Help with Moisture Content meaning and calcs needed.

[openspaceman]

3 minutes ago, BeePeeAitch said:

Thank you very much both for the detailed replies. I need to digest them to make sure I understand. I'm still trying to see the relationship between the result I'd get doing a calculation after oven drying, that's presented as %MC WWB, and the reading that a (imaginary perfect) moisture meter might give? 

 

 

...assuming these figures really were my 'oven dry' results - and I know you've both said they probably aren't - then for this piece of wood, would a theoretically perfect Moisture Meter reading be 0% ?

 

If not, why? 

 

Thank you, thank you, I realise this probably seems pointless and I really should just buy a Meter 😂

 

 

Yes if the wood is oven dry there is no moisture left in the sample. I had been experimenting with wood drying for over 25 years before I bought a meter and even now I would only use it to get a general indication and would revert to oven drying for a definitive answer.

 

My moisture meter gives up at less than 10% and I'm not sure that it measures wwb or dwb. For us wood burners 25%dwb is 20%wwb  so not a great difference as the figures get lower.

[openspaceman]

2 minutes ago, Dan Maynard said:

This bit however might get you laughed out of town, willow is great to burn when dry.

The mantra is "any wood is good wood if it is dry" but there are a few caveats in that many woods that have a high moisture content when fresh felled are not dense when dry so occupy a lot more space in the woodshed (and firebox) so you are continually stuffing the fire whereas a dense hardwood like beech keeps burning longer.

[daltontrees]

21 hours ago, BeePeeAitch said:

brain... ache... 

Yes this can be hard to understand.

But think of it this way. When wood is growing it might be typically 50% moisture (but depends on species).

Your calculation so far ony shows that the wood has lost 38% of its weight. Call it 40% for round numbers.

IF, and only if, you know that it was 50% moisture at the start, you could say that it's weight is now made up of 50% of its original weight in wood and 10% of its original weight in water. The ratio of these two 10:50 is 100% x 10/50 = 20%. That's the moisture content.

But, you can never really know if the original 50% is right. It might be 55%, it might be 45%. This would give you different moisture content figures. (18% or 22%).

The only way to be sure is to oven-dry the piece, get it's 0% moisture weight and then let it rehydrate and weigh it again. But who would ever do that?

I tried to calibrare a pile of cedar once. I found that different parts of the same ring had quite significantly different moisture contents. It's pretty futile.

Measurement by meter is a different matter. It goes on basis that electrical conductivity of wood increases in proportion to moisture content. No calculation or calibration is required.

So, weight loss is is a good indication of water loss but tells you nothing absolute.