Lumber weight as a drying signpost

Calculating weight and moisture loss in a dehumidifcation kiln. July 24, 2001

I got to thinking that I can calculate the weight of, say, 1,000 feet of wood at 20% MC and then calculate the weight at 6%. I would then know how many pounds of water I need to get rid of to dry the wood to 6%. I measure every morning how much water is coming out of my small DH kiln. Would this be a valid way to determine the average MC of lumber in the kiln?

At present, I put two nails in two sample boards and run wire to the outside of the kiln so I can use a pin meter to monitor MC. When the meter says I am dry and the water extraction falls off, I open the kiln up and retest with a pin-less meter and oven dry. Can you comment on this procedure? I am new to DH drying and don't want to open the kiln door because it has a whole bunch of clamps holding it shut and I don't want to fill it back up with cool, humid, outside air.

Forum Responses
Calculating the weight of lumber and then figuring the MC loss by measuring the weight of condensate is a valid procedure. We do have some concern about overall accuracy and also the fact that the average MC may be okay, but there can still be a few wet pieces in the load. Therefore, using your procedure, it is critical that you check the MC of each piece with a moisture meter to confirm that the wood is dry, and not too dry or too wet.

Your nails, wire, and MC meter measuring technique is used within the industry. Above 20% MC, it is not too reliable, and yet that is where most defects occur. You do need to make appropriate temperature corrections, too.

In short, be aware of possible shortcomings in both methods and adapt to these risks.

Gene Wengert, forum technical advisor

Measuring the water coming out the drain is an indication of how the wood is drying but is not usually a good way to determine drying rate. All the water does not leave that way. Some will leak out through cracks and openings and vents. People think they have the world's tightest kiln and there is no air leakage, but there often is. Do you have to run heaters to maintain the temperature, once a certain temperature is reached? If you do, your kiln leaks. A well-constructed kiln will have more heat generated by the fans and dehumidifier than is needed to maintain temperature.

The amount of heat loss or moisture gain by the exchange of air when the door is opened is miniscule compared to the amount of heat and moisture in the wood. It is not a concern unless the door is left open for a long period. It is more important to get a good handle on the drying than to worry about the little heat loss from exchanging 100 cubic feet of air.

It looks like it would be possible to weigh your whole kiln. Calculate the water loss per day by change in weight. I don't know how much 1,000 board feet of oak would weigh or how much the scale would cost, so price may be prohibitive.