Measuring Moisture Gradient at the End of Kiln-Drying
Core and shell moisture content can be important things to know in certain cases, but it's usually not vital information for running the kiln. June 13, 2014
Question
I realize that at times during a kiln charge it is important to know the moisture gradient of the lumber, so an oven test of both the shell and core is done. At the end of a kiln charge what method of oven testing is to be done? Is a 1" section the width of the board an accurate way to get the final MC? I heard before this is just an average of the shell and core and is not acceptable and that a shell and core test must be done. Is it necessary to know the moisture gradient at the end of a kiln charge? I would like to know how others cut their oven tests to determine an average final MC. It seems to me that with dry lumber trying to get an accurate oven test of the core would be rather difficult as it is much lighter, which makes it more difficult to get an accurate reading. In that situation would a sample wider than 1" be used?
Forum Responses
(Commercial Kiln Drying Forum)
From contributor F:
Usually the shell is lighter than the core because it is cut thinner. Either way if you have a scale with enough precision you will have no problem.
From contributor K:
The shell is going to be lighter because it is usually drier than the core as well. Lumber dries from the outside in, so you need to know the MC of the core at the end of drying in order to prevent under or over drying the lumber. An oven test of the combined shell and core may show that the lumber is at 7%, but the core could be at 10% and the shell could be at 4%. If the lumber was conditioned at this point, you would probably have lumber that would test around 10% after conditioning. Also, as Contributor F stated, if you have a precise enough scale you shouldn't have any problem. The scales I use to weigh my wafer samples, weighs in 1/10 of a gram. A heavier sample could be used but it will just take longer for your oven sample to get dry.
From Gene Wengert, forum technical advisor:
I have not seen any standard kiln drying text that uses the moisture gradient, shell to core, as a basis for operating a kiln. I have seen a few people suggest that a certain gradient should not be exceeded, but I have not seen any research or large data base that indicates the relationship between gradient and defects, including stain, warp, and checks or honeycomb. In fact, the gradient is not as important as the stress (and the strength of the wood itself). Even if the gradient were useful for control, the shell to core difference is not sensitive enough - you would need intermediate values. Further, the critical gradient is sensitive to temperature and MC level too. Any connection between the gradient and quality would be rather complex. The shell core test is designed and used at the end of drying to assure (for some products) that the shell and core MC are within narrow limits. I cannot recall anyone using the shell core tests in the many visits that I make to kiln operations in the past few years. I think that last time was for bowling alley stock.
From Gene Wengert, forum technical advisor:
Regarding the final MC test, almost everyone uses a 1" thick piece (along the grain). If you went to a standard shell core test, your pieces would be half the weight, so if your original testing with a full cross section is accurate to 1/2% MC, the shell core will be accurate to 1% MC, as the weight is 1.2 of the one inch piece. As a 1" along the grain piece often weighs about 70 grams, oven-dry, it would be best to have a balance that weighs to 0.01 grams to obtain good MC accuracy.
From the original questioner:
I now do have a better understanding. In the example where I stated that the moisture gradient was calculated throughout the kiln charge, it was done so a certain gradient was not exceeded (8/4 hard maple). It neither decreased time nor quality as compared to our normal procedures. Also, I was asking about size of oven tests because for example our scale measures to the thousandths (0.000) but the thousandths is only measured in even numbers. For example if an oven test were cut that initially weighed .034, and the final weight was .032 the mc is 6.25%. The next possibility would be a final weight of .030, or 13.3%. I believe the actual MC falls somewhere in between. That seems to be a very large range based on the fact that the samples are being cut way too small (.034 green weight of the core test).
From contributor N:
To the original questioner: You mentioned a 1" sample board weighed so small. Isn't that used just to get your sample board of 30" estimation? Then you in turn use that weight from the 30" board to determine final MC?
From Gene Wengert, forum technical advisor:
I believe that if your balance of scale weighs to 0.001 and a moisture section (which is the name for the 1" along the grain piece cut to measure MC and placed in the oven) that weighs 0.034, then you are measuring in kilograms and not grams. In other words, your moisture section weighs 34 grams, which would be a typical value, although a little light weight. You need to measure to 0.01 grams, or 34.xx grams in this case.
From contributor H:
I’m currently using a Delmhorst j-200 to check moisture content on my inbound kiln-dried lumber. Would it benefit me to do oven-tests? If so, how do I get the conversion tables to determine MC in exotic species?
From Gene Wengert, forum technical advisor:
The meter you are using will give you readings that are within 1/2% MC of the true, oven-dried MC. I do not think that you would benefit from an oven test, especially because you can measure many more pieces with the pin meter and in a much shorter time without destroying the wood. The correction for different species at the low MC you have is so small that it would not be necessary to use a correction. The only concern would be when you get readings under 6.5% MC, as the meter does not work well at low MC’s, so you only know the wood is very dry (maybe over-dried), but not the true MC.
