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

[openspaceman]

33 minutes ago, Rob_the_Sparky said:

Sorry, a bit late with this reply, but what you are describing is the difference between bias and noise.

Yes they are the terms in your industry  but accuracy and precision are more engineering terms, @Stubby may remember but is probably not interested as he will burn wood that is dry enough without worrying about measurement.

33 minutes ago, Rob_the_Sparky said:

 

 

  As far as I can tell the meter is set-up to measure high resistances then uses a look-up table (or a simple equation) to convert that resistance reading into a moisture reading.  They are not directly measuring moisture, the reading is an estimate of moisture based on the look-up table.

That was my guess too

33 minutes ago, Rob_the_Sparky said:

 

Those who have done a load of oven drying would know though.

Oh I've done lots but have never owned a moisture meter

 

I'm still basing my thoughts on the chips for moisture meters mostly being aimed at measuring moisture or dampness in the building or carpentry businesses.

 

But as has been said there is a direct calculation that can swap between measurement systems.

 

To my mind the problem will be with customers owning a meter but logs delivered with an equilibrium moisture content in 90% RH should still fall under 20% mc wwb.

 

I used to be involved in pellet making and one of the requirements was a moisture content of 10% and at this level wet weight basis and dry weight basis converge, the thing was that pellets depend on lignin fusing to form a skin and the sawdust particles being forced close enough together for weak hydrogen bonding to glue them, this is also how paper fibres bond. In a normal British winter in an unsealed container the pellets would fall apart as the ingress of moisture destroyed the bonding.

 

 

[openspaceman]

1 hour ago, Paul in the woods said:

As has been said, I gather it's the building trades so construction timber would be dry basis. Your planks I would assume would be sold dry basis.

There is still some level of conjecture on this point so while @Woodworks did check dry a sample after using his moisture meter and the result did lead to this conclusion we still don't have it from the horses (manufacturers) mouth, nor am I sure how farmers measure grain moisture, @Billhook perhaps knows?

 

The thing about selling planks is I doubt you get the opportunity to measure other than on the surface whereas you split a log to expose what you believe will be the wettest point for measurement. Of course people buy planks on volume and seldom worry about moisture unless they start growing mould, cup, split or check in use.

 

[openspaceman]

2 hours ago, Dan Maynard said:

Take a 5kg log off the stack. Put it in the oven for a day or two, dry it completely and then find it weighs 4kg, so there was 1kg of water.

Oven needs to be above 100C and I use 120C, nowadays I use a microwave

2 hours ago, Dan Maynard said:

 

 

Or at least that's the way I picture it. How they can have written the law without specifying this baffles me.

They did:

 

""prohibited level”, in respect of a relevant unit of wood, means a moisture content of more than 20%; "

 

Pretty well defines it as water must not exceed 20% of the whole weight.

 

The confusion arises with the moisture meters and at best these are only checks, for proving an offence oven drying is probably bound to be used if indeed anyone is ever brought up for it.

 

 

BTW I supplied myself with 2.25m3 of freshly felled and split beech billets yesterday, had them delivered in a Citroen relay horsebox, I estimate it must have been overloaded at about 1.5 tonnes.

 

I have not given myself a letter explaining I must keep it for two years before burning.

[Woodworks]

2 hours ago, openspaceman said:

 

 

 

BTW I supplied myself with 2.25m3 of freshly felled and split beech billets yesterday, had them delivered in a Citroen relay horsebox, I estimate it must have been overloaded at about 1.5 tonnes.

 

I have not given myself a letter explaining I must keep it for two years before burning.

No need for that until May 😀

[Billhook]

3 hours ago, openspaceman said:

There is still some level of conjecture on this point so while @Woodworks did check dry a sample after using his moisture meter and the result did lead to this conclusion we still don't have it from the horses (manufacturers) mouth, nor am I sure how farmers measure grain moisture, @Billhook perhaps knows?

 

The thing about selling planks is I doubt you get the opportunity to measure other than on the surface whereas you split a log to expose what you believe will be the wettest point for measurement. Of course people buy planks on volume and seldom worry about moisture unless they start growing mould, cup, split or check in use.

