of wet wood, 20% MC WET basis

Dry basis: 1 pound of water compared to 4# of dry wood, 25% MC DRY basis, same stick.

If you paid less than $400 for your moisture meter it is almost definitely showing you a wet basis reading.

 

[johneh]

You guys realize this is way above my pay grade

 

[Ashful]

Imagine a a four pound piece of wood oven dried to 0.00% MC. It is absolutely dry.

Put it in a big zip lock bag with one pound (one pint ) of water and zip the top closed.

Come back in a few days, the wood has absorbed all the water. There is no water left in the bag. You weigh the wood, it weighs five pounds now.

Wet basis: 1 pound of water of water compared to 5 # of wet wood, 20% MC WET basis

Dry basis: 1 pound of water compared to 4# of dry wood, 25% MC DRY basis, same stick.

If you paid less than $400 for your moisture meter it is almost definitely showing you a wet basis reading.

Great point, and one that very few have ever even considered. I never even thought about it, before.

 

[XmasTreefarmer]

A few years ago I harvested small diameter branch wood (up to 2" diam.), cut to length, stacked in cardboard box, and recorded
weights.
1) shingle/ sawtooth oak mix
11/5/16 (C/S/S) - 30.5#
11/25/17 - 21.0#
3/11/18 - 21.0#

2) Metasequoia
Cut 11/24/17 - 29.0#
3/11/18 - 20.0#

Lots of water in wood.

Cincy - That is a great little experiment. It's helping me to get my head around this (at least a little bit). With your Shingle/Sawtooth Oak mix you lost 9.5# of water in that year of seasoning - about 1.2 gallons! That helps me to picture the water - I'm picturing your wood in that cardboard box at the end of your test with a gallon jug of water in there, which is now not in the wood.

 

[XmasTreefarmer]

Here you go kiln operators manual
(broken link removed to https://www.esf.edu/wus/documents/DryKilnOperatorsManual.pdf)
have fun chapter one everthing you don't need to know about wet and dry wood
Have not looked at this in 20 years now I remember why

Hey John - I downloaded that and I'll see if I can get my head around it. It does look like a lot to digest. One thing I noticed was some info about the "wet basis and the dry basis" which Poindexter mentioned. That is helpful, because it does account for a 5% difference in his example depending on if you are using wet basis or dry basis - and that is quite a bit!

And I also think that this is way above my pay grade too!

 

[XmasTreefarmer]

You must be one of them thar softwood guys. An 8 lb piece of oak 20” long is only 4” x 4”, based on 45 +/- 2 lb/ft3 at 20% MC. That’s a mighty small split.

Ashful - This is really getting somewhere! I think I finally got it Here's what I think I know.

Part of my confusion here has been that I had forgotten the weight of the wood itself - that it is still "heavy" even in the absence of moisture.

So, for your example - Green Oak weighs between 60-70#/cubic foot according to Penn State. So your example piece at 4"x4"x20" is 0.18 ft3 so would start at 10.8# on the low side and 12.6# on the high side. Let's go with 11# to keep the math simple. So your piece goes from 11# when first cut to 8# when seasoned to 20%MC. That is a loss of 3# of water which is 3pts.

If you take that piece and picture that being all water - it would be 11pts. I can now see how 3pts of water could come out of that size piece of wood! I just could not see that. Now I can see that and picture that piece of wood with more "air space" in it where the water used to be! Ouch - my head hurts! Thank goodness for a moisture meter - that's enough thinking for me.

 

[Ashful]

Ashful - This is really getting somewhere! I think I finally got it Here's what I think I know.

Part of my confusion here has been that I had forgotten the weight of the wood itself - that it is still "heavy" even in the absence of moisture.

So, for your example - Green Oak weighs between 60-70#/cubic foot according to Penn State. So your example piece at 4"x4"x20" is 0.18 ft3 so would start at 10.8# on the low side and 12.6# on the high side. Let's go with 11# to keep the math simple. So your piece goes from 11# when first cut to 8# when seasoned to 20%MC. That is a loss of 3# of water which is 3pts.

If you take that piece and picture that being all water - it would be 11pts. I can now see how 3pts of water could come out of that size piece of wood! I just could not see that. Now I can see that and picture that piece of wood with more "air space" in it where the water used to be! Ouch - my head hurts! Thank goodness for a moisture meter - that's enough thinking for me.

... and the student becomes the Master.

 

[XmasTreefarmer]

... and the student becomes the Master.

Ha! Ashful, I will always be "the Grasshopper" to you!

And to sort of "close out" this thread - Let me put in one more thing for your consideration:

With our example of the 4"x4"x20" piece of oak.

It's going to weigh 8# at 20% MC, I think that means that at 11# green it will be at 28% MC and on the upper end of green oak, at 12# for that piece, it would be at 30%MC. Interesting that if I am correct, oak does not start with a lot of moisture, but it sure gives it up slowly after it's cut and split. I always give my red oak 2 years CSSC (Cut-Split-Stacked-Covered). Sometimes after one year it will be down near 20%, but to get it consistently below 20%, I need 2 years here in Southern WI.