Q: How close can you equalize lumber in a kiln? We have one customer that is asking for every piece in the load to be between 6.7 percent and 7.2 percent moisture content. Is this reasonable? We are drying cherry, hard maple, red oak and a few other hardwoods. Most is 4/4 and 5/4.
A: The very good drying operations will achieve a standard deviation (SD) of MC when drying lumber of about 0.4. The SD is useful in that 2/3 of the MCs will be within + or -1 SD around the average MC and about 95 percent will be within + or -2 SD. Three SD will contain 99.9 percent of all the values.
In your case, the average target MC is 6.95 percent. I seriously doubt that even the best kiln drying operations can hit 6.95 percent MC right on the head. They probably will be within 1/4 percent MC.
The best hardwood lumber drying operations with an SD of 0.3 will achieve 2/3 of the pieces between 6.65 percent and 7.25 percent MC. Further, 95 percent of the MCs would be between 6.35 percent and 7.55 percent MC. So, right away, you can see that even the best operation cannot achieve the narrow range your customer wants, even if it can get the correct average MC.
There is another issue that has to be addressed, however. That is MC measurement. The moisture meter itself is not very accurate. Most manufacturers will state that it is within + or - 1/2 percent MC. That is because it is measuring electrical resistance and then relating the resistance to MC, which is not a perfect relationship. (Or if you use the pin-less MC meter, then the relationship is between the dielectric coefficient and MC. This relationship is slightly more variable than the resistance relationship.) So, if every piece of lumber had exactly the same MC, the SD of the pin meters would be on the order of 0.3. In other words, you would get a variety of meter readings even though each piece had the same MC. That's just the way wood is!
Finally, even if you keep lumber at an EMC of 6.95 percent for many days, each piece of lumber will equalize to a slightly different MC (perhaps SD=0.05) due to the variability of wood itself.
So, what is the answer or solution? The only way I can see that you can keep the customers happy is to agree to an equalization period at 160 F and 6.8 percent EMC of 4 or 5 days, charging them perhaps $25 per MBF for each day, after the first two. You may also wish to factor in that you will try to keep the incoming MC of the lumber as uniform as possible, so that equalization will not be prolonged due to poor kiln loading procedures. With this procedure, you will have done the best possible. (I wonder if finding a more reasonable customer might also be a good solution?)
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