Checking Moisture and Achieving MC Targets

      Higher moisture near the core has a lumber dryer doubting his moisture meter, but drying conditions are the more likely concern. September 6, 2010

Question
I previously posted a question here about drying lumber indoors, and am currently attempting to do so in a heated room with a dehumidifier running. I purchased a $50 moisture meter which seems to give consistent and reliable results that make sense. The MC of recently milled boards goes down steadily to below the 8% limit of the meter when measured from the face or edge of the board with the pins pushed in with moderate pressure. However, if a fresh cut is made, the MC is considerably higher. Is it standard practice to make a fresh cut every time you want to check the MC? I haven't seen much discussion of this issue in my reading so far.

Forum Responses
(Sawing and Drying Forum)
From contributor L:
Sounds like you are checking the MC in the wood's end grain? Trying checking at least 2' from the board end/any knots. If you are checking 1" lumber push pins in 1/2".



From Gene Wengert, forum technical advisor:
In dry air, the surface will dry out and give you a dry reading while the wood underneath the dry zone may still be at a higher value. How deep do your needles go? Are you testing side grain or face grain? Does your meter have a digital readout or a light that lights at 8% MC?


From the original questioner:
I am checking the boards all different ways as I get used to the meter. Both face and edge. The meter is digital, but its lower limit is eight. I am pushing the pins in as far as I feel comfortable without snapping them, but probably not to a half inch as the previous response in this thread suggests. Maybe that's a meter feature that should be more strongly highlighted - the durability to be pushed in enough to measure the core moisture. I am dealing with all one inch or less boards.

As I 'pre-plane' my surface-dry boards down to 7/8 or so, the meter will then register anywhere from 10 to 20 %, depending on how long the original log has been milled. I then re-stack them in my moisture controlled room. We plan to start woodwork in a couple of weeks.

So, is it safe to say if the core MC of my boards is not eight or less, I will kick myself when my trim joints open up in a few months? I can mask some of the potential shrinkage, for example by notching my header casings into the side casings. The face trim around doors will be tougher and more visible. I am planning on using a lot of this lumber for flooring, which as I understand it shrinks and swells quite a bit with the seasons anyway. Any suggestions for better dealing with this would of course be appreciated.



From Gene Wengert, forum technical advisor:
If the meter is correct and the core is 10% to 20% MC, then the pieces are not ready to use, as there will be shrinkage in use, open joints, and perhaps warping as well. Can you try cutting off the end (at least 12") and then use the meter in the freshly sawn end, getting a core, shell and in-between readings?

The in-use MC this time of year is often 6.0% MC (30% RH) in homes and offices. As a general rule, wood shrinks 1% in width and thickness for each 4% MC loss. So, you can see why with 10% to 20% MC in the core, you will get substantial drying and shrinkage. You mention that you have a moisture controlled room. If you are trying to dry lumber for interior use, it should be at about 20-25% RH in order to achieve the required low MCs. Note that wood products in use generally range from 6 to 8% MC. That includes flooring.



From the original questioner:
Gene, thanks for the advice. I am having trouble getting the RH below 30%, even with heat on and a dehumidifier running. I don't leave the DH running when I am not there, though (still gun-shy after my recent fire). I think I will crank up the heat as that seems to drive down the RH pretty reliably. My fresh cut core MC on some of my stock is still 10 to 20 after being in the house for about six weeks with heat/DH for three to four weeks.


From Gene Wengert, forum technical advisor:
It will be tough to get the core under 8% MC at the RH and temperature you have, with or without good air flow. You do need to stir up the air, but under 20% MC, air flow is not an important factor in drying speed. Temperature and RH are the keys.



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