Q: We make kitchen cabinet doors with various hardwoods. Recently, we seem to be having a problem with moisture content. Sometimes we will see moisture as high as 11.5 percent MC on our meter. Our supplier is amazed when we report these values to him. It is rare, but we do see this, and when we do, we might see 25 percent of a pack that is fairly wet. Can you provide some input?
A: Let's approach this in a logical manner. First, we need to make sure that you are measuring the MC correctly. Make sure you are following the moisture meter manufacturer's instructions precisely. Generally, errors in measurement will cause only a percent or two of error, so it is obvious to me that you really do have wet lumber. To assure that your readings are correct, you might take a reading on a small sample and then have this sample oven dried to determine the MC by weight. If there is a delay between your electric meter measurements and the oven test, put the sample in a plastic sealed bag.
Second, part of the normal kiln drying process is called equalization. Equalization is done at the end of the drying cycle. Let me explain in more detail. Prior to starting the dry kiln, the operator will attempt to find the driest piece and the wettest piece of lumber in his kiln load. During the drying run, he will allow the driest lumber to proceed with normal drying until the driest is 2 percent MC below the target moisture level; in your case, 7 percent target - 2 percent = 5 percent MC. At 5 percent MC, he would then increase the humidity in the kiln to 5 percent EMC; this means that none of the lumber will be able to dry under 5 percent MC. The dry pieces stop drying, but the wet pieces will continue to dry. This equalization setting of 5 percent EMC is held until the wettest pieces reach the target MC, or 7 percent MC in your case. At this point, if done correctly, all pieces of lumber in the kiln will be between 5 and 7 percent MC. In conditioning (also called stress relief), the operator would add about 1 percent MC to the lumber, giving a final of 6 to 8 percent MC. (Note: The kiln operator may fine-tune all the previous numbers slightly to achieve the desired results specific to his situation.)
Finally, your data shows that the lumber was not equalized properly. As this problem of high moisture shows up rarely and then within one particular pack, I suspect that the operator is not doing an adequate job of looking for the wettest lumber when the kiln is being loaded. Perhaps he is too busy or perhaps he is not fully aware of the correct equalization procedures.
Having said all this, there is also the small chance that your high moistures are due to catastrophic regain of moisture after drying. The truck drove through a rain storm without a tarp over the lumber, etc. If this is the case, use a pin-type moisture meter and check the MC at the surface and at the core. The core moisture will be unchanged from when the lumber left the kiln.
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