Determining MC by weight

      Tools and techniques for measuring moisture content by weighing lumber cuttings. January 4, 2001

I’d like to learn more about cutting, weighing and drying lumber samples for determining MC.

1) For samples to be weighed before and after oven drying, what sort of resolution and accuracy are we looking for?
2) With respect to sample size, does one shoot for green or dry dimensions?
3) Does length of the sample matter?
4) I still don't quite get the purpose of retaining the 30 inch piece after cutting the two (why 2?) pieces from both ends.
5) When the larger piece is returned to the kiln, presumably to experience the same conditions as the entire charge, are the ends re-sealed? If so, doesn't that bugger the weight?
6) Am I correct in understanding that at any point in time the ratio of weight loss to dry weight equates with MC of the un-dried sample?
7) For those who have done this, what kind of repeatability can one get over the course of a complete drying cycle?
8) For example, oven drying a sample right off the mill (about 100% MC green), and another sample from the same piece of wood, say after a month (80% MC green). Will the resultant dry weights be the same?
9) What kind of scale/balance do you use?

Forum Responses
1) Weigh to at least 1/2% of the weight--100 grams needs to be weighed to at least 0.5 grams accuracy and resolution.

2) The size is measured when you cut the piece. It is full thickness, full width, and 30 inches along the grain.

3) The suggestion is 30-inches. The sample is used only for MC calculations and so it is not necessary to have 1 BF, 2 BF, etc.

4) The 30" piece is weighed daily and the MC is calculated based on the initial MC of the two moisture sections.

5) Yes. The weight change is so small that it doesn't matter when calculating MC--the sample might weigh 10 lbs, while the end coating weighs 0.002 lbs.

6) Not exactly; the weight loss compared to the oven-dry weight is the moisture content loss. Also, the weight at any time compared to the oven-dry weight is the current MC. The oven-dry weight is an estimated value based on the MC of the two moisture sections.

7) We cut a mid-course correction sample (called an intermediate) when the wood is under 25% MC. With this technique, the MC will be nearly perfect--closer than anyone requires--at any time during drying.

8) No, the weights will be different, but the MC will be the same if the two pieces were identical, which doesn't happen with a variable material like wood.

9) You can buy some scales in the $200-$400 range that will work well. If the DH manufacturer doesn’t have some scales for sale, go to Nyle; they have auxiliary equipment available, or go to Graingers, or similar. Do not go cheap on MC measurements. Also, you need to spend at least $200 for a moisture meter. And you should attend a class on drying--probably $400. These are expenses to protect your investment.

Gene Wengert, forum technical advisor

Instead of cutting samples and weighing them , would it not be suitable to only use a moisture meter - perhaps with a probe into the kiln stack?

Most drying quality loss occurs at MCs over 50%. The drying rate at high MCs is a good tool for monitoring the risk of loss.

Unfortunately, moisture meters do not work well above 25% MC. They also have an accuracy of 1% MC, so often they are not accurate enough. The pin-type meter does not work well below 6.5% MC, so it is hard to use this type of meter when running a kiln near the end of drying.

Gene Wengert, forum technical advisor

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