Is my MC calculation process right?

nrp0450 · August 26, 2015, 08:54:50 PM

My dad and I built a pineywoods solar kiln a while back. Here's a link to that thread . We've run 4-5 loads through it and it seems to be working well. I've built a few pieces of furniture with wood that it dried and nothing has shrunk or warped. But I had never checked the MC. So I did that today.

It's a load of red oak that we put in straight from the mill (no air drying) about 40 days ago. It's been hot (95+ degrees) most days during that time so should be getting plenty of BTUs to remove moisture from the wood. I also have a dehumidifier in the kiln set to only run at night. The fans only run during the day. Today I pulled out a 1x4 board from the front of the stack about half way down. I cut an inch off of one end and then cut a 3" piece off. I weighed the piece on my little digital scale and it was 101.7g. I then microwaved it in 1 minute increments on 20% power. I watched it to make sure it didn't smoke and weighed it between each time it went in the microwave. It went down by 0.2g a few times and then 0.1g a few. After 5-6 one minute sessions it stopped losing weight at 100.9g. If I'm calculating right, that's less than 1% moisture content (0.8% to be exact). Is that possible or am I doing something wrong? I didn't think a solar dh kiln could get the MC that low.

In the hour since I did this, the weight has gone up to 101.1g.

WDH · August 26, 2015, 09:01:59 PM

Doesn't sound right. Try again with just a 1" wide slice. Maybe your sample is too large, and after you evaporated the surface moisture, the oak is giving up the moisture too slow for the time you allotted. However, just a supposition on my part.

GeneWengert-WoodDoc · August 27, 2015, 07:10:52 AM

I agree with WDH. Also, you need to get the piece very hot, so longer exposure to the microwave is necessary between weighing.

One special concern with a smaller piece is weighing accuracy. If a smaller piece weighs 30.0 grams and the loses 0.1 grams down to 29.9, that will be a 1/3% MC loss. So your scale will be measuring in 1/3% MC segments for each 0.1 gram loss.

Let's also check your scale. Cut two pieces, weigh them separately, and then weigh them together. It should all add up.

Denny · August 27, 2015, 07:56:18 AM

Turn that microwave up to full power and keep nuking and weighing in one minute increments till it stops losing wight or you get er smokin'

.

I was taught not to cut oven samples near or at the end of the board and like WDH said they should only be 1 inch wide. Sacrifice a board and cut out four nice 1 inch wide pieces spread out along the length but closer to the middle of the length of the board instead of at the very ends.

Oven weight subtracted from the "Green" weight gives you the loss. Now divide the loss number by the oven weight number to get your MC.

nrp0450 · August 27, 2015, 09:07:00 AM

Thanks for the help everyone. I cut a one inch slice last night and tried it in the microwave at low power and got the same results. It lost a small amount of weight and then quit losing. I also checked my scale like Gene said to do with my two pieces and they added up correctly. So I think it's working. I started drying both pieces in the oven last night and they both lost weight down below where they stopped in the microwave. They were still losing weight when I was ready for bed. So I turned the oven off and have restarted them this morning. I guess I wasn't using enough power in the microwave like Denny said. Or 1 minute wasn't long enough, like Gene said. Lesson learned. The pieces were getting warm, but not hot in the microwave. At 200 degrees in the oven they're definitely hotter. I'm checking their weight every 20 minutes. We'll see what happens.

That makes sense about taking the slice from the middle of the board. I just couldn't bring myself to sacrifice the board. If these numbers still don't look right I'll do that.

nrp0450 · August 27, 2015, 12:20:26 PM

OK, the 1" piece quit losing weight in the oven after about 3 hours and my wife needed to use the oven, so I took them both out. The 1" piece was 4.4% MC and the larger one was 3.3% (but still losing weight). And the larger piece came from closer to the end of the board. So the numbers seem plausible. Is 4.4% realistic with a solar dh kiln for a slice toward the end of a board?

