Tag: microwave ovens

How do the new Cyclonic Wave Microwave Ovens work?

Lou Bloomfield Leave a comment

How do the new Panasonic Cyclonic Wave Microwave Ovens work and are they better than ordinary microwave ovens?

I just did a quick check of the patent literature to see if I can figure out what those new Panasonic ovens are doing differently. I found an international patent application WO 2009011111 A1 and 5 national applications, including US 20100176123 A1. Assuming that these patent applications describe the new microwave ovens, they look interesting.

One of the basic problems with microwave cooking is that the microwaves reflect from the metal walls of the cooking chamber and establish a standing wave pattern. Standing waves appear whenever vibrations are confined to limited region of space, such as in musical instruments on the surface of water in your drinking glass. The vibrations “dance in place,” which is why they’re called standing waves. In some places, the motion of a standing wave is strong and in other places, it’s weak.

The standing waves that form in a microwave oven’s cooking chamber aren’t vibrations; they’re electromagnetic standing waves. Nonetheless, they have that same characteristic of having more intensity in some places than in others. Those variations in microwave intensity produce uneven cooking and are thus a nuisance. Two conventional solutions to the uneven cooking problem are to move the food about, typically on a rotating platform, or “stirring” the microwaves with moving metal objects that dither the standing waves about.

What Panasonic appears to have done is develop a more sophisticated microwave source, one that can steer the microwaves as they first