Question about moisture in wood

[Paul Williams]

I have been working on some twice turned and once turned oak bowls and got to thinking about if I could calculate when a bowl had reached a particular moisture level. I don't think it is this simple, but if a bowl weighed 4 pounds and the moisture meter said it was 25% water would it weigh 3 pounds at zero moisture. Perhaps more realistically if it we removed half of the water or 1/2 pound would it then be at 16.7% moisture content.

Of course that all assumes that dry wood and water weigh the same and that a moisture meter is really saying that at 25% moisture content 25% of the weight is water. I doubt that is true, but there is probably some relationship and I might be able to calculate how dry my bowl is by it's weight.

[Tom Wilson66]

The percent moisture content of the wood is calculated as the weight of the water divided by the weight of the wood, then this number is multiplied by 100 to make it a percent. In the example you gave, the moisture content if you have 4 pounds of total weight of which 1 pound is water would be 1 divided by 3 times 100 which is 33 percent. (To get a 25 percent moisture content the original weight of the water needs to be 0.8 pounds) Rather than go through all this math, most turners will concern themselves with the equilibrium moisture content, which is when the wood no longer takes on or gives up moisture from the atmosphere. This varies with the relative humidity of the area the piece is in, but is determined easily by the piece neither gaining or losing weight over a period of time. The number the moisture meter gives is dependant on a number of factors, and can be off a bit. The only way to find the actual moisture content of the wood is to weigh it while it is wet, then dry it out in an oven, and weigh again. The actual moisture content can then be calculated. Hope this helps.

[Paul Williams]

Thanks for the correction Tom. I normally do what you suggest as far as drying until it no longer looses weight. It is more of an interesting exercise than a desire to know when to return the bowl. What started my thinking about this was being in a hurry with one bowl for a friend. I turned it thin from wood that measured 25%. I got to thinking about what the moisture content might be as it dried. I was hurrying it along in the microwave, and wondered if I could calculate when would be a good time to stop.

[Tom Wilson66]

Since you know the percent moisture of the wood, you can calculate the dry weight of the wood in the bowl. If your friend has an air conditioned house, you probably want about 8 to 10 percent moisture. You could use these to determine the weight you need to get to, if you weighed the bowl before you started microwaving it. For instance, if your bowl weighed 1 pound (calculated DRY weight) and you want 10 percent moisture, you would need to have 0.1 pound of water left. So when the bowl got to 1.1 pound, it would be at 10 percent moisture.

[Michael Mills]

As Tom said, you need to look at the equilibrium moisture content. Here is a link which gives equilibrium by city starting on about page 4.
For me about 14% is good to go, however, inside a house is normally lower and sometimes I bring it in for a few weeks or a month prior to the final turning. My meter has guides/charts for about 30 different wood species but I really don't bother to change the generic setting. I would think basswood and ebony would lose different % of weight going from 30% to 10% but maybe not.
http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf

[Tom Wilson66]

" I would think basswood and ebony would lose different % of weight going from 30% to 10% but maybe not."

Both would lose 20% of the weight of the dry wood, whether it is basswood or ebony. Moisture content does not depend on the specific gravity of the wood involved, only the relative weights of the water and the wood. By the way, the Forest Product Laboratories has lots of wood information besides the equilibrium moisture contents charts. Worth a look for anyone interested in the science of wood.

[charlie knighton]

the wood blank dries outside to inside......not evenly