Help with Moisture Content meaning and calcs needed.

[BeePeeAitch]

Hello. I fear I'm about to get laughed out the place, but I've read quite a bit today on measuring and calculating MC (wet or dry basis) and I still have a significant mental blockage... I think it might be because it seems the phrase 'Moisture Content' can refer to several different things? maybe.

 

An example: 

I have some very recently felled Willow (yea I know it isn't good to burn, but that's another discussion, this is just a 'theory' question).

I cut/split a piece and weighed it at 757g

Dried it around the house, oven, over the boiler for a week. It now seems to have stabilised at about 468g for a couple days.

 

So... the internet (and this thread) tells me that the Moisture Content calc I think I need is:

((wet wood - dry wood) / (wet wood)) * 100 = ((757 - 468) / 757) * 100 = 38.2%

[or alternatively]

(weight of water / weight of wet wood) * 100 = (289 / 757) * 100 = 38.2%

 

Can anyone tell me, is that saying that this particular piece was 38.2% moisture (in weight) when freshly felled? Could one also say that when this wood is 38.2% of it's original weight, there has been a 100% moisture drop relative to the humidity indoors? 

 

If so, when I'm looking to get wood to a magical "20%" figure I see floated around, is that 20% of this weight loss?

So using these figures - sorry I know this is painful to watch - when the piece of wood was about 526g (dry weight + 20% of the water weight) it was fine to burn? 

 

brain... ache... 

[GarethM]

Just buy a moisture meter, no maths required.

[BeePeeAitch]

26 minutes ago, GarethM said:

Just buy a moisture meter, no maths required.

 

Thanks, yes it's on the list of things to buy one day for sure. But I'd be very interested to know how it's calculated if anyone else can help please.

[Mik the Miller]

https://www.amazon.co.uk/wood-moisture-tester/s?k=wood+moisture+tester

[GarethM]

They are £9 on eBay!.

[BeePeeAitch]

Yes, thank you both.

 

If anyone else is interested in - or has experience with - the calculations behind the results a moisture meter might give, and can help me work out the maths, please chip in. 

[Paul in the woods]

What's the question again?

 

Firstly I'm not convinced the wood sample has lost all it's water as I would expect willow to be greater than 40% water when felled. Perhaps it had started to season before your experiments.

 

Assuming it has lost all it's water then the log was 61.8% dry matter and 38.2% water. 468g of dry matter and 289g of water so a 780g wet log.

 

To get down to 20% moisture you would need 468g dry matter and 117g of water so a 585g log. (To check your numbers just feed them back into your calculation).

 

I think that's right. 

 

Cheap moisture meters will only give you an approximate idea as well. They will be dry basis and you'll get different readings depending on where you push the pins in and how far.

 

Just cut, split, stack and leave the logs to get the sun and wind during the spring and summer and they should be under 20% by next winter. (And back up to 30% if we get another autumn like this one with endless rain, mutter, grumble...)

Edited November 27, 2023 by Paul in the woods

[openspaceman]

11 minutes ago, Paul in the woods said:

Firstly I'm not convinced the wood sample has lost all it's water as I would expect willow to be greater than 40% water when felled.

Yes I would expect fresh felled willow to be up to 60% water and 40% dry matter, maybe a bit less moisture in winter, especially in the middle of a larger diameter stem.

 

Given the OP's figures if in an unheated building it would tend toward 17% mc wwb  but in a warm room about 10%.

 

The way to check is heat it to 120C for 24 hours to get a oven dry weight.

 

Then

757-468=299

299/757=0.3817=38% mc wwb

 

But if it is still 10% then the oven dry weight is 468*.9=421.2 and the original water content is 335.8

 

so the original mc =335.8/757=.44=44%mc wwb which is equal to 335.8/421.2=79.7% mc dry weight basis which is how a joiner might look at it.

 

In practice my wood dries way below 20% mc wwb in a summer so I seldom get the meter out

 

 

 

[BeePeeAitch]

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? 

 

28 minutes ago, openspaceman said:

289/757=0.3817=38% mc wwb

 

...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 😂

 

 

[Dan Maynard]

4 hours ago, BeePeeAitch said:

yea I know it isn't good to burn

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