Your microwave is a moisture meter

[cgallery]

If you liked my Cheerios vs. dust collector experiment (which I still need to summarize), you're going to LOVE this!

I don't have a moisture meter. I recently purchased some soft maple at a very reasonable price. I tried to resaw some and it started to warp/twist/bow quite a bit shortly after being cut. Sure, could be tension in the wood. But it could be moisture. "What is the moisture content in this wood, anyhow?" I wondered to myself.

So this morning, I cut two pieces and weighed them w/ my food scale. One was 9 grams, the other 36 grams. I then microwaved them for approx. two minutes and weighed them again and got 8 grams and 28 grams.

Then I did this math. For the smaller piece, moisture content was computed as (9-8)/9 = .11, or 11%. For the larger piece, moisture content was computed as (36-28)/36 = .22, or 22%.

Quite a variance, I know. However, I suspect my food scale isn't precise enough and that if I had used a more accurate scale I would have found the smaller piece had actually shed more like 2 grams of water, which would be (9-7)/9 = .22 or 22%, just like the larger piece.

I have an old balance that belonged to my photographer father (he used for missing chemicals in the darkroom). I am going to experiment with that tonight and see if I can get more accurate readings.

If you try this at home, be careful. I blasted the larger piece enough to actually cause scortching. I think the key is a very precise scale and a small sample of wood that is easy to get to bone dry very quickly, w/o the risk of catching fire.

And now I want a good moisture meter to compare results. So I don't know if I've actually gained anything. But I do intend to try a few more samples in the microwave this weekend and compare results.

[scorrpio]

I kinda doubt microwave is accurate. Kilns that dry wood never heat it enough to cause scorching. You might recall that charcoal is a lot lighter than original wood. Microwave temps go beyond water-they likely begin to break down wood itself.

P.S. A regular oven set to something like 250F might be a better way.

[Russianwolf]

And how do you know your microwave burns a certain amount of water away in 2 minutes to match your math? Or are you assuming that after two minutes ALL water content is gone (aka 0%)?

[cgallery]

I kinda doubt microwave is accurate. Kilns that dry wood never heat it enough to cause scorching. You might recall that charcoal is a lot lighter than original wood. Microwave temps go beyond water-they likely begin to break down wood itself.

P.S. A regular oven set to something like 250F might be a better way.

Well, the most accurate way to determine moisture content of wood is apparently (from what I've read online) to measure its weight before and after drying the wood to bone-dry in a 105-degree celcius oven for twenty-four hours. I believe this is far more accurate than any commercially-available moisture meters, just much less convenient.

What I'm trying to accomplish is a faster test by quickly using a microwave to bring a very small piece of wood to bone-dry. So I don't want to give up on the microwave. Just need to experiment w/ it a little. The smaller piece, which cooked for 1.5 minutes, didn't scorch. I'll get it. Or not.

[cgallery]

And how do you know your microwave burns a certain amount of water away in 2 minutes to match your math? Or are you assuming that after two minutes ALL water content is gone (aka 0%)?

Yeah, the idea is that I've reduced the wood to bone-dry (0%). In my next experiment, I will attempt to microwave a piece in one minute increments (letting it rest for a few minutes between each nuking) until it no longer changes in weight. Then perhaps I'll be able to make a generalization about how much I need to nuke it.

[jking]

Something else to keep in mind here is the difference in size of your samples may throw off your results. The microwave likely can "cook off" a larger percentage of the moisture in the smaller piece than the larger in the same amount of time.

Think about when you cook a steak. If you take two steaks of equal size and cut one of them in half & then cook the three pieces on the grill or in the oven, what happens? My experience has been that the smaller pieces will cook faster, because the heat less volume to cook. This is assuming you are trying to cook completely through.

I'm thinking you would need to establish cooking times based on the size of of material you want to test. I don't know if this would need to be done based on species of wood, also (though I think it might).

John

[cgallery]

I'm thinking you would need to establish cooking times based on the size of of material you want to test. I don't know if this would need to be done based on species of wood, also (though I think it might).

John

Exactly! More experimentation is required. For example, a smaller piece of wood releases moisture faster. But weighing it accurately is problematic. Am I better off w/ three 10 gr pieces? I suspect I am. But I'm going to experiement w/ it a little this weekend and find out.

