Nightly Moisture Reabsorption or Redrying Wood Over and Over Again

I figured that wood had to be reabsorbing moisture from the air at night so I set out to see how much it was.

I weighed a piece of red oak morning and evening for the past month. It started at 3824 gm. and finished at 3758 gm. for a loss of 66gm which is about 2.33 oz, or a little over 1/4 cup. (1.2% of the original weight of this piece of wood in April). What surprised me was how much water it took in every night. It was on top of my stack and covered with an old storm sash window. It often took up 15 gm. or 1/2 oz. The most it took up was 26 gm, nearly an oz. Adding up all the daily decreases in weight my test piece of wood had to 'lose' 427 gm. or 15 oz. in order to lose 66 gm. The total loss was 6.5 times the actual.

It was the lightest on 9/28 at 3753 gm, 5 grams less than todays nightly weigh in.

I plotted the morning and evening weigh in. Note that on some days the piece of wood gained weight during the daytime. It never lost weight at night and only once did it stay constant. During runs when I had good loss three days in a row, the morning high was often almost the evening low from 2 days before. It also hasn't rained here much more than a sprinkle for the past two months.



This piece has already lost a little more than 1/4 of its weight since April so it is getting dry. I'd estimate it's moisture content to be in the low-mid thirties. I also checked a piece of very wet standing dead hickory that I got mid month. It exhibited similar behavior with the daily loss being much greater. I'd expect that from a piece of soaking wet wood.

 

#scifri

I love it!

 

Great stuff. Thanks for sharing.

 

What you observed is fairly consistent with the readings that are taken at fire weather stations to determine fire danger. Dead down woody fuel is divided into size classifications based on how long it takes to absorb and release moisture. 10 hour fuels are 1.4" to 1" in diameter. The moisture content is changed by relative humidity, sun exposure, wind and precipitation. Picture below is of fuel moisture sticks that would be weighed 3-4 times per day to check for weight change and see how he weather would affect fire behavior. I believe they have more modern ways of measuring fuel moisture in an automated fashion now. Great data and very interesting.

 

I have known girls that gained weight overnight! Down right scarey in the morning!

Gary

 

Science... I'm out.

 

You sir have to much time on your hands

Think I'll go have a beer now.

 

I'll have one with you. Cheers, clink. Bottoms up.

 

Good test
Looks like the downward trend is variable, with some days an overall gain.
You said no rain for 2 months, what caused some days to have increases ?
( overcast, cool, high humidity days. ? )

 

Thanks for this study and post. At your location having almost no rain for two months is opposite to here, with a very rainy summer. Interesting how overnight moisture was mostly gained - do ya suppose temperature related?
Fortunately, the 3 year plan seems to overwhelm those daily zig-zags.

 

Drying was occurring only on sunny days. I think the reason for that is two fold but related. The first is the rise in temperature that causes the RH to drop. The hourly forecast was often showing RH as high as 90% at sunrise which would fall into the 40s or 50s by mid afternoon while the dew point was remaining nearly constant. (When the dew point is constant it means the absolute amount of water in the air is unchanged.). And the second reason would be that the radiant energy from the sun was providing the BTUs necessary to get the water to evaporate. On days that the wood picked up weight during daylight hours it would have been cloudy with a higher RH.

 

Very interesting thread.
It appears to disprove the wind is more important than sun theory. And my stacks are in the shade 90% of the day, but my red oak got down to ~19% in two years. We did have an extremely Hot, Dry summer one of those years. Who knows but if we all have a system that works, good enough.

 

(where is that darn "thumbs down" icon when you need it?)

 

Very interesting PB. It's crazy how much wood absorbs and releases water during the year on just inside things like trim, doors, cabinets, etc. Expanding during the warm wet months and shrinking during dry winters.

 

How about filling in the stories and pictures behind this comment.................?

 

You are much more diligent than myself to keep up with this

your findings very much coincide with Uncle Augie findings in his solar kiln.
The porous nature of wood allows the internal passages that would normally transmit water to the tree and CO2/nutrients downward are now open on both ends, warming during the day increases the temperature within these capillaries raising the dewpoint(wetbulb depression) thus forcing water out. When these spaces cool they create a reverse depression - they can know hold more moisture that exists in the ambient air, drawing it back into the split.

 

Dude, you're a scientist. I love charts and numbers like this. Keep them coming. Enjoyed the post on the amount of moisture the piece of oak lost too.

 

that's what crossed my mind when i read this.

 

Couldn't edit my post above but saw an error. 10 hour fuels are 1/4" to 1" in diameter. In context there are 1hr, 10hr, 100hr and 1000hr fuels. They indicate how long it takes for a dead down piece of wood to get to equilibrium with its environment. It applies after the wood has been dead for a while and if one was to chart it as Paul bunion did above, it would be a jagged chart and lie flat. Since we mostly burn 100 hour and 1000 hour fuels, (1" to 3" and 3"+ respectively) you can see why it would be important to top cover your stacks as any moisture let into the pile from above would under the right conditions (high humidity and low temps for several days) be able to go back into the wood.. I'll stop before I end up talking about fuel loading and fire danger rating..

 

The more I look into this the more I find that it is heat as the most important factor. When the RH is low that is only affecting the water that is at the edges of the split, while heat will affect all of the water in the split from the edges to the middle. This is why the solar kiln works without vents to remove humid air. The heat warms the water and air in the middle of the split causing it to expand and drive moisture out of the wood.

Heat is the key.

By the time the wood cools the ambient air surrounding the wood will be cooler already, and cooler air holds less water,so any air that is reabsorbed has less water in it that the water that was expelled during the heating cycle.