Thread: Moisture meter jumps between two readings

Originally Posted by BobL

.....Any ideas? ......

Nope.

Well, no very helpful ones. My first thought was that the meter circuitry has blown a transistor or two, but then you told us it's working as per usual on other woods, so logic says the meter is ok & the wood is the culprit. However, I cannot think how the electrical resistance of the bit of wood between the pins could alter significantly in rapid succession as described. So the next thought is that the meter IS the culprit, and it's become unstable at the part of its range you are working in. I guess the best way to narrow it down with certainty is to borrow another meter. If it behaves erratically on the pine, then you have identified the culprit, but why it would behave like that beats me....
Cheers,

Thanks Ian.

10 days later, with the long hot spell we've had the NIP seems to have dried out a bit more and the jumping display meter is now typically reading 8-12%

Maybe there was enough current from the meter when the timber was wetter to intermittently vaporize some water molecules making the circuit?

Originally Posted by Fuzzie

Maybe there was enough current from the meter when the timber was wetter to intermittently vaporize some water molecules making the circuit?

I though about this but doubt there is anywhere near enough current to vaporise water.
However, you have got me thinking that there may be some polarization of unbound molecules, ie they align forming a conductive path which they can hold for a short period but cannot maintain that alignment ?

You're the physicist, Bob - I was thinking it might be something to do with the ion content of fresh, wet wood but my first-year physics was a very long time ago & I bamboozled myself trying to come up with a plausible explanation. There are certainly plenty of ionisable molecules in the cytoplasm of any cell, animal or vegetable. Nerve conduction works by a wave of de-ionisation of the axon membrane (much slower than direct current) and perhaps something similar is occurring along the cell membranes in wet wood, but in a less smooth way than for an intact axon.

I agree that vaporisation of water molecules is highly unlikely, we'd be looking at current densities of fractions of fractions of a milliamp oer square mm. That wouldn't get too many molecules hot under the collar at NTP....

Cheers,
Ian

Originally Posted by IanW

You're the physicist, Bob - I was thinking it might be something to do with the ion content of fresh, wet wood but my first-year physics was a very long time ago & I bamboozled myself trying to come up with a plausible explanation. There are certainly plenty of ionisable molecules in the cytoplasm of any cell, animal or vegetable. Nerve conduction works by a wave of de-ionisation of the axon membrane (much slower than direct current) and perhaps something similar is occurring along the cell membranes in wet wood, but in a less smooth way than for an intact axon.

I think we may be onto something. I'm going to see if I can get hold of a fresher piece of NIP for testing.

Originally Posted by BobL

I think we may be onto something. I'm going to see if I can get hold of a fresher piece of NIP for testing.

Bob

I think you need to replicate the test with different species, different examples of the NIP (as you have said) and different moisture meters if you can beg borrow or steal such devices. I am sure you, and Ian, are much more versed in the ethics of testing than I so I don't wish to tell you how to suck eggs.

Amazingly, everything you have stated points to the phenomena, if I can put it so demonstrably, lying with the NIP! I look forward to hearing what happens.

Regards
Paul

Could you please replace the 15 year old battery? I have a bit of a theory which involves internal battery resistance so can we eliminate the battery from the equation?

Originally Posted by NCArcher

Could you please replace the 15 year old battery? I have a bit of a theory which involves internal battery resistance so can we eliminate the battery from the equation?

Sorry I should have mentioned I've since tried new batteries and no difference. Also that the <3V in my OP should have been >3V, (3.05 - 3.08V)
The new batteries were all 3.2V

I have some ~15mm jarrah offcuts I milled about 2 months back and they're showing a 14-15% MC in the thin sections and a steady 16% MC in the slightly thicker sections.
A branch I cut off a Chinese tallow a couple of weeks back shows a steady 36% - I don't necessarily believe the number but it is steady.
All of the other wood in my shed shows <8% and anything (unless living) outside is <5%

I dont use moisture meters at all anymore. I cut a sample piece. If it is a big piece, I cut it into slivers (3" * 1/2" square is good)
weigh it (or the combined bunch of slivers), & record the original weight.



then repeat
microwave it for 30sec increment
record weight
Until weight remains static

divide original weight by end_weight to determine moisture content

This method leaves me in no doubt, and is always different to both our moisture meter's readings

T91 - given Bob's professional background & experience I doubt he would be relying too much on the reading derived from a moisture meter being an absolute value! No matter how sophisticated the circuitry, the MC reading you get is always derived from statistical samples of the same or similar species. Biological variation ensures there's always a renegade or two in the bunch. The main value of any meter, el-cheapo or expensive is that when you get the same steady values repeatedly, you can be reasonably confident the wood has equilibrated to your conditions - the absolute MC is pretty much immaterial in that case.

Indeed, total drying is the only reliable way to get a precise measure of absolute MC (of your sample), but it's also a destructive method (unles you are a pen-turner & can use the samples ). "Sampling error" is always a major concern given the potential variation of MC in a large board & any method is equally subject to sampling error.

I used to do quite a bit of total drying in my working days when I had access to highly accurate balances & drying ovens. Since I retired I've had a few occasions when I wish I still had that convenience, but by & large I don't miss it too much. I've found that by the time I get around to actually using any wood I harvest it has had more than enough time to equilibrate in any climate.....

Cheers,

Originally Posted by BobL

Sorry I should have mentioned I've since tried new batteries and no difference. Also that the <3V in my OP should have been >3V, (3.05 - 3.08V)
The new batteries were all 3.2V

Thanks Bob, I can sleep now.

Sure ,I've done the drying method quite a few times but that was more as a basic curiosity thing.
When you have 100's of slabs, logs and cants etc in various heaps all over the place I'm not going to dismantling stacks and cut up individual slabs etc.
The MC meter is just an indicator usually for when customers come in and ask - whats the MC of that. I so tell them I can measure it more accurately but it will cost them

Mostly I just just need to know roughly how dry something is, and "is this drier than that".
Its just I have never see a timbers MC readings oscillate so much that has my curiosity aroused.

OK my curiosity has been tweaked to the point that I went and explored this further.

Up until now I have been using the MC meter on the NIP timber by
a) Just pushing the meter pins into the surface, both along and across the grain (penetration is about 1mm - not that good I know)
OR
b) drilling 2mm diameter holes into end grain and pushing the pins most of the way in.
All produced oscillating readings.

Using method a) on all other timber in the shed produces stable or near stable readings.

What I had not done was to drill pin holes and measure across the NIP grain. When I did this . . . . .
WHALA - stable or close to stable readings.

I even made a video.

First 5 seconds show MC meter pins in holes in ingrain. Readings oscillate between 5 and 11% (remember the 5 could be down at zero)
Next few seconds shows MC meter (upside down - sorry about that) it oscillates between 12 and 13% - I call it stable
Last 4 secs of vid shows MC content of a thin NIP strip measured across the grain - steady 12%.



Notes:
thin strip is from same log and milled at the same time as the bigger lumber BUT not stored under the same conditions.
Lumber stored in warm-hot shed - strip was on front veranda, probably just as hot during daytime but cooler at night

MC of strip as measured by drying in microwave method was 8%
The difference from the MC reading of 12% seems high but I've seen it this before - all the strip needs is a thin streak of MC down inside the inside middle and the meter will register this as a higher reading. Now the strip has been dried out by the microwave MC meter measures <5%