High Voltage in your Kitchen: Unwise Microwave Oven Experiments , page 4
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UNWISE MICROWAVE OVEN EXPERIMENTS
High Voltage in the Kitchen

William J. Beaty      Electrical Eng.


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Some Microwave Oven Myths

Q: Do Microwave Ovens cook from the inside out?
A: 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 partly absorbed by the food. Usually 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. This effect is different for different foods of course. If a food is mostly water, then only the outside inch cooks at all. If a food contains both air and water (like bread, cake, etc.,) then the radio energy penetrates all the way through, and the food gets heated everywhere, even deep inside.
Q: If I put a fork in the Microwave, will it destroy the oven?
A: Nope, this is a myth, but it has some roots in reality.

In order to safely use metals inside a microwave oven, the cook has 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, where the staple heats up and sets fire to the bag. And if a metal object in the oven is lightly touched to another one, or touched to the metal wall of the oven, an electric arc might ignite at the contact point. If not stopped it can set fire to the oven. Also, sharp metal edges can cause problems. In the higher power ovens (800W and above) when the amount of food is small, sharp points and sharp edges on metal objects can initiate a corona discharge, a "Saint Elmo's Fire," which behaves the same as a flame and can set fire to the food and the oven if allowed to continue for long. Aluminum foil can become a blow torch!

So, it's much easier to totally ban the use of metals in microwave ovens. The alternative would be to send everyone to school to learn the complicated rules!

Q: What about the dangerous radiation?!!
A: Microwave ovens don't use "radiation." Instead they use radio waves.

Usually when we talk about radiation we mean radioactivity or "Ionizing Radiation," gamma rays and high energy subatomic particles. Microwaves aren't high energy, the microwave photons less energetic than visible light. Yes, if the source of microwaves is a very "bright" source, it can cook food. If normal light is very bright, light can also cook food (think of those solar ovens used by campers.)

Note that technicians and scientists talk about "microwave radiation" in the same way that they talk about "optical radiation." Optical radiation is just another word for VISIBLE LIGHT. They're using the word "radiation" to mean "waves." In this sense light is radiation, the warmth of an electric heater is radiation, even radio waves are radiation. And when you speak, some "acoustic radiation" comes out of your mouth. But radioactivity is an entirely different thing. Microwave radiation doesn't sterilize your reproductive organs like high-energy X-rays would. (An oven with a damaged door can only cook you!)

Q: Can a fluorescent tube detect microwave leakage?
How can you tell if your microwave oven is safe? There's one simple way to detect a major microwave leak. Get a straight fluorescent tube (not a CCFL bulb) and hold it against the edges of the oven's door when the oven is on. High levels of microwave leakage can make the bulb glow. Do this in a darkened room so you'll see the slightest glow. This works much better if there is nothing at all inside the oven. If you own an older oven (pre-1980s) you might want operate it only briefly (ten seconds.) That, or include a glass of water.

However, this method only shows the larger leaks. Your oven might be leaking too little, so it won't light a fluorescent tube. Instead try using a neon pilot light: the small orange kind such as "NE-2" bulbs from Radio Shack stores. These bulbs concentrate the voltage-field between their tiny electrodes, so they'll glow at lower microwave watts. Or, to catch even the smallest leakage, you need a "microwave leakage detector." Search the web for these. I've sometimes seen them on eBay for $10.

If you do have a Microwave Leakage Detector, hold it near an operating cellphone to get an idea of the accepted level of microwave wattage. Just like ovens, cellphones use microwaves in the GHz frequency range. If your microwave oven is leaking far less than the microwave output of your cellphone antenna, then you need not worry about your microwave oven.

Q: Aren't these ovens tuned to a special frequency so they only heat water?
A: No. The usual operating frequency of a microwave oven is far below the resonant frequency of water vapor... and liquid water doesn't have a resonant frequency. The peak is 10GHz to 50GHz depending on temperature, while microwave ovens run at 2.4GHz or 0.9GHz. Also, the radio energy in a microwave oven can heat many other substances besides H2O. Water isn't special. For example, drops of grease on a plastic microwave dish will be heated far hotter than 100C, and this causes the mysterious scarring which frequently occurs on plastic utensils. Any molecule which is "polar" and has positive and negative ends will be rotated back and forth to align with the changing e-field of the radio waves in the oven. The vibrating electric field of the radio waves vibrates the oil and water molecules and any other polar molecules within the food. Jostling molecules equals heat! Microwave ovens have difficulty melting ice, presumably because the water molecules are bound together and cannot as easily be rotated by the e-fields.

