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

[Dan Maynard]

Still don’t understand.

Examples please

[emoji106]

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.

 

1kg / 4kg * 100 = 25% dry basis (water compared to dry wood)

 

1kg / 5kg * 100 = 20% wet basis (water compared to wet log starting out)

 

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

[Paul in the woods]

14 hours ago, Rough Hewn said:

Still don’t understand.
Examples please

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%

 

Edited February 16, 2021 by Paul in the woods

[Rough Hewn]

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%
 

Who uses dry basis?

[carbs for arbs]

4 minutes ago, Rough Hewn said:

Who uses dry basis?

 

As I understand it (someone correct me if I'm wrong), the building trade uses dry basis.  And almost all moisture meters measure on a dry basis.  

 

 

Some great replies in here, thanks everyone.

Edited February 16, 2021 by carbs for arbs

[Paul in the woods]

10 minutes ago, Rough Hewn said:

Who uses dry basis?

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.

 

Firewood, biomass woodchip etc on wet basis.

[Wonky]

Umm, now I understand 

 

[trigger_andy]

So my cheap Sthil meter is most likely a dry basis? So when Im measuring timber for use in the house if it reads 25% I should read it as such but if I was to stick that same piece of timber in the fire I should read it as 20%?  

[carbs for arbs]

@trigger_andy - yes, I think you're right, as bizarre and daft as it sounds!

[Woodworks]

25 is the new 20 😆

[Rob_the_Sparky]

On 05/02/2021 at 10:23, openspaceman said:

I was trying to point out that if any meter gives a consistent result it is still usable even if the result is wrong because a correction can be applied, whereas if it gives results with a wide scatter even if they are centred on a single result it is worthless.

 

Not having used one I am still in the dark.

 

Take target shooting, if you strike the target in the same place and every time it hits the same spot within a couple of inches but several inches from the bull it is precise and you need to offset your point of aim from the bull to be able to consistently hit it. If the shots scatter randomly around the bull by a couple of feet then it is accurate but imprecise and not usable.

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

 

Noise is the scatter and if you average out noise then you get the correct answer.  This is what gives you the variation measurement to measurement.  Bias gives you an error in the reading no matter how much averaging you do, it is an offset from the genuine value you are trying to measure.  The noise is most simply described using standard deviation but as complicated as that is, it is still a simplification.  It is a case of the more you know the more complex it becomes I'm afraid and probably not something you want to go to deeply into.  The bias is best calculated by taking the mean of a lot of readings.

 

Note: all of the above applies when trying to measure something constant.  With wood the material is not constant from tree to tree and species to species.

 

I'd be surprised if you get any sort of bias or noise data from a moisture meter manufacturer.  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.  Poor match between the look-up table and wood being measure = poor accuracy (bias).  I don't know how much variation there is in moisture for a given resistance but I suspect it is quite a lot given all the variations in the wood.  Those who have done a load of oven drying would know though.