Moisture content oven tests

Microwave ovens versus drying ovens for MC testing. October 30, 2002

How big should oven samples be? I'd like to try using a microwave for testing, but wonder if a cheap drying oven would be better?

Forum Responses
For lumber, the moisture sections should be 1" along the grain and full thickness and width. This would be about 100 grams. You need to weigh to 0.05 grams or better.

Any microwave with a carousel tray and various power settings will work.

Are you being penny-wise and pound-foolish? Getting the correct MC is very important and not a thing to go "cheap' on.

Gene Wengert, forum technical advisor

From contributor F:
I advise not to use microwave ovens. The people that call me up that use microwave ovens consistently have some kiln samples that they "can not get below 15%" and other samples in the same run end up with "negative MCs". This is the result of over-drying or burning the wafers or not drying them enough. Drying the samples completely without burning them seems to be difficult for many people.

I have at least 50 clients that use microwave ovens 100% of the time. There is no burning if you follow the instructions, including med-low power, carousel tray, not touching another piece, paper towel underneath, and near the outer edge.

It is like saying that you do not recommend a kiln because some people surface check oak lumber in a kiln!

How can you get wood very hot and still have 15% MC in it? They are not following the instructions.

Negative MCs result because they did not dry the samples completely. The instructions say that you weigh every 2 minutes to assure weight loss has stopped, etc.

Incidentally, I dried hundreds of samples in a hot air oven and in a microwave before the technique was published and recommended. Although it is more time consuming than a hot air oven, it works perfectly if the instructions are followed.

Gene Wengert, forum technical advisor

I also have had inconsistent results from microwave drying. Specifically, it is with bacterially-infected oak. Even following the directions carefully, there will be pockets of burned material long before the rest of the sample(s) are ready.

Even if everything seems okay, I still am not comfortable with microwaved results in red oak. Too much is riding on the answers I would get. I conventionally oven test all oak.

Burning results because you are putting too much energy into the wood. You need to use low power so that the heat will not get excessive (over 400 F). Drop the power setting to low for just one piece of wood or if the wood is bigger than the standard moisture section size.

Gene Wengert, forum technical advisor

From contributor F:
I know what causes burning and under-drying and over-drying of wafers to get bad MC estimates of kiln samples. However, maybe you're an Indy 500 driver trying to get an average person to drive an Indy 500 car at Indy 500 speeds.

If people are consistently having a hard time driving their microwave ovens, why insist on using them? Give them an old fashion station wagon and they will get to their vacation destination safely.

I do not think that using medium low to low power in a microwave oven is that difficult to avoid burning. I do not think that using the paper towel, avoiding having the samples touch each other, and putting them near the outside is that hard, either. What is hard is the labor intensity of using a microwave oven. You need to weigh, dry, weigh, dry, etc. to assure that the oven-dry MC is reached. Too many people want the speed of a microwave, but do not want to put in any labor or skill. Use a hot air oven for a low labor MC determination, but even then you must assure that you reach 0% MC by weighing twice.

I would like to talk to someone who cannot get their samples below 15% MC. This would mean that their oven-dry MC was 15% too low, or that when they burned the samples in the microwave, they lost about 15% of their weight to burning. This is absolutely amazing, as even smoking samples will not lose that much extra weight - 15%. If the samples get to 15% MC and no drier, then they made a weighing or calculation error, but not a "burned them in the microwave" by 15% error.

You should contact these people and let them know that there is another error. (As an example, take an oven-dry piece of wood that weighs about 100 grams and light it on fire. See how long it takes to lose 15% of its weight!)

Did you know that sometimes lumber will dry to about 15% MC and then appear to stop drying? This is not uncommon and is discussed in DRYING HARDWOOD LUMBER. It has nothing to do with microwave ovens. Maybe this is your customers' problem. I have seen this often.

Certainly, negative MC samples are because the sections were not fully oven-dried. This happens in a microwave and in a hot air oven. I have seen it in both.

Microwave ovens for sections have been used in the lumber business for over 21 years.

Gene Wengert, forum technical advisor

From contributor D:
I agree with the professor. Microwaves work fine with a little care. I have a microwave in my office and, while I'm working on other things, I'll heat samples for a couple minutes and then let them cool down with the door open. Make phone calls and answer email between heating periods, but don't try to get them done instantly. Weigh them once in awhile and you'll see them come dry.

Dry loads (like popcorn) are hard on magnetrons. I keep a bottle of water in a plastic soda bottle with a rag for a stopper in the microwave.

Are you saying that a 400 g. x 0.1 g scale is not accurate enough for 1 inch samples?

Regarding the resolution of a scale or balance: Let's assume that you have a piece that weighs 100.0 grams. Actually, with more resolution, it could weigh between 99.95 to 100.05 grams, but would read 100.0 grams. Now you oven-dry the piece and it weighs 90.0 grams. (It could actually weigh 89.95 to 90.05 grams.) On the average over many readings, you will find the MC to be 11.11% MC. But sometimes you will have the true weight to be 100.05 and the oven-dry to be 89.95. This will give 11.23% MC. So, the resolution of your equipment is approximately + or - 0.1% MC. Not too bad. But if you have a 50 gram sample, your resolution is now + or - 1/4% MC. Now, we also have an accuracy issue which I have not considered, but it is probably of the same magnitude, so overall, double the + and - numbers.

Gene Wengert, forum technical advisor

From the original questioner:
I have done a couple tests in my microwave now. I put 8 pieces in. I followed the instructions, then got impatient (always my downfall). Also, I didn't have the proper sample sizes and ended up smoking 2 of them. Lesson learned! I measured the pieces to 0.01 g and after 3 bakings at power level 3 for 2 minutes, on a scale of 1 to 10, the weights didn't change at all. My moistures were from 8.5 - 9.5%. I measured 9 - 11% with my Wagner and Delmhost pin meters. I'd say that was close enough for me. I'm happy with the results. My kiln claimed their samples came in at the high end of the spec'd 6-9%. I do believe the procedure works. Don't get impatient and over-heat the samples - they do smoke easily if you over-power them.

Contributor D, if you put water in the microwave, what is your procedure? I've seen on the Discovery channel someone put a cup of water and a beaker full of fruit flies in a microwave. The flies were flying around while the water was boiling! How about that for science? Microwaves in the oven are tuned to water molecules. Wet food heats up faster than dry - ever notice that?

Why are dry loads hard on magnetrons?

From contributor D:
The high frequency oscillation of the magnetron causes high-frequency movement of water molecules and that makes the heat. The water is a part of the electronic circuit. It keeps the oscillation in the correct frequency. Dry loads can let oscillations run damagingly high. For all I know, "new and improved" microwaves may have this fixed.

My idea might be all wet but I haven't blown a magnetron for awhile. I simply put a plastic soda bottle with water in it into the microwave with the wood samples. I put a rag in the top of the bottle as a stopper that will allow it to vent.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

We have been using microwaves for over 15 years to dry our kiln samples and the key to doing it right without burning the samples is the power setting on the microwave. Use it set at 30% power level and don't go over 15 minutes on each go around of cooking. Dry the sample till it stops losing any weight.