Est. 1998 — 27 years of woodworking knowledge
a caution to microwave wood driers
Bill Tindall, E.Tn.
Microwaves are designed to match the microwave radiation to the microwave absorption of the water molecule, for obvious reasons given their normal function. If the oven is used to dry a small piece of wood or wood to 0% water the 1000 watts of microwave power will only be partially absorbed by the small piece of wood or the dry wood. The rest will be bouncing around and may result in the damage described in the attachment.
I'm curious how to offset this risk.
Chuck Nickerson
Would either A or B work?
A. Don't try to take it down to 0%.
B. Have a small bowl of water in the oven.
Re: I'm curious how to offset this risk.
Clint Searl
If you're gonna use it to dry wood, put the cat in there, too.
Re: a caution to microwave wood driers
Walter Ahlgrim
The First Law of Thermodynamics tells us that energy can neither be created nor destroyed. Something will absorb that energy.
I do not think it is as scary as Bill’s quote, after watching this YouTube video it seems the energy spread more or less evenly around the oven. After 30 minutes no apparent damage.
https://www.youtube.com/watch?v=AsaW5xnOkCA
Walta
Re: I'm curious how to offset this risk.
Bill Howatt
A makes the most sense, keep some moisture.
The bowl of water in B would likely absorb most of the energy and lengthen the drying time. This is similar to the potatoes in your supper getting hot and the meat doesn't get as hot (or vica versa).
Damage to the magnetron would depend on what standing waves are set up and how much power gets reflected back and if there is any safety circuits. When I worked with large klystrons we had reflected power detectors, arc detectors, etc in the waveguides to shut the power off immediately if there was a detection. Another memory lane experience!
Bill
Re: a caution to microwave wood driers
Larry Clinton At Frankfort, (Central) Indiana
You are correct Bill, however since the 80's most if not all microwave ovens have mitigated this effect with redesign of the wave guides.
They can still be damaged if they are allowed to run for a few minutes with nothing in the oven, although they are far more tolerant of damage. They can also be damaged if there is no moisture in the food or other material in the oven.
My company designed and built microwave timers / controls, and we tested the controls usually with a neon light laying in the oven and put a small container of water in the unit. As long as there is moisture in the material of the oven it will adsorb the energy.
microwave energy distribution
John K Jordan
>>> ...it seems the energy spread more or less evenly around the oven...
I didn't watch the video and am no expert, but I learned a tiny bit about the energy profiles inside microwave cavities. At the Lab I created graphics and animations based on microwave research data. The energy distribution depended on the microwave frequency and the cavity size and shape. At worst case there was one huge hot spot in the resonant cavity. Various designs try to even out the energy. One successful but expensive method was to vary the frequency over time which changed the resonance and evened out the heat over time. This has been used in applications such as creating diamond film. A cheaper method is tune and spread as well as reasonable then use a rotating table so the object being heated is always moving through the hot spots.
BTW, for microwave entertainment try watching while cooking old CDs.
Drying to 0% moisture?
John K Jordan
>>>Chuck writes: I'm curious how to offset this risk.
Would either A or B work?
A. Don't try to take it down to 0%.
B. Have a small bowl of water in the oven.
Maybe I missed something, but I don't understand the need to dry wood to 0% unless destructively testing a sample for absolute moisture content. Under some circumstances drying to 0% may even physically damage or destroy the wood as noted below. Regardless, as soon as an absolutely dry piece of wood is out of the oven it will immediately start gaining moisture until it reaches EMC for that environment.
People do sometimes use the microwave ovens to dry small pieces of wood for use but that's best done by zapping in many small steps, each of maybe 20 or 30 seconds, letting the wood cool between each zap. Otherwise the wood can get very hot and char or even catch fire (and stink up the kitchen). I've done this when in a big hurry and wanted to turn a small blank on the lathe that day rather than wait for weeks or months for the piece to air dry. I did NOT try to dry all the way to 0% moisture content.
To dry to 0% to measure the moisture content a better and perhaps safer method is to use a toaster oven on the "warm" setting. A resistive-heating oven can apply a even heat without hot spots and the chance of overheating the wood from the inside out.
To monitor the temperature of the toaster oven I use a thermocouple and an inexpensive reader: https://www.amazon.com/gp/product/B001EUB6BM.
