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Conductivity and wood density

Started by C_Miller, February 14, 2019, 08:19:07 PM

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C_Miller

I run a Nyle LM200.   Sometimes I have a probe reading that  indicates moisture content considerably higher relative to other probes in the same stack of lumber. 
I am usually drying live edge 2 1/2" thick walnut, soft maple or oak.I haven't been able to establish a pattern. was wondering if wood density in the figured slab would affect the reading.
CJM

doc henderson

I use a pin-less meter and it is definitely true with that.  Not sure, as I am no expert, with other meters.  I think @GeneWengert-WoodDoc , will have good info for you!
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

Southside

Walnut is known for holding water pockets. I don't know why, but it does. 
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YellowHammer

The probes measure electrical conductivity or electrical resistance between the probes, so anything that affects that will give a different reading, whether its from the wood, or the kiln sensors themselves.

Wet spots can be a problem, especially in walnut and white oak. However, there are other possible physical factors that I like to eliminate first as well.  

Sometimes the probe spacing is off, (30 mm between probes), sometimes there is reduced electrical contact on the probe and wire banana plugs, (I like to use a screwdriver to slightly expand the banana plug to more securely fit the probe holes, and I clean the contacts regularly), and sometimes its a function of where the probes are placed.  Closer to the end of the board will sometime give a different reading than in the middle, especially when in the higher than fiber saturation point or so MC range.  Also, its important for all the pairs of screw in probes to be set at the same depth.

If you swap the probe wires with another adjacent set then that will tell you about half the story.  Sometimes the high reading will go away, sometimes it will follow the wires.  Then if the  high reading remains, get a longer (or shorter, depending on the original probe length) set of screw probes and drive them in right next to the ones already in the wood and put the leads in them.  You should see the same reading or pretty close, based on the depth of the probes.  The shorter probes should read slightly drier than the deeper one, if there is a wet spot in the wood. Sometimes if I have a wet spot I will move one set of leads to the duplicate screw in electrodes, as that is a spot of interest, and since the Nyle uses average values for its algorithms, I like to concentrate the reading on the anomaly.  

Every now and then, I will get a block of wood, drive two probes in it, and insert each pair of wires in it, they should all read the same.  That will give confidence the machine is reading properly.

Once you are convinced you have wet spots, it won't be much of a problem until you get to the final drying stage where all the readings do need to match up.  If you can't clear the wet spot by the end of the cyclce, start raising the temperature until just under the compressor safety.  If the rest of the stack is at 8% or so and you still have wet spots, turn off the compressor and go into a sterilization cycle, raise the temps to 150, and hold for 24 hours, and then let it slowly drop for 24 hours.  Then turn the compressor to 100% by setting the wet build to 70%, and the dry bulb to 100% and give it a day os so and the wet spot will resolve.  
YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

K-Guy

Once again a reply from me is unnecessary as Yellowhammer probably explained it as well or better than I can. :D 
Nyle Service Dept.
A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.
- D. Adams

C_Miller

Thanks Guys, I appreciate all the info. 

Put my pin meter on the probes and got the same reading as on the control panel.

Yellow hammer what did you mean by "compressor safety"?

Sounds like the water pockets will clear up with a thermal hammer tunk. I have been holding temps to high to long must be. I'll try just thumping it next time.
CJM

GeneWengert-WoodDoc

Water pockets are almost always the result of anaerobic bacteria in the tree.  These bacteria creat a bacterial slime that inhibits the movement of moisture enough, so there there are small regions of wetter wood.  Sometimes, these regions are called bacterial wetwood, or just wetwood.  You will also see that the initial moisture is higher than normal.  For example, in red oak, normal green moisture is 75% MC, but bacterial Infected wood will have as high as about 110% MC, with 90% being commonly seen.  So, this higher starting MC and the restricted movement mean wetter wood zones.  It is worse with thicker pieces...seldom seen in 4/4.  Patience is the only cure.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

YellowHammer

The compressor has two safeties, a refridgerant low pressure shut off and a high pressure shutoff.  The high safety trips a little above 130 degrees in the kiln.  
YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

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