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Author Topic: Drying samples in a microwave oven -  (Read 291 times)

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Offline btulloh

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Drying samples in a microwave oven -
« on: May 19, 2017, 01:11:19 pm »
I just scored a free microwave from a neighbor that I plan to use for drying wood samples.  In a gov't publication I read that using a microwave oven can lead to over-drying a sample quite easily.  The threshold between well-done and burnt-to-a-crisp can be crossed pretty quickly.  ( I know this is true for food as I have crossed that threshold on many occasions. )

No doubt there are some of you using microwave ovens to dry samples.  Are there any tips or advice on avoiding over-drying samples?

Thanks -
HM126

Offline Ianab

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Re: Drying samples in a microwave oven -
« Reply #1 on: May 19, 2017, 04:26:23 pm »
Keep the power low and zap it in short bursts. Let it rest for a minute If its getting too hot. If you start seeing smoke or charring then you have over done it.

If it starts turning to charcoal the weight will change and you get a wrong measurement.
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Offline btulloh

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Re: Drying samples in a microwave oven -
« Reply #2 on: May 20, 2017, 08:41:18 am »
I found this discussion on another forum that is useful.

http://www.woodweb.com/knowledge_base/Moisture_content_oven_tests.html

The name of one of the contributors sounded mighty familiar.  ;)
HM126

Offline GeneWengert-WoodDoc

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Re: Drying samples in a microwave oven -
« Reply #3 on: May 20, 2017, 06:19:21 pm »
I pioneered using the microwave for small pieces, called moisture sections in kiln drying books.  The section should be about 1" x 1" and full length of the 4/4 piece of lumber.  The exposed end grain and short distance along the grain means they will dry quickly.  This is true for hot air or microwave.

I did a hundred tests to assure people that there was no difference between the microwave and a hot air oven. 

The key for a microwave is using a rotating trey and medium low power.  You can put one or more pieces in, but put them on a paper towel and do not let them touch each other.  Run them for 20 minutes.  It does take longer if you have more than one section, if the section is large, or if the section is wetter.  Then weigh and run for 2 more minutes.  If the weights (0.01 grams scale resolution) are the same, you are done, but usually more time is needed.  A few more minutes if nearly dry or 20 minutes or more if still fairly wet.  Never walk away from the oven, as if the section does begin to smoke, shut off the oven and remove the sections to outside, as they could smoke more and the smoke is really smelly. Maybe put them in a bucket of water.  The risk of smoking is almost zero if you follow the rotating trey, not touching, and med-low suggestions.

They do make an rotating trey you can add, if it is in the original unit.

Smoking means one spot has gotten over 450 F (approx.), but other spots can have moisture, so this is not oven dry when smoking.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline btulloh

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Re: Drying samples in a microwave oven -
« Reply #4 on: May 21, 2017, 09:39:31 am »
Thanks for the info Gene.  Fortunately this mw oven has the rotating tray and it is in good working order.  I'm going to test a few samples and see how it goes.  Right now it's hard to spend time watching the samples dry, but I'll get over that.

It seems like a conventional oven would be a lot more convenient, but acquiring that seems to be more difficult.  The only thing I see that is appropriate is lab-type ovens.  That's a lot of expense for a hobby operation.  I will keep looking for bargains.

It wouldn't be too hard or complicated to build an appropriate oven (or modify something) by using an appropriate temperature controller.  Factoring in safety measures, etc. is important since it would be running unattended, so it starts to become a bigger project.  The last thing I need right now is another project!
HM126

Offline GeneWengert-WoodDoc

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Re: Drying samples in a microwave oven -
« Reply #5 on: May 22, 2017, 06:20:33 am »
The hot air oven does need a safety switch to avoid over heating. 

It also should have a s¾mall fan to stir the air.  Otherwise, it will be hotter near the heating element.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more