Firewood moisture meters, and "wet basis" vs "dry basis"

[daltontrees]

OK I get it a bit more. A dry meter only needs to be an analogue to digital converter but a wet meter needs to have a calculator in it too.

FR_BEC_Wood_as_Fuel_2012.pdf FR_BEC_Wood_as_Fuel_Technical_Supplement_2010.pdf

[openspaceman]

33 minutes ago, daltontrees said:

Just when I thought I had it...

 

I will need to dig out an old publication I have somewhere about moisture content. My recolledtion was that green wood was 65% water, and hearth-dry was closer to 20%. Call it 66% or 2/3rds. If so, 1/3 weight reduction on drying brings wet % to 50%?

If we take fresh felled 1tonne  beech at this time of year, drop it in water and it sinks. It contains 52% wood and 48% water.

 

Dry it till it weighs 2/3 its original weight i.e. 666kg and we have 520kg of wood still but now only 146kg water, so 22% mc wwb actually not quite dry enough but close.

[openspaceman]

14 minutes ago, daltontrees said:

OK I get it a bit more. A dry meter only needs to be an analogue to digital converter but a wet meter needs to have a calculator in it too.

FR_BEC_Wood_as_Fuel_2012.pdf 1.65 MB · 1 download FR_BEC_Wood_as_Fuel_Technical_Supplement_2010.pdf 3.61 MB · 1 download

That is if there is a linear relationship of moisture to conductivity. Thing with resistances  where there are multi-paths with variable resistances in parallel it gets a bit complicated so I though probably a look up table and empirical results.

 

BTW that last FC leaflet shows the pellet burner we first imported in our Welsh unit near Bridgend.

[Paul in the woods]

11 minutes ago, daltontrees said:

Is there a wee mistake in the presentation of the formula, i.e. should it not be

Mwet = (100 * Mdry) / ( 100 + Mdry)?

 

Yes, I've used * rather than +, I'll edit it.

[Paul in the woods]

On 16/02/2021 at 07:52, Woodworks said:

Can we have that excellent explanation pinned on the firewood forum? 

@Woodworks has asked for something to be written that can be stickied in the firewood section. I've written a first draft that may be over simple for some but is aimed more at people who don't know much about moisture content, perhaps a log customer who is checking a delivery of logs.

 

Can I ask people to read though what's written and make any suggestions for changes. I can them make them in a new post for Steve to sticky.

 

Many thanks.

 

 

 

 

[Paul in the woods]

Moisture content of a piece of wood is defined in two ways.

 

Wet basis which is a measure of the water in a log, expressed as the weight of water as a percentage of the wet/unseasoned log. (i.e. wood and water)

 

Dry basis which is a measure of the water in a log, expressed as the weight of water as a percentage of the completely dry log. (i.e. just the wood)

 

Firewood is measured on wet basis but wood for other uses such as construction may be measured on dry basis. Many wood moisture meters do not state what basis they use and are likely to measure on a dry basis.

 

The Air Quality (Domestic Solid Fuels Standards) (England) Regulations coming into force soon stop small quantities of firewood from being supplied where the moisture content is above 20%. The law does not mention what basis is used but as it is relating to fuel it can be assumed the moisture content should be 20% wet basis.

 

If you have a cheap moisture meter which measures on a dry basis it is worth noting

25% dry basis = 20% wet basis.

 

Some useful calculations

 

Wet basis moisture content = weight water / ( weight dry wood + weight water) * 100

Dry basis moisture content = (weight water / dry weight) * 100

 

You can convert from dry basis to wet basis using the following formula:

 

Wet basis moisture content = 100 * Dry basis moisture content / ( 100 + Dry basis moisture content)

 

Further information including how to test the moisture content by drying test pieces of wood in an oven can be found in the following link. Note, this method could be used to confirm what basis a moisture meter uses to measure: https://www.forestresearch.gov.uk/documents/1979/FR_BEC_Testing_Moisture_Content_Simple_method_2011.pdf

 

Edited February 20, 2021 by Paul in the woods

[Woodworks]

Thanks Paul. I have spoken with Woodsure and even though we are pretty clear on whats to be expected this was the reply I got "We are hoping for some guidance to come out from DEFRA so may be worth waiting till then"

[carbs for arbs]

I think the write up is clear and covers everything perfectly.  Good work   

 

57 minutes ago, Paul in the woods said:

The law does not mention what basis is used but as it is relating to fuel it can be assumed the moisture content should be 20% wet basis.

 

That's the only bit I'm not certain about.  I thought I had read that the law (or official guidance) of 20% definitely is wet basis.  But I can't find that now and maybe I remembered it wrong, so take what I say with a pinch of salt!  

 

Cheers

 

[arboriculturist]

Categorically   -   WET BASIS  and a Meter that measures on a WET BASIS will cost you

circa. £300.

Edited February 20, 2021 by arboriculturist

[Woodworks]

This is where wet basis is officially mentioned. Page 7 

technicalBulletin130121PRINTWEB (1).pdf

Edited February 20, 2021 by Woodworks