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

[arboriculturist]

Approaching £300 to buy a MM that can be programmed to measure wwb - bearing in mind you will be lucky to find any supplier whatsoever in the UK that has a clue what you are talking about.

 

Don't bother contacting Woodsure.

[Rough Hewn]

Had a regular turn up last week with his new field MC meter.
Air dried 1” ash measured 35%.
Bollocks...
Then he recalibrated it.
12%.🤣🤣🤣[emoji106]
That’s for milled timber.

For firewood yiu can get one from stihl fir £10-£15.
Cut your log in half, stick prongs in.
Less than 20% it’s good.
More than 20% it goes back in the shed.
Fir firewood it’s really not rocket science.
[emoji849]

[Woodworks]

17 minutes ago, Rough Hewn said:

Had a regular turn up last week with his new field MC meter.
Air dried 1” ash measured 35%.
Bollocks...
Then he recalibrated it.
12%.🤣🤣🤣
That’s for milled timber.

For firewood yiu can get one from stihl fir £10-£15.
Cut your log in half, stick prongs in.
Less than 20% it’s good.
More than 20% it goes back in the shed.
Fir firewood it’s really not rocket science.

Its not rocket science but it's a fine line between being compliant with the new regs and not so worth being accurate 

[Paul in the woods]

25 minutes ago, Rough Hewn said:

Fir firewood it’s really not rocket science.

I cannot find any info what basis that one uses either. As it can be used for construction it's likely to be dry basis, so we're back to the same problem.

[Rough Hewn]

I cannot find any info what basis that one uses either. As it can be used for construction it's likely to be dry basis, so we're back to the same problem.

It’s firewood.
You are measuring moisture content.
Stick the prongs in a piece you’ve just split.
That’s it.
The variation is +\-2%?

Measuring timber mc is very different as you can’t split the piece you want to measure.

[Paul in the woods]

8 minutes ago, Rough Hewn said:

It’s firewood.
 

You're missing the point. The new regs means sellers of <2 cubic meters of logs must sell under 20%. After a bit of research that's wet basis. Most, if not all, cheap meters use dry basis. 25% dry basis = 20% wet basis.

 

So, if you deliver your logs at the legal limit of 20% wet and some know it all whips out their cheap meter it'll say 25% and they'll start complaining etc, etc.

[arboriculturist]

55 minutes ago, Paul in the woods said:

You're missing the point. The new regs means sellers of <2 cubic meters of logs must sell under 20%. After a bit of research that's wet basis. Most, if not all, cheap meters use dry basis. 25% dry basis = 20% wet basis.

 

So, if you deliver your logs at the legal limit of 20% wet and some know it all whips out their cheap meter it'll say 25% and they'll start complaining etc, etc.

There is no shortcut, which is why I decided that the price of the correct meter is just another business expense to be absorbed.

 

We all know that splitting a log in 1/2 and just testing the middle of a log is an inaccurate way of assessing the true MC of a piece of timber, but that is what we are all told to do and know one is interested to hear otherwise.

 

For those who oven dry test, my point is clearly reinforced.

 

I find it easier just to tow the line sometimes and this is sadly one of those occasions.

 

 

[openspaceman]

54 minutes ago, arboriculturist said:

 

 

We all know that splitting a log in 1/2 and just testing the middle of a log is an inaccurate way of assessing the true MC of a piece of timber,

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?

 

If readings taken by the meter are consistent and the logs are then oven dried one could decide with what precision the meter measures.

 

For most cases we are discussing here we just want to assure ourselves and customers that the product is less than 20% mc wwb so a reading of 25% mc dwb is good enough for that.

 

Anyway if the reading is marginally close just keeping a sample log in a normal room will increase its chance of passing the grade for a few hours till a second test 😉

9 hours ago, carbs for arbs said:

 But do you think it is safe to say that that meter is definitely dry basis?

I've no way of telling but it's easy enough to check as @Woodworks did recently. It only takes minutes with a microwave but get it wrong and your readymeal has a wood-fired flavour till you buy a new cooker.

9 hours ago, carbs for arbs said:

 

 

 

https://www.stovesonline.co.uk/firewood-moisture-meter.html

This article implies that his meter also uses a generic chip that measures the resistance of the log and has a look up table that compares the resistance with known moisture contents and displays a dwb reading. There are other ways, like dielectric that can be used.

 

These chips were developed by companies selling snake oil damp proofing products to otherwise  intelligent but gullible home owners. They were so successful that home insurers fell for them hook line and sinker and old houses which had been inhabited for centuries became difficult to insure unless work was done

9 hours ago, carbs for arbs said:

 

I have been thinking though.  If part of the calculation is based on the wood weight, how does the meter determine this?  Is it because wood has a consistent weight?  And if so, is that why some meters allow you to select what type of wood you are measuring, so it can adjust the weight accordingly?  

The moisture in a log (sap) has various dissolved salts in it, pure water is a good electrical insulator but the salts plus water make an electrolyte, which conducts electricity. I don't know how the resistance of this electrolyte varies as the concentration changes as the water is lost but imagine it gets higher resistance as it dries. As the pins are a fixed distance apart this removes one variable. So with lots of measurements one can see a correlation with resistance and dryness. This can then build a table by comparing resistances of logs that are then dried in an oven and comparing this oven dry weight with its original weight, the difference being the water.

[carbs for arbs]

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     

[Dan Maynard]

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.