A small digital scale is so cheap it's almost free: https://www.amazon.com/gp/product/B002SC3LLS
A couple of years ago I posted more information about using the "oven dry method" to measure the absolute moisture content here, on the Turning section of WC:
Note, to be clear this is not necessarily a good way to dry wood for use, but a destructive testing method best used to test the moisture content of a number of boards or pieces of similar thickness that have been cut at the same time and dried or stored under the same conditions. It's sometimes used to test a kiln load of wood. When complete, the test sample is bone dry at 0% moisture and may even develop cracks from the process, depending on the wood and starting moisture. The sample is best taken from the center of one piece.
R. Bruce Hoadley's "Understanding Wood" is a good book for understanding all this and why it is important, whether you work with green wood or dry. Lots of woodturners like to turn green wood but I mostly turn dry.
https://www.amazon.com/Understanding-Wood-Craftsmans-Guide-Technology/dp/1561583588
Hoadley is a craftsman as well as a wood scientist so his book is more readable and useful than some of the technical literature. I also like his book "Identifying Wood" - I use it so much that my copy is falling apart!
JKJ
to answer your question
Bill Tindall, E.Tn.
The topic is measuring the moisture content of a wood sample. To do the calculation one needs the weight of the wood at 0% moisture. Some advocated using a microwave to get to 0%. I am pointing out another reason why an oven is better.
If the moisture content of the test piece is high this test will destroy the test piece. As it begins to dry it will fracture severely. That's good. It will let the water out quicker.
Re: to answer your question
John K Jordan
>>>Bill replied with "To answer your question"
The topic is measuring the moisture content of a wood sample. To do the calculation one needs the weight of the wood at 0% moisture. Some advocated using a microwave to get to 0%. I am pointing out another reason why an oven is better.
If the moisture content of the test piece is high this test will destroy the test piece. As it begins to dry it will fracture severely. That's good. It will let the water out quicker.
For completeness: "To do the calculation" one also needs remember to record the starting weight as well as the weight at 0% moisture.
You provided a reasonable summary of what I just posted.
I still do not find the thread or messages on the topic that prompted your post "a caution to microwave wood driers" that started this thread. Perhaps you can post a link to the earlier discussion. I would like to read that.
JKJ
Re: to answer your question
Bill Tindall, E.Tn.
http://www.woodcentral.com/woodworking/forum/messages.pl/page/1/md/read/id/531535/sbj/i-do-not-recommend-the-microwave-method/
Re: to answer your question
John K Jordan
>>>
http://www.woodcentral.com/woodworking/forum/messages.pl/page/1/md/read/id/531535/sbj/i-do-not-recommend-the-microwave-method/
Thanks, that helps. I agree with both you and Richard, better to use the non-microwave oven method but the microwave will work fine with the method stated.
I've never seen or experienced the problem you illustrated but I have heard of such from others. The combo of your photo and description is a word to the wise. If OK with you I may save both for if this comes up elsewhere.
JKJ
Re: to answer your question *LINK*
Sgian Dubh
John, now that my name has been raised in this thread (one I had no intention of becoming involved in, to be honest), I suspect it's possible Bill's comments were prompted by the reference to microwave oven drying of wood I made in the thread he pointed you to earlier, as well as by the fact there is a description of the microwave oven drying methodology to establish wood MC in my book, Cut & Dried: A Woodworker's Guide to Timber Technology, a copy of which Bill has.
Anyway, at some point during the earlier writing stages of my book (2009 I think), I extracted from it the description of microwave oven drying for wood MC and turned it into a page on my website which you're welcome to have a look at at the attached link. Slainte.
Testing for Wood MC with a Microwave Oven
Re: to answer your question
John K Jordan
I'll look. BTW, I bought two copies of your book and intended to auction one as a WC benefit. However, I haven't heard back from Ellis when I asked about the current mechanics of doing a benefit auction.
Do you know? I haven't seen an auction in a while, at least a couple of years. If we are no longer doing those pass it on elsewhere.
JKJ
Re: to answer your question
Sgian Dubh
John, I very much appreciate you buying one copy of my book, but to buy two goes above and beyond what I'd hope for in anyone, so thank you very much indeed. I do hope you've found it interesting, and even perhaps useful.
As to the question about auctions here at WoodCentral, I really don't have an answer. My suggestion would be to try and contact Ellis again.
Sorry I can't help more. Slainte.
Re: to answer your question
Don Stephan
Well Richard, I was starting to jot some notes for a talk in a few weeks, and was using both your Cut & Dried and Hoadley's Understanding Wood. My opinion they complement each other extremely well, and I'm glad I have both. I plan to recommend both during my talk.
Re: to answer your question
Sgian Dubh
That's very kind of you Don. I can now look forward with certainty to significantly enhanced royalty payments in the near future ... perhaps, ha, ha.
What's the subject of your talk, just as a matter of interest. Slainte.