Monitoring Moisture Content During Kiln Drying

      At the critical early stages when wood moisture content is high, moisture meters are not accurate. Here are tips on keeping track of moisture throughout the drying process. May 17, 2011

I am drying some 8/4 white oak. I am using a meter with the long insulated needles. When checking the MC on any particular board, I check at the surface and the core. I am trying to keep the surface above 12% while the core equalizes to something similar. What is the name of the range of MC differences between surface and core and how big should the differences be allowed to get? I mixed some air dried and fresh off the saw in this charge and it has been a bear to bring down evenly.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
I am curious... Where did you come up with the idea about shell core MC differences? Are you aware that moisture meters with needles do not give accurate values above 28% MC? Usually we would not use a surface of 12% MC until the average MC is under 30% MMC when kiln drying. How are you drying?

From contributor B:
As I read it, it seems like a great idea. Probably take a while, but it seems that is a good idea with white oak. Interested in how it turns out.

From Professor Gene Wengert, forum technical advisor:
Do you have a copy of "Drying Oak Lumber"? Lots of great info in that book.

From the original questioner:
I came up with the idea of shell/core differences partly because I can't do oven testing with these boards and partly because I am a novice trying to put all of the attention I can on the drying history. Is it not something normally observed? Is there no name or informational standards for this depression? Yes, I am very aware of the limits of the meter. The boards are all unedged, through and through cut slab wood, 36" to 52" wide or 17' stringer stock. Not going to oven test one. Would a smaller 8/4 board work as a sample?

The charge has been running since July in a small DH kiln. Our chamber is insulated with closed cell blown insulation, pretty tight. When it started I had a hard time holding the temperature down, so I ran fans only and vented manually. It was the only way I could stay about 110 F. I have no way of reading RH in the kiln so I would vent during the night mostly, two or three times a week for eight to ten hours, just trying to not overheat the charge. After the meter started to come on line, about a month ago, I set the DH compressor at 20% unless it overheated, but ambient temps were more forgiving.

I have a copy of U.S.F.S. "Drying Hardwood Lumber" and Forest Products Utilization Technical No. 8, "Handling, Drying and Storing Heavy Oak Lumber." Shell readings are 12-20% and core readings are 18-30%. Currently I am bringing up the temp with no venting or compressor.

From Professor Gene Wengert, forum technical advisor:
I do not know of any standard drying text or any research that has developed operational techniques using shell and core MC gradients for controlling the drying process. For at least the last 60 years, we have dried based on the MC of the entire sample and not just shell, core or the difference. This technique has worked extremely well.

Because the most critical time in drying is the loss of the first 1/3 of the moisture (65% MC to 44% MC for white oak) and because the moisture meter does not work at these high MCs, the average MC technique is the best technique we have for controlling quality at high MCs. At lower MCs, the risk of quality loss is very small and so a meter can be used, but it is not critical to control the process precisely at this lower MC level.
A smaller 8/4 would be better than a meter, but appreciate that your wide pieces have a lot of quarter grain (slow drying), so a smaller piece might not represent the wider pieces perfectly.

From the original questioner:
Thank you Dr. Wengert. So when reading with a meter, the probes should be in the center of the board? I figured that too much drying along the shell would collapse the cell structure for water moving from the core and stress the wood.

From Professor Gene Wengert, forum technical advisor:
1/4 of the way in gives the average.

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