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

[carbs for arbs]

Hahaha!  Burn them at the stake!  

[carbs for arbs]

But only if the firewood is less than 20%....

Edited February 4, 2021 by carbs for arbs

[carbs for arbs]

Wet basis, no less

[difflock]

I had commented on another thread, as to how dry my firewood was, according to my GEM moisture meter and averaging readings.

I kinda knew the readings were too unlikely low to be correct, so I "investigated" a little.

I had somehow, inadvertently clicked the wrong range for timber specis.

Back on the correct range it gave a more likely to be correct 20%!

I am oven drying a sample log to check the accuracy.

marcus

 

[arboriculturist]

10 hours ago, carbs for arbs said:

Some fantastic replies in here, so thanks to all.  And big thanks to @openspaceman for addressing my musings one by one.  

 

There's some other things I've been thinking which I'll write up when back from the night dog walk.

 

But aside from saying thanks, I was going to add that we don't have a microwave and I'm not sure I'll be allowed to cook a log in our newish range cooker for 24 hours! 

 

Back soon     

"You need to define accuracy. If the log has been drying the reason you split it is so the meter measures the wettest (worst case). Not having a meter I cannot experiment but I imagine one could get a feel for the average moisture content  by comparing an outside reading as well? "

 

Yes I agree, as you said -  "You need to define accuracy. " Mine was a general comment and was based on comparing pin test centre log with oven dry.

 

There is a correlation between the outside MC and centre MC which can give a reasonable overall MC of the log. But there are of course a lot of variables that effect this, species, time of year, hardwood, softwood, size of log, length of time drying.

 

I have developed my own data for this which works well as an indication of the MC of a batch of drying timber.

 

All this said, basically time and airflow is your friend when it comes to lowering the MC of timber, as has been pointed out by many many times.

Most people by now know how long they need to store their timber and under what conditions to achieve sub. 20% MC.

 

[openspaceman]

1 hour ago, difflock said:

I had commented on another thread, as to how dry my firewood was, according to my GEM moisture meter and averaging readings.

I kinda knew the readings were too unlikely low to be correct, so I "investigated" a little.

I had somehow, inadvertently clicked the wrong range for timber specis.

Back on the correct range it gave a more likely to be correct 20%!

I am oven drying a sample log to check the accuracy.

marcus

 

That's a posh meter then?

 

I've been using my multimeter and it's giving readings in the 15-30 MOhm range with the pins about 10mm apart on dry wood and about 6MOhm on fresh.

 

I dry smaller samples in the microwave but back in the day I would leave a whole log in the fan oven at 125C, I would log the electricity use and this gave a direct relationship of  the constant heat loss from the oven plus a close match of 0.7kWh per kg of water lost

Edited February 5, 2021 by openspaceman
spelling

[openspaceman]

1 hour ago, arboriculturist said:

Yes I agree, as you said -  "You need to define accuracy. " Mine was a general comment and was based on comparing pin test centre log with oven dry.

 

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.

[Khriss]

12 hours ago, Dan Maynard said:

Surely if there will ever be a prosecution then this stuff has to stand up in court? I think eventually there will be a need for impounding logs and oven drying in forensic labs, just like drink driving blood samples.

It's going to make a less exciting episode of Silent Witness though.

In these lockdown times itll still get a watching 😋  k

[difflock]

10 hours ago, openspaceman said:

That's a posh meter then?

 

 

I kinda doubt it?

No idea what I paid, but guessing sub £100.00?

Nicely made robust thing though, in a wee cordura carry case, with an optional  plug in temperature probe. 

Cheers

Mth

 

[carbs for arbs]

Well, the Amazon MM I got (link in first post) has started conking out already, so think I'll be sending that back and trying something else.  

 

I have been wondering whether a table exists that shows different species of wood and what adjustments to make to the reading for what species is being tested.   Does anyone know of such a table?  I guess it would be quite useless if you don't know what type of wood the meter is set-up/calibrated to in the first place, but it would be interesting to see nonetheless...  

 

@difflock - the only GEM meter I found was for testing grain and cost around a grand!