Adjusting Moisture Meter Readings for Hot Lumber

      Moisture meters aren't accurate when used on hot wood fresh from the drying kiln. You have to allow for that. June 13, 2014

Question
I am trying to reduce drying times for steam high temp kilns. I'm drying loblolly pine to meet a KD19 moisture content. I find that I am drying to 10% moisture content or less to keep planed lumber under the 19% moisture content. From hot checking at kiln to after planer, I am getting a 6% gain in moisture. I believe it's due to case hardening, where outer part of lumber is extremely dry, core is very wet, and planer cuts off the dry part, exposing wet wood. I want to dry for less time, could I do this by conditioning lumber at end of dry cycle to allow inner moisture to escape the case hardened shell - while not adding time to the cycle? Also, I'm using a wet bulb depression of 60 degrees plus. Is this too much for pine?

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
I believe that the MC change you are seeing is perhaps due to moisture meter errors and is not what is actually happening. The gradient you mention is not common for HT SYP drying. I will bet you that the heat is giving you incorrect readings; in fact, I have not seen any temperature corrections for 230 F. Corrections for MC meters for temperature are only a rough guideline; they cannot be used with any accuracy. Casehardening is a term used to describe the presence of stresses. It has nothing to do with final MC gradients. Casehardening does not affect MC readings, it is purely a stress condition.

Your final 60 degree depression is quite common. I see a lot of 230 and 165. Of course, it takes a special wet-bulb to function at these temperatures - water flow must be adequate and the wick cannot lose much moisture before the wick is at the bulb itself. Many hardwood kiln WB sensors will not work at HT. You need to work on getting the correct MC when the wood leaves the kiln. Because the MC meter is not giving you a good value, you need to make an adjustment for that. So, dry until the MC meters reads 10% or 9% or whatever value gives you the correct MC after planing. This is because the meter does not work at these temperatures accurately. You are not actually over-drying or drying to these low MCs. Many people will also use the WB depression level and the TDAL level, and even a Wagner in-kiln meter sometimes, to help determine the end point.



From the original questioner:
I was not considering the impact of temperature adjustments of the moisture meter. I was establishing the relationship between the hot check and the planer results in moisture to establish a guideline for the kiln operators to dry the charges to. So, case hardening will not prevent moisture equalization from occurring - or slow it? Is time the only (major) factor in equalization?


From Gene Wengert, forum technical advisor:
Time, species, thickness, grain, temperature and RH, as well as the amount of equalization (moisture uniformity that you start with and that you want), all determine the time.



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