Originally Posted by
John TenEyck
In my prior post I noted that the maximum daily drying rate for 4/4 cherry is 5.8% and for 8/4 it's 2.3%. It doesn't matter much how you do it. So, put the wood in a plastic tent so the RH is constant for all the wood. Do not do this w/o the tent because the boards on the outside of the stack will dry much faster than those in the center. OK, put the dehumidifier, a small oil filled heater, and a fan inside the tent. The fan will circulate the air so all the boards see uniform RH. The heater drives the moisture out of the lumber, and the dehumidifier gets rid of it.
OK, to run it you need to cut at least one specimen cut from the middle of one of the boards that you weigh daily. Yes, daily. Make it about 24" long. Now cut off a piece about 2 or 3 " long and weigh both pieces immediately. Write the respective weight on each piece. Put the small piece in a toaster oven at 220F until the weight is constant for several hours. The moisture content of your lumber is:
(Wet weight - dry weight) / dry weight x 100 = % moisture content.
You can throw the little piece away now. Put the larger piece in the outside row of your lumber stack at mid height, with the other pieces from that board on each side. Use thinner stickers under the specimen so you can remove it for daily weighing. Button up the enclosure and turn on the dehumidifier, heater and fan. Start with the dehumidifier at 90% RH, and the heater at 90F and the fan on.
Let's say the MC was 60%. OK, if it's 4/4 stock, you can dry it at a rate of 5.8% per day. That means the MC needs to be no lower than 60 - 5.8 = 54.2% after one day of drying. The following day it can be no lower than 54.2 - 5.8 = 48.4%. And so on. To determine the daily % MC, weigh the specimen, take it out of the enclosure and weigh it. The MC that day is:
Calculated final dry weight = Original Wet Weight - (Original Wet Weight x % MC/100). So for a specimen that weights 2000 gms with 60% MC the calculated final dry weight is = 2000 - (2000 x 60/100) = 800 gms.
Now calculate the daily %MC from: (Daily weight - Dry Weight) / Dry weight x 100 = Daily %MC
It's not as hard as it looks. But if you don't want to do that, then get a moisture meter that can read both 1/4" and 3/4" deep, and measure the % moisture at both depths each day. Take several readings at the 1/4" depth with 4/4 stock and at both depths for 8/4 stock. The average of your readings is a reasonable estimate of the moisture content, though not as accurate as the calculated method.
As the daily %MC goes down it will stall at each condition you set for the dehumidifier and heater. Lower the dehumidifier in 5% increments until no water is removed, then increase the temperature to 100F. Some more water should be removed by the next day but if not, lower the RH by 5%. Keep increasing temp and lowering the % RH, but do not go higher than 120 - 130F, or lower than about 20% RH. Do not exceed the maximum daily % moisture loss. At some point the daily % moisture content will get down to 6 - 8%, which for your 2000 gm specimen means it will weigh 800 x 1.06 to 1.08 = 848 to 864 gms. At that point lower the temp., but no more than 40F per day, until cool. You're done.
If all that sounds too hard, then let it air dry until it's down to the equilibrium moisture level of your area (12 - 14% where I live) and then bring it indoors, sticker it, and let it dry until it's 6 - 8% or whatever moisture content you want.
John