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.
(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?
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.
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.