Elevated Moisture Readings after Planing

Wood that is not uniformly dry will read higher after planing; and the results vary between pin meters and pinless models. June 4, 2012

Question
I am drying a load of 1 1/2 inch thick white oak for outdoor use. I dried it to a moisture content reading of 13% at 3/4 inches deep. I planed several boards and now I'm getting readings of around 20%! Any ideas on why this is happening?

Forum Responses
(Sawing and Drying Forum)
From contributor D:
Maybe the boards were down in the pile? You said you were drying - air or kiln?



From the original questioner:
The boards were at the top of the pile. I took the MC readings of the rough boards and got 13%. After planing, the exact same boards were 20%. It was not more than an hour from kiln to planer. These were kiln dried in a Nyle L200.


From contributor L:
Did you take the two MC reading at the same place on the boards? If not where did you take the reading (end grain or where)?


From contributor D:
Maybe when you planed down you were into the real wood? I use a Delmhurst meter. It sounds like the charge was taken out too soon.


From the original questioner:
Yes, I took the MC readings on the same place of the boards. Near both ends and in the middle of both faces. I did not check the end grains or edge grains. I'm using a Lignomat pinless meter. No probes.

The boards were not very rough, but maybe just enough to give a false reading. I went pinless after having depth limitations with a pinned meter.



From contributor R:
Your wood isn't dry, that's why you're getting the higher readings. Pin meters are the most reliable, and the industry standard for kiln operators. There are no depth limitations to a pinned meter - you just buy longer pins for whatever thickness you want to read.


From contributor B:
The pinless meters are sensitive to density. Does your pinless meter have a species setting for white oak, and are you using it? If your white oak happens to be of greater than average density it will read a higher than true %MC, lower than average density a lower than true %MC. Pinless meters do have depth limitations. One of the best hand held pinless meters can read moisture about an inch into wood, so it can realistically be used with 2 inch thick lumber. If your meter cost less than $500 I would be suspicious if it could detect moisture deeper than 1/2 inch.

I am surprised you earlier had depth limitations with a pin meter as probes are available with 3 inch long pins; the hardwood industry commonly uses over one inch long pins. I would bet your meter was not reading moisture as deep as you thought, and then when you planed the wood you were able to pick up the core moisture. I avoid suggesting that either a pin or pinless moisture meter is better than the other. Each one has specific advantages, limitations, and characteristics.



From the original questioner:
I set my meter to a specific gravity of 68 for reading white oak. I purchased it for about $300. My pinned meter was a cheap, hobby one. So the pins weren't replaceable. This is the first time I've dried oak straight off the mill and never this thick before. Your suggestion that my meter is only accurate to 1/2 inch could explain the problem. I'm really not reading the core until I plane the board.


From Professor Gene Wengert, forum technical advisor:
Let's assume that the oak rough was about 24% MC in the center and about 8% MC on the surface. When checked with the pinless meter, you would get a reading of about 14% MC, although the dry, rough surface may give you a bit drier than actual, as the meter is more sensitive to the surface MC than the core and the surface contact was not perfect with the roughness.

Now you plane the lumber and remove the dry outer layer and you have a wetter piece of wood remaining. So you will get a higher reading.

Note that the pinless meter is not suggested for running a kiln. The pin-type meter is also not effective until lower MCs (perhaps 35% MC average) are reached. For this reason, the sample board method is the only reliable method. This is not because of the skill of the manufacturer, but because these meters are actually measuring an electrical property of wood and this property is not well related or correlated to the MC when you are above 30% MC.



From contributor B:
As stated before, your boards were not dry throughout. I have a good pin meter, which I never use when I am drying wood. I take the time to use test boards and pre-dry samples from those boards, then compute and dry by the book. I also have a L-200 and dry up to 3000 bf at a time. My customers and myself have always been satisfied. Air drying can show low MC and be wet in the middle if you use a pin meter.
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