Double-Checking Moisture Content
You can check your moisture meter by drying wood samples and weighing them, but the method has its own uncertainties and imprecisions. August 29, 2006
Question
I may have over-dried 2000 bf of 4/4 white birch. My Delmhorst J-2000 read 8.5% surface and core at 130 degrees F. Upon cooling to 60 degrees F, I'm getting nothing but negative readings, which indicates the wood is less than 6%. I replaced the battery and checked the meter in my house. It appears to be working. It looks like I put too much trust in Delmhort's temperature compensation. It's been pretty close in the past. I am using a Northland kiln. Can I save the wood by getting it back to 130 degrees and adding water with the vent shut down to steam it?
Forum Responses
(Sawing and Drying Forum)
From contributor A:
Out of curiosity, what would be the difference between doing what you are thinking and just pulling the wood out and letting it acclimate? I don't think adding steam will help, as the drying process destroys the cell walls or something of that sort. Did you try to use it and see what happens yet? Maybe there aren't going to be any problems with it.
From the original questioner:
Thanks for the response. The wood is scheduled to be used for kitchen cabinets in two weeks, which does not provide much time for acclimating. Since posting my message, I have been continuing to check a sample board. I get readings in the core that vary from "negative" to 6.6 to 8.5 without moving the pins. While I'm still curious about my original question, I am now also wondering if my wood is not completely cooled, or if there is a static charge, or if it is time to get a second meter.
From Professor Gene Wengert, forum technical advisor:
Could it be that you had some (just a little) moisture on the surface that gave you a high reading? Do you have a water spray system? I doubt if you can get the core up very high quickly. Regarding your varying MC readings, check the cable and the needle nuts. Sounds like a poor connection.
From contributor B:
To get the real MC for a lot less than a new meter, try microwave drying. I got a scale from Walmart for dieting (digital to +/- 1 gram) for under $30. A $5 used microwave from a yard sale. Cut a slice that weighs about 100 g or a little more. Run it for 2 minutes, then weigh it every 30 seconds or so until the wood starts to smoke. Use the formula from "Drying Hardwood Lumber" (very easy), and you have the right number. My Delmhorst R-2000 is never more than a percent or so different in the mid ranges, but at the low ranges it starts getting hard to bang the pins in and they start breaking, etc... and I just use the oven dry number.
From contributor B:
Forgot to add this. If you want to take the mc back up... I stress relieve with a leaf blower (yes, another yard sale item) and a 5 gallon bucket of water and a length of tubing. I hang the bucket high, run the tubing to the leaf blower and start the water flowing. I blow the misted water into the circulation fans, and it evenly mists the load. Turn the heat back up for a few hours and you can put some water back into the wood, plus it will be stress relieved and should rip straight.
From Professor Gene Wengert, forum technical advisor:
Let's assume that the sample weighs 100 grams (+ or - 1 gram) and the oven-dry weight is 92 grams (= or - 1gram), with the piece really being oven dry and there is not a gram of moisture (a dollar bill weighs one gram) still in the piece. The real oven-dry is 91 grams.
First, if the piece actually weighs 100.00 and 85.00, then we get 17.65% MC. If the piece weighs 100.49 and 84.51, which would still register 100 and 85 on the scale mentioned, this would give 18.91% MC. And if the piece weighed 99.51 and 85.49, then 16.40% MC. So, without being able to measure to the 0.1 grams, the closest you can get is + or - 1.5% MC. If we then add the + or - 1 gram accuracy of the balance, we will have even a larger potential error.
The bottom line is that you must weigh moisture sections to 0.1 gram if you want to know the MC to within several percent.
From Professor Gene Wengert, forum technical advisor:
The previous example is based on 100.00 and 85.00 and not 92.00.
For 100.00 and 92.00, then 8.7% MC. For 100.49 and 91.51 (readings would be 100 and 92), then 9.8% MC. For 99.51 and 92.49, then 7.6% MC. So, again we see that the best we can do without 01.grams is + or = 1.2% MC, no matter how good we weigh and calculate and oven-dry.
Now, if we also consider that there is still one gram of moisture in the sample, and the true OD weight is 91.00 and not 92.00, then we get 9.89% MC and not 8.70% MC. So a one gram error means 1.2% MC error.
From the original questioner:
What if I take a larger piece of lumber, crosscut it into many 1 inch pieces, weigh them all together, oven dry them, and then re-weigh them all together? That should get me the precision I need on the scale, but still allow quick and thorough oven drying because of all of the end grain. Does this make sense? Thanks for your input.
From contributor I:
I agree with Gene's math. The cheap scales will only get you an approximate measurement. But in many cases, that is enough. In general woodworking you don't need to measure MC to less than 1% accuracy. You are still as accurate as most moisture meters, which will give slightly different readings, even on the same board. And boards in the same stack, or different parts of the same board, will vary by that much anyway. If you increase your sample weight (to say 200g), then you will also improve the accuracy. If you just need to check if wood is kiln dried to approximately 8%, then 1g accuracy will tell you that. If you want to calibrate your moisture meter, then .1 g will be needed. If you work in the CSI lab, then you can use .01 accuracy and tell if the last person to handle it was sweating ;)