I think I'll go ahead and take some slices from the middle and do them. And also try longer or higher power nuking. Gene, is there a rule of thumb for how long and at what power the piece should be microwaved? Or is it just trial and error hoping not to burn it? And are you saying that for 1" slices I really need a scale that reads to the 1/100ths of a gram?

Denny · August 27, 2015, 01:04:25 PM

For what its worth, I use a small lab oven running somewhere between 210 and 220 to get my initial MC numbers. These oven samples stay in the lab oven for 24 hour or close to it. On the rare occasions that I need to know exact MC right away and have to use the microwave, I usually cut two 1 inch pieces from each of my sample boards and microwave em on high at one minute intervals. I don't microwave all the samples at once, just one set of oven samples at a time. After about the 3rd minute of getting nuked, the pieces are very very hot, almost to hot to handle. Don't walk away from the microwave ! One time I figured I could go take a leak while my samples were cooking in the oven and low and behold, I smoked em. Talk about stink !!

I had that burnt RO stench in my control room for days. And its easy to smoke em when your mic's on high too.

Also, I've never used the microwave with green lumber, its only used on dried lumber, lumber thats usually below 8%. I rarely use my microwave to check MC. I always use my lab ovens to find initial MC.

nrp0450 · August 27, 2015, 01:32:28 PM

Well, I cut a 1" slice from the middle of the board. I nuked it on high for one minute and it lost just under 3%. So I did another 45 seconds (still on high) and watched it. It started smoking right before the end of the 45 seconds so I took it out. It calculated at 6.1% MC. But that's probably off because of smoking it slightly. Next time I think I'll try 1 minute intervals at half power. Should I be letting the pieces rest between microwave sessions? How long?

GeneWengert-WoodDoc · August 27, 2015, 07:24:23 PM

The suggestion is to put one or two pieces in the microwave at medium low power. With green pieces, it will take 45 minutes. With dry pieces, usually 20 minutes without weighing until then. With high power, the outside heats and starts to smoke while there is still moisture in the core. It takes time for the moisture to travel out of the wood, so lower power is needed for that reason. Even in the hot air oven, it takes time for the moisture to move out. If you were to use higher power, then use a rest period between energy inputs of several minutes...not suggested. Also, with higher power you will go over 215 F and then more than water will be evaporated, so the results are not as accurate. Medium low is suggested for a good reason. Note that "medium low" is around 20 to 30 % of the time on full power and then a 80 to 70% off or rest period of no energy...that is how a microwave works. The MW does not actually reduce the power output, but just the percentage of time that it runs with full power.

To see if the wood is dry, put the piece on a mirror or glass plate and look for moisture on the plate in a half a minute. If "oven dry" then you will see no moisture.

To get 3% or 4% MC, you would have to have around 10% RH, which is not likely in a solar, solar-DH, or even in a steam kiln. So, your readings are still off.

WDH · August 27, 2015, 08:26:05 PM

In my experiments with a microwave, I would cook a 1" wide slice for 20 minutes on 20% power. Weigh. Then, I would nuke for an additional minute, weigh. Nuke for an additional minute, then weigh, and so on. I never waited very much between weigh-ins. Probably should have.

With my dehumidification kiln, when there is little water coming out of the kiln, about a cup a day or so, and when the wet bulb depression is over 30°, and when the moisture meter reads 8%, when all of those line up, I know that I am done. Then I sterilize.

When I wring out all the water, that is the best that I can do

. Measuring the water output from the wood is a good way to know what is going on inside the kiln. Some water vapor is lost through the vents, but the water leaving the kiln, or lack of it, is a good indicator.

nrp0450 · August 31, 2015, 06:13:07 PM

This thread has been very helpful. Now I feel like I know what I'm doing.

It looks like I need to get a scale that weighs down to the hundredths. If it takes 20 minutes to go from 6% moisture to 0% and the process is linear (I know it's not) then each minute only removes 0.3%, which is less than the 1/3% that 0.1g on my scale equals. So it might look like the piece is done losing weight when it's really not.

Do you guys use 0.01g scales?