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TUCoPS :: Hardware Hacks :: micro_ex.txt

Unwise microwave oven experiments.





Microwave Oven Experiments

Over the years I've indulged my fascination with microwave ovens by doing
unwise experimentation.

Disclaimer: This information is presented for your information only. Anyone
who tries to duplicate these demonstrations does so entirely at their own
risk. There is a chance that you will damage your microwave oven. There is a
chance that you will cause a fire. 

They're Heeeeere!

Years ago I was living with roomates, and while working in the kitchen I
noticed that the florescent light over the sink was about 8 inches long. A
light went on in my brain ;) because I'd always wondered what would happen if
a flourescent tube was placed in a microwave oven. In theory the RF energy
should have enough voltage to ignite the mercury vapor into a plasma, and the
lamp should light. But standard ovens put out at least 500 watts, so the tiny
flourescent tube should light quite bright, to say the least. I'd never
encountered a flourescent tube which was short enough to fit. So, I pulled out
the tube, stuck it in the oven, said "THEY'RE HEEEEEERE!" , and punched the ON
switch. Sure enough, the kitchen was lit up by a blue-white blaze of light
coming from the front of the microwave oven. I only let it run for about 1
second, but this was enough to heat the flourescent tube so it was too hot to
touch. 

Snifter of Neon

While working on a microwave article for an encyclopedia, it crossed my mind
that it might be possible to map the pattern of RF energy in the oven by
filling it with low pressure gas. The gas would glow in proportion to the RF
electric field in various parts of the oven's volume. This would be an
involved bit of construction to pull off, so I did the next best thing. I
grabbed a big bag of NE-2 neon pilot lights and stuck them into a wineglass,
hoping that this small volume would show some patterns when the glass was
rotated by the oven's turntable. I filled the glass with water, to give the
oven something to heat so it wouldn't be damaged by the small load presented
by the bulbs. I ran the oven, and the bulbs glowed REALLY BRIGHT. As the
turntable turned, various bulbs extinguished and others lit up. However, I
could see no coherent patterns. When I emptied the glass, I discovered that
several of the bulbs were stuck together. The short metal leads of some bulbs
had melted into the glass of adjacent ones. Also, several of the bulbs had
small holes melted through their glass, and were full of water. Apparently the
plasma temperature was so high that it heated the glass to melting. Or,
possibly some corona discharges developed between the inside and outside of
the bulbs and burned through the glass.

Foil-eating Plasma

I'd seen flames produced by microwave ovens before. In the strong RF field,
even the tiniest flame will absorb a large percent of the many-hundred-watts
oven output. Thousand watt candle? So, I decided to try initiating an
electrical flame-discharge intentionally. I tore aluminum foil into 2"
squares, crumpled it lightly so it didn't lay flat, then placed it on the oven
turntable with the two pieces in light contact. Sure enough, when the oven was
turned on there was a loud buzz and a bright light, and a flame erupted from
the contact point between the two pieces of foil. When I looked in on them, I
found that the brief flame had eaten a bite about the size of a dime out of
both pieces. 

Mapping the Energy Nodes

Microwave ovens cook unevenly because a pattern of standing waves forms inside
the oven chamber, and the pattern creates an array of hotspots throughout the
oven's volume. An operating frequency of around 2000 MHZ will produce a
wavelength of around 10cm, and the hotspots should be at halfwave points, or
every 5cm, but in a complex 3D pattern. I'd always wondered how this could be
visualized. Perhaps fill the entire oven with raw eggwhites, then let the oven
cook them into an interesting white, rubbery 3D sculpture? Or fill the oven
with solid wax, and let the RF hotspots melt out a 3D structure of holes?
Finally someone figured it out: 

   Alistair Steyn-Ross and Alister Riddell, STANDING WAVES IN A MICROWAVE 
   OVEN, The Physics Teacher October 1990, Vol. 28 No. 7 pp474-476

Steyn-Ross and Riddell were stimulated to investigate the pattern of melted
cheese on an oven-cooked pizza. They hit on the use of Cobalt Chloride soaked
paper. When wet, CoCl solution is pink, but turns sky blue when dry. (It's
sometimes sold as "weather indicator" paper) They discovered that this worked
beautifully, and a large square of the paper would give varying patterns of
pink and blue when supported at different heights on a tile of cork within the
oven. 


Some Microwave Oven Myths

Do Microwave Ovens cook from the inside out? 

   Nope. Food is partially transparent to the radio waves, so the energy is
   able to shine through it, but at the same time the waves are absorbed by
   the food. Most of the heat is produced in an outer layer about an inch
   thick. So, large pieces of meat will be quickly cooked to a depth of about
   an inch, while the inside portions are cooked by heat conduction, just like
   in a conventional oven.

If I put a fork in the Microwave, will it destroy the oven? 

   Nope, this is a myth, but it has some roots in reality. In order to safely
   use metals inside a microwave oven, the cook would have to learn numerous
   complex and mysterious rules in order to avoid fires and undercooked food.
   For example, thin metal will heat up fast in the oven, and may cause fires.
   The famous problem of the staple in the paper popcorn bag comes to mind. If
   a metal object is touched to another one or to the metal wall of the oven,
   an electric arc might ignite there and if not stopped it can set fire to
   the oven. Sharp conductive points can initiate a corona discharge, a "Saint
   Elmo's Fire" which behaves the same as a flame and can set fire to the oven
   if allowed to continue. So it's much easier to totally ban the use of
   metals in microwave ovens.

Aren't these ovens tuned so they only heat water?

   No. The usual operating frequency of a microwave oven is nowhere near the
   resonant frequency of water, and the RF energy will heat other substances.
   For example, drops of grease on a plastic microwave dish can be heated far
   hotter than 100C, and cause the mysterious scarring which frequently occurs
   on plastic utensils. Any molecule which is "polar" and has positive and
   negative ends will be rotated to align with the electric field of the radio
   waves in the oven. The vibrating electric field vibrates the water
   molecules (and any other polar molecules) within the food.




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