# Wood drying 50% humidity in air 25% wood moisture content?

#### bigealta

##### Minister of Fire
Hearth Supporter
So if the Outdoor Humidity level is say 50%RH than will, let's say. 25% interior moisture content wood splits gain or decrease moisture levels either on the surface or the interior of the splits? It must still be drying because even in our 80-85% RH summer days the fresh cut wet wood does seem to be drying some.

It depends a lot on temperature. And air flow.

Here's a chart of the equlibrium wood moisture vs air RH:

bigealta
Here's a chart of the equlibrium wood moisture vs air RH:

Wow interesting. I didn't think there was such a bid delta.

I know relative humidity is the ratio of the mass of water in the air to the saturation (100% RH) mass of water in the air. How is the wood moisture number defined?

I know relative humidity is the ratio of the mass of water in the air to the saturation (100% RH) mass of water in the air. How is the wood moisture number defined?

It's straight up percent by weight. That's why there is such a large difference between that and RH- full saturated air is still mostly air (by weight).

That chart is awesome. Always nice to see the data that supports our drying process to confirm our wood can dry to 15% in almost any circumstance.
Let’s hope we never have to verify those 130 - 150 temps though except for those who kiln dry their wood.

It's straight up percent by weight. That's why there is such a large difference between that and RH- full saturated air is still mostly air (by weight).

I'm not trying to be difficult, but by what weight? The dry weight of the wood and the water? Assuming that to be the case, one (1) lb of wet wood containing one half (½) lb of water is at 50% moisture. Then, if half the water evaporates (¼ lb), leaving ¾ lb of wet wood, the moisture content would be 33⅓%. Are those correct?

@River , excellent question. Imagine a 4 pound stick of wood baked in an oven at 240 degrees for hours and hours and hours. There is zero moisture in the 4 pound stick of absolutely "bone dry" wood.

Now put your dry stick in a big enough ziploc bag, add one pint (one pound) of water, seal up the bag and come back in a couple days. The wood absorbed all the water. The stick now weighs five pounds.

For a dry basis measurement you would compare the weight of the water (one pound) to the weight of the dry stick (4 pounds) and find the stick is at 25% MC - dry basis.

For a wet basis measurement you would compare the weight of the water (one pound) to the weight of the wet stick (5 pounds) and find the stick is at 20% MC- wet basis.

Measuring MC dry basis is a pain in the neck, requires a lot of time and specialized gear. The pin type moisture meters mere wood burners like us use, the ones that run on a single nine volt battery - all those are expressing moisture content in wet basis.

That chart is awesome. Always nice to see the data that supports our drying process to confirm our wood can dry to 15% in almost any circumstance.
Let’s hope we never have to verify those 130 - 150 temps though except for those who kiln dry their wood.
And with enough time

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