I do wish I had a good moisture meter to compare my results with.

[onedash]

does a normal kiln heat wood beyond the boiling point of water? I would have guessed no but I have no idea. A microwave is definately going to boil the water in the wood which should be seen as steam. Will it get to zero? Im sure it would eventually but doesn't a kiln do more than heat the air????If its only heat you would have a steam room. How do they take the moisture out of the air? They must dehumidify the air don't they? Either with a device or by replacing the wet air with dryer air. Because heat alone would just make it hot and moist instead of room temperature and moist....Right??

[Sam Conder]

does a normal kiln heat wood beyond the boiling point of water?

No. Water will evaporate at any temperature, but the rate of evaporation is dependent upon the temperature. It also depends upon the relative humidity. A kiln not only regulates temperature, but humidity as well.

[jackellis]

Reduce the power to something like 30% and run it three times longer, watching the wood carefully to see that it does not burn. What you want is a relatively slower process. Turning down the power reduces the cycle-on time of the magnetron for a given time interval, which is the same as turning it on for a minute and turning it off for a minute.

If you use full power, wood with a high enough moisture content could also explode and damage or destroy the oven (and things nearby). It takes a little time for the moisture to escape, even at elevated temperatures. If you do this indoors on a day when the outside temp is really low, the indoor relative humidity will also be quite low.

[cgallery]

I hate to keep blathering on about this, but it just occurred to me that I can continue to use my digital scale and keep the process simple. Assuming the scale resolves to 1 gram, I need fifty grams of wood to resolve moisture content to within 2% (1/50 = .02, or 2%). If I want to be accurate to 1%, I would need 100 grams of wood.

So I am going to slice approx. fifty grams of maple (which my gut tells me is wet) and fifty grams of cherry and walnut (which were kiln dried and seem very stable) each into 10 gram slices and cook those using various methods on the microwave and record their before and after weights and compute their MC based upon the results.

I won't be able to compare the results to an expensive meter. However, if things check-out like I think they will, I may borrow a meter from someone around here and repeat the test.

To make drying go faster, I could even empty the separator, run a board through the jointer a few times, and subject the shavings to the microwave. Although, the volume may grow too much. But lots of ideas.

I think I may even try to get this published (if it works out).

[sacherjj]

Rather than starting and stopping manually, just run a lower power level as was stated. Microwaves don't really have power, the magnetron is on and off. So 30% power will run the magnetron on full for 30% of the time. I think you will have to include time to open the door of the microwave to remove the humidity. I still don't see you getting as close as a good oven. The microwave boiling will allow you to get all the free water, but I'm not sure that all the bound water will come out, due to the high humidity.

[cgallery]

I still don't see you getting as close as a good oven. The microwave boiling will allow you to get all the free water, but I'm not sure that all the bound water will come out, due to the high humidity.

I think the key to success is to cut the wood into small enough pieces. If I cross-cut at 2x4 into 1/8" slices, for example, until I have several pieces that total 50g, then I should have no problem nuking the water out. I would think the wood fibers will work like straws and literally spew the water out.

[crokett]

I think I'd just go get a moisture meter. Not least because I wouldn't want to explain to SWMBO if the microwave broke.

[cgallery]

I thought I'd post a little follow-up on some my my experiments. I tried two samples (53g of cherry and 54g of soft maple). Each of these samples was in the form of seven pieces, cut so maximum end-grain would be exposed.

I weighed each sample, microwaved it at 30% for five minutes, and weighed it again. The 53g of cherry dropped to 50g, and the 54g of soft maple dropped to 51g. I nuked each sample an additional minute at 100%. I did this a few times each, weighing the samples after each "hit."

I gave up after about four 1-minute sessions. The cherry was down to 49g, and the maple was down to 50g. They hadn't moved after the first 1-minute "hit." If this was bone-dry (I have no illusion that it was), that would indicate my initial cherry sample MC was 7.55% ((53-49)/53). The maple would have been 7.4% ((54-50)/54).

I have no moisture meter to compare results.

This week I will be visiting a small mill with an onsite kiln. I'm noing to take a sample of the cherry and the maple and see if they can check them w/ their meter.

Also, due to the aroma of microwaving wood, I think I'm going to try a heat gun and a smaller sample (with a scale that can resolve to .1g rather than my 1g kitchen scale).

Kinda fun, though.