If liquid water really did have narrow resonant frequency, and if the oven was tuned to this frequency, then the water would be far more opaque to the wave-energy. The water in just the food's thin surface would absorb all the energy. In that case only the outside surface of foods would be heated. The end result might resemble the browning and slow cooking of normal electric ovens. Or, perhaps the thin outer surface of meat would become a blast of steam, and the inside would remain ice cold. But because water does not resonate with the microwave frequency, the waves can travel an inch or so into the meat before being absorbed. Microwave ovens heat a thick layer of meat, not a thin surface layer.

Another note: single H2O molecules have a sharp resonant frequency, but liquid water does not. In order to have a distinct resonance, a water molecule must be alone in space, not bound to billions of identical neigbors. The bonding to neighboring water molecules spoils the sharp resonance and greatly widens the absorption-frequency band. Liquid water has a huge, wide absorption band, not a single resonant frequency. In other words, water absorbs all short radio waves. Typical microwave ovens don't even use the best frequency. They should be up around 10GHz frequency rather than the usual 2GHz, but that would make the microwave tube more expensive. Here's an article about it:


Q: Don't you need distilled water to cause superheating and "explosions?" Impure tap water only boils, it won't explode.

Ummm. What?

Of course impure water can "explode." That's why people get scalded; they re-heat their coffee in microwave ovens and then quickly add sugar. Coffee is extremely impure water! To cause "coffee explosions," you don't need special pure water. All you need is water that lacks micro-bubbles.

It seems that some recent TV show tries to prove that only pure water can superheat and "explode." This is wrong. Probably the creators of that show did not know an important fact: nucleation centers for boiling are actually tiny bubbles. While solid crystals will grow upon solid nucleation centers found in impure liquids, the gas bubbles in cola, and in boiling pots, grow upon microscopic seed-bubbles. You can't grow crystals unless you have a solid microscopic seed. And you can't boil water unless you have invisibly small seed-bubbles present. It doesn't matter how filthy your water is, or how many crystal nucleation centers it contains... if it lacks seed-bubbles then it will not boil normally, instead it will superheat.

Clearly this has nothing to do with distilled water. In fact it's very easy to boil ultra-pure distilled water normally, without explosions. First chill and shake up your bottle of distilled water thoroughly. This creates lots of nucleation centers for boiling. And it's easy to make "impure" water explode; impure water called coffee and soup. If tap water has had all the bubbles cleared out by earlier boiling or by simply sitting for days in an open container, then it will superheat, and when sugar is dumped in, it will go DOOSH and spray all over.


Q: Why do Spagetti-O's explode all over the inside of the oven? BOOMPH!

Heh. I finally figured this out. Took me years. It's the same as with coffee explosions. If a liquid doesn't have any microscopic seed bubbles, then it will superheat and explode. There are several common liquids that totally lack seed bubbles: canned goods, eggs, meat, and vegetables.

Canned goods are boiled and then sealed. The boiling will remove all the dissolved gases, and as the food cools, the live steam condenses back into water, creating a fairly good vacuum. That's why the lid on the jar of spagetti sauce goes "poink" when you break the seal. In a vacuum... THERE ARE NO SEED BUBBLES. Duh! So Spagetti-Os and jars of applesauce are just like the explosive coffee which has been boiled and cooled several times. They're like bombs waiting to go off. There's a possible cure: stir them first. Stir them well, whisk them up so there's lots of little bubbles mixed in. (I don't know how well this works, I've only just started experimenting.) The only other cure is to stick a bunch of wooden chopsticks into the food. You need several, since unlike coffee, the food doesn't stir itself as it heats.

Eggs also explode. Did you ever try frying an egg in a microwave? The yolk tends to go BOOMPH! It seems that biological systems can adjust the amount of dissolved gases within themselves. As long as the dissolved gas is a bit lower than the natural amount, this will cause all tiny bubbles to shrink and vanish. Tiny bubbles will dissolve. This makes sense where living things are concerned. You don't want your tissues to fill with foam! Given the choice, Nature would rather make bubbles inside plants and animals slowly shrink, not slowly grow.

So eggs, meat, and vegetables are "exploding coffee" too. Unless you whisk some air into your eggs, they will tend to superheat and explode. Same with potatoes, beets, etc. At least with vegetables you can slice them thin. That gives them large surface area, and destroys any large volumes which are far from the air. Meat is the same: it can explode if overheated. Add some air by slicing it, or use a needle to poke it full of holes.


Q: I want to buy a microwave space heater!