 

We always used to measure grain by grinding it in a coffee grinder then compressing it in a small chamber on top of the actual meter.  I think that modern combines can do it fairly accurately without having to do all that but I am not sure of the process

[Paul in the woods]

8 hours ago, openspaceman said:

There is still some level of conjecture on this point so while @Woodworks did check dry a sample after using his moisture meter and the result did lead to this conclusion we still don't have it from the horses (manufacturers) mouth, nor am I sure how farmers measure grain moisture, @Billhook perhaps knows?

 

The thing about selling planks is I doubt you get the opportunity to measure other than on the surface whereas you split a log to expose what you believe will be the wettest point for measurement. Of course people buy planks on volume and seldom worry about moisture unless they start growing mould, cup, split or check in use.

 

Going off topic a bit, but if anyone is interested, I found an American site that has some details about their wood working moisture meters.

 

Details in the FAQ section here: https://www.delmhorst.com/faqs/woodworking-industrial-and-mill

 

It states "Resistance-type meters express moisture content as a percentage of the oven-dry weight of the wood." So that's dry basis.

 

It also talks about driving pins a quarter of the way in to a board or using a pinless, magnetic field meter.

 

Also, I posted this up a while back, https://www.forestresearch.gov.uk/documents/1979/FR_BEC_Testing_Moisture_Content_Simple_method_2011.pdf

 

That has the statement:

 

“Dry basis” is expressed as the percentage of the oven dry weight of the wood. For example, if the wood in a piece of timber weights 50kg and the water also weighs 50kg then the dry basis moisture content is 100%. The main advantage of this method is that the oven dry weight of the wood remains constant. This method is the standard used by many of the organisations doing research on wood, as well as building surveyors and architects. (It is rare to use dry basis measurements when talking about woodfuel) The MCdb = (Weight of water in a sample/ oven dry weight of sample)x100

 

I have assumed that if an architect is specifying moisture in dry basis then a chippy will be working with dry basis. I could be wrong of course.

[daltontrees]

Thanks for explanations everybody, Head-spinning stuff. I agree the new rules seem to be on a 'wet' basis. The 'wet' basis is the most intuitive one I think as it doesn't re;ly on knowing the hypothetical oven dry weight, the important bit being how much drier is it than the green weight.

Or in words, the wet basis is now much of the log is water. The dry basis is how much heavier is it because of water content.

Dry basis could be a useful indication of re-humidification. Ther's no doubt that wet basis is most obvious measure of how much the wood has been dried from green.

PAUSE FOR THOUGHT

The electrical resistasnce of bone dry wood is high. Compared to wet it is very high. I think that effectively conductivity is very nearly proportional to free moisture content. I don't know how meters can automatically calibrate to measure dry weight %, whereas wet weight % would require no calibration. basically a resistance meter.

[openspaceman]

29 minutes ago, daltontrees said:

Thanks for explanations everybody, Head-spinning stuff. I agree the new rules seem to be on a 'wet' basis. The 'wet' basis is the most intuitive one I think as it doesn't re;ly on knowing the hypothetical oven dry weight,

We have mostly known that all along, the point of the thread I thought was deciding which moisture meters measured on the wet basis and I don't think we have found one that does yet.

 

If we put aside poplar, elm, willow and softwoods then the worse case hardwoods are oak and beech, if people have crane scales then one could be fairly certain if 1/3 of the green weight is lost the average moisture content will be below 20%

[daltontrees]

1 minute ago, openspaceman said:

We have mostly known that all along, the point of the thread I thought was deciding which moisture meters measured on the wet basis and I don't think we have found one that does yet.

 

If we put aside poplar, elm, willow and softwoods then the worse case hardwoods are oak and beech, if people have crane scales then one could be fairly certain if 1/3 of the green weight is lost the average moisture content will be below 20%

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%?

[daltontrees]

12 hours ago, Paul in the woods said:

Say you have a fresh piece of tree.  Green Weight = 2,000g which consists of 1,200g of dry wood and 800g water.

 

Wet basis the moisture would be

= weight water / ( weight dry wood + weight water) * 100

= 800 / ( 1200 + 800) * 100 = 40% wet basis

 

This can be more simply written

=weight water / green weight * 100 = 800 / 2000 * 100 = 40%

 

Dry basis the moisture would be

= (weight water / dry weight) * 100 = ( 800 / 1200 ) =  66.7%

 

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

 

Mwet = 100 * Mdry / ( 100 * Mdry)

 

which in the above example is 100 * 66.7 / (100 + 66.7) = 40%

 

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

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