Heh. Me too. Why aren't they available? One main reason springs to mind: in your eyes, the cornea and internal lens are very sensitive to heating. Unlike most other body parts, they have no blood vessels to keep them cool. Intense radio waves can give you instant cataracts, so a microwave heater would require that we use special goggles to prevent blindness. Another problem: a normal heater heats only your skin surface, while a microwave heater heats a very thick layer of flesh. So, with a normal heater you can yank your hand away when you feel pain, and your thin skin cools down almost instantly as the surface heat migrates to fill a thicker layer. But with a microwave heater, you might receive internal burns at the same time that you feel pain on your skin. And when you yank your arm away, then the thick layer of heated flesh wouldn't cool instantly, causing even more tissue damage. And last, a microwave heater is an intense electrical source, and it would probably destroy any radio, cellphone, PDA, or laptop that came near. Lawsuit city!

But here's an interesting link found by Marcus:

Heating homes with microwaves
http://www.personalmd.com/news/a1996122603.shtml


Q: Doesn't microwave energy lower the food's nutrition?

I don't know.

On the other hand, I've heard lots of crazy rumors along the lines of "microwave energy turns food into slow poison." Maybe it's true, or maybe all the rumors are just some BS made up by bored storytellers. However, because these crazy rumors exist, we must be on guard against believing them, and we should only trust information if we can get it from people who are up front about where the information came from in the first place. Anything else, and we'd end up believing the liars who have fun by starting rumors, hoping their rumors will catch fire and scare huge numbers of victims.

Just follow the same rule that you follow for crazy spam emails that give you all sorts of dire warnings about various topics. The rule: if the warnings were real, wouldn't it be dead easy for the original email author to include several URLs pointing to many articles about the danger? So, if an email doesn't link to real live websites, it's almost always a false rumor being spread by hoaxers. And with any health warning, if warnings about microwaves don't link to news articles or perhaps actual medical journals, almost certainly the author is passing on third-hand rumor rather than a story about a genuine hazard.

Below are a few of my own rumors! With no links to research or online news!

:)

I vaguely recall seeing something in the papers long ago about microwave ovens harming food vitamins. But I don't remember if they said that microwave-cooking is worse than REGULAR BOILING. I don't know if they said that the problem comes from simple overcooking rather than from any weird stuff with radio waves. I've always heard that eating raw vegetables is best, and cooking destroys vitamins. What happens when you feed people on overcooked or canned food for months? If somebody says that microwave cooking is much MUCH worse than a canned food diet, I'd like to find out WHY they say that. Maybe they're really just warning us about boiled vegetables, but trying to make out like the problem is with the microwave ovens rather than with the boiling. Microwave ovens cook food deeply and quickly, so if you boil your vegetables for five minutes on a stove, they'll still be green and crunchy, but if you boil your veggies for five minutes in a microwave oven, they'll be way overcooked. With microwaves, it's hard to cook only the outside surface of the vegetables. (Now that I mention it, even boiling water doesn't do such a great job. It's better to stir-fry veggies so the inside is almost raw but the outer layer is cooked.)

If we need to be warned about any cooking, the warnings must be precise. And fake warnings themselves aren't innocuous. Don't forget, Halloween trick-or-treating in the USA was ruined by a rumor-storm of fake warnings about poisoned candy and razor blades.

Another issue: microwave cooking is unsafe... COMPARED TO WHAT? Life is unsafe. People die from taking showers or stepping on toys, not from eating microwaved food. Think a minute. When you eat browned meat or even baked bread, the browned parts are full of heat-shredded biomolecules which form all kinds of NASTY CARCINOGENS. Yet they seem not to harm people very much (perhaps we've all gotten used to the stuff over the millenia because we've been eating charred food ever since humans discovered fire.) As I understand it, browned food is a dark secret of cancer studies, and if they applied the current FDA regulations to normal foods like bread or BBQ, the government would have to ban cooking. Browned foods have been "grandfathered in" to the cancer regulations. Yet perhaps the carcinogens in browned food do occasionally cause things like colon cancer? If they do, then perhaps microwave cooking... is much SAFER than normal ovens? Since microwave cooking is akin to steaming? It's almost impossible to force your microwave oven to shred the molecules on the outside of your steak into tasty brown carcinogens. Instead you get steamed steak. MMMMM, healthy.

Does this mean that microwave ovens are health-food devices? Maybe stores should've been selling microwave ovens next to the carrot juicers and wheat-grass kits? I don't know. First I'd want to know if anyone performed any microwave-cooking nutrition studies at all, even with lab animals. And as a control, compare the results against such common health dangers like going swimming or (gasp) DRIVING A CAR... or at least compare it against baked bread and fast food hamburgers.



Q: Corona discharges? High voltage? Balderdash! The watts per cm^2 is too small!

A: Wrong, melted pyrex breath! You're thinking about cooking huge roasts. What happens with EMPTY microwave ovens?

Yes, a typical oven's output might only be 1000 watts or so. And yes, 1000W spread over a few hundred square centimeter does not proudce strong e-fields. The fields will only be a few hundred volts per cm. (For corona discharge we need around 30,000V/cm, a hundred times more intense.) But you're forgetting something important: the voltage rise for resonant circuits and standing waves. If there's a huge roast (a significant electrical load) then... 1000W leaves the magnetron, and 1000W is absorbed by the meat. There's a 1000W energy flow between magnetron and food, and the max. e-field inside the oven stays low.

On the other hand, if there's no food in the oven, then the 1000W bounces back and forth, yet the magnetron still puts out more energy. This adds to the waves already there. It's like wiggling your hand in a full bathtub: energy is stored as standing waves, and the waves build up higher and higher until frictional losses finally halt their growth. Inside an empty microwave oven you might have 50,000 watts in one direction and 49,000W in the other (with the magnetron supplying the 1000W difference.) The interior of the oven is a resonator, and the peak wattage within that space can become HUMONGOUS, since it only depends on the "Q" factor of the resonator; it depends on the "friction" of the system. Q is high, so huge wattage gives huge voltage. If there's no food and no glass of water in the oven, then the e-fields become intense, and the tiniest burr on a metal object can trigger the formation of a large "Saint Elmo's Fire" which consumes hundreds of watts of RF energy and resembles a blowtorch.

In fact, the empty chamber of a microwave oven is very much like the secondary coil of a Tesla Coil. The only major difference is the operating frequency. RF energy is injected into the resonator, and the output voltage rises and rises until finally the conductors get hot (or until finally an electric arc breaks out somewhere.) Tesla coil secondaries do this. Microwave ovens do this too. With nothing in the oven chamber, either the metal walls and glass parts get very hot, or an electric arc bursts forth from a sharp metal point somewhere inside the oven cavity.



Q: How do microwave ovens work?

A: OK, this question doesn't involve microwave oven myths, so I guess this section is becoming a FAQ. Microwave ovens are weird, they were born in a military "black project" dealing with exotic physics. The microwave vacuum tube had its birth in England during World War II, and was the central part of a new secret weapon: radar. Eventually the secret military technology was declassified, and it ended up in appliance stores everywhere. One is led to wonder how many other incredible military discoveries are still sitting unused in that (perhaps) non-mythical government warehouse seen briefly at the end of the first Indiana Jones movie!

The Klystron and Magnetron microwave tubes both rely on nonlinear effects of density waves in particle streams interacting with solid surfaces and tuned cavities. That's right, they are identical to that bizarre resonant-cavity standing wave generator known as the EMPTY BEER BOTTLE.

A microwave vacuum tube is like a whistle. Blow across a glass bottle's orifice, and tiny sound waves within the bottle will cause the air jet from your lips to move slightly. Motions of the air jet create pressure waves in the bottle. Pressure waves wiggle the jet even more. Runaway feedback takes place, and a loud sound is created. If we replace the air with an electron stream in a vacuum, and use a hollow metal bottle, then radio waves will build up in the bottle, and they will deflect the electron stream back and forth. A microwave tube is an electron-whistle which creates a loud "sound" in the form of radio waves. Intense sounds can heat objects, and intense radio waves do the same.

If we start our electron-whistle operating, and then if we try to extract energy, the resonance is ruined and only a little wave-energy comes out. This problem was solved by using multiple "bottles" and a magnet to direct the electron stream across their "mouths". A magnetron tube consists of a central electron-emitter, one or two disk-magnets which cause the electrons to swirl in a whirlpool motion, several tuned cavities with their open mouths pointed into the whirlpool of charges, and a high voltage power supply which moves the electrons along at high velocity. Energy is extracted from just one of the tuned cavities, and this has only a small effect on the resonance of the others.

If you wanted to create an acoustic model of a microwave oven, you could attach a vacuum cleaner to the center of a heavy cylindrical box. Put slots around the edge of the box. The resulting tornado acts to supply the jet of high-speed air that will "play" the bottles. Several glass bottles could then be poked through the sides of the box, inserted into the tornado, and adjusted to give a loud sound. With luck, you might even be able to connect an "exit tube" to one of the bottles, connect the tube to a sealed metal cabinet, then actually heat any objects which are placed in the cabinet. Use really thick, heavy construction materials, otherwise the intense sound would not stay trapped inside your beer-bottle-tron device. It would sound like an air-raid siren.

The speed of light is about one million times faster than the speed of sound. However, audible sound is about a million times lower in pitch than microwaves. The effects cancel out, so the wavelength of the sound waves in glass bottles would be very similar to microwave wavelength. So instead of giving out 1,000 MHz microwave radiation, your device would give out sound radiation that's a few inches in wavelength (just like microwaves,) but a million times lower in frequency, or about 1000 Hz.


Q: I nuked the fruit flies on the food, but they survived!

I noticed the same thing. There are several possibilities. First, the pattern of heating inside the oven is NOT uniform: there are hot spots and cold spots, and most important: the hot spots don't touch the metal walls. If a bug crawls on the oven's inner surface, it's fairly safe. Also, if you're cooking a large hunk of food at the time, then this food absorbs the RF energy like mad, and insects won't get as hot as when the oven is totally empty. Also, insects have built-in behavior to avoid being cooked by sunlight... if they feel hot, they crawl faster, and if the heat stops, they stop too. Perhaps when you turn on the oven, all the bugs move until they hit a cold spot in the radio wave pattern, then they stay in that spot. (So if a bug was on the rotating glass platter far from the metal walls, then it's out of luck.)

Q: I tried making "microwave ball lightning." It didn't work.

For better results, try it with Matt Crowley's discovery, carbon fiber: carbon "veil" from a plastics supplier or fiberglas supplies store. JL Naudin finds that the sharp carbon stick from a mechanical pencil also works well.

The demo with the burning candle requires a microwave oven with a turntable inside. The turntable carries the candle into a "hotspot" in the oven, then plasma bursts forth. Without a turntable you might have to spend twenty minutes moving the candle to different spots. Also, I find that my 1000 watt oven makes plasma flares very easily, but an older 500-watt version might not have the gusto. Try using a newer, "hotter" high wattage oven. Also, sometimes all the hotspots are a certain distance above the glass plate. If your stubby candle won't make BL, try propping it up on a wood block or an overturned bowl. Try different heights.

I suspect that the "ball lightning" needs both a Saint Elmo's fire and a source of chemicals in order to get going. Sharp pieces of metal sometimes produce microwave arcing, but it's unreliable. A charred toothpick is a conductor, and also it has incredibly sharp points (the carbonized wood fibers.) This might be why a normal candle doesn't make fireballs, but a candle with charred toothpicks near the flame DOES. Or perhaps the charred toothpicks contribute ions? Maybe instead use uncharred toothpicks which are wetted with salt water?

Here's a question of my own: if you try to catch a fireball in an upside-down glass chemist's funnel, does the plasma pour up through the funnel's thin tube? Could it even escape from the oven if you drilled a hole in the oven top to pass the glass tube? PLASMA TORCH! I haven't tried this one myself.

Q: The turntable rotates randomly? Why?
A great mystery within microwave ovens: WHY DOES THE TURNTABLE SOMETIMES ROTATE BACKWARDS? I always wondered about this. The obvious explanation is that the turntable motor is a 60Hz synchronous induction motor. But why? Synchronous motors aren't as good as the normal kind. One thing might make sense: it forces your turntable to end up in the same position as it started. That way your coffee mug will be at the front, or the handles on the cassarole dish will be positioned correctly. But my microwave oven doesn't do this. Most of the time the mug ends up in a crazy position.

Testing is required. I heated a mug of tea at work for a minute, and for the first time I actually watched the clock as the turntable rotated. AHA! IT ROTATES ONCE EVERY TEN SECONDS!!!! I verified the effect and it does work: as long as you punch in multiples of 10 seconds, your food will come back to its original position. But something's screwy. My oven at home doesn't do this, yet its turntable randomly starts off clockwise or CCW, so it must contain a synchro motor. So I timed the oven at home. Bingo: it rotates every 20 seconds. That explains everything. At home, if I punch in 30 seconds, or 10 seconds, then the turntable rotates an extra half turn, putting the soup bowl on the opposite side. Not too smart. How many people cook things for 20 seconds, or 40 seconds? A 3RPM turntable speed only works if you cook something for one minute. But now that I know about the problem, I can start only using multiples of 20 seconds.



Try Dr. L. Bloomfield's Microwave Oven FAQ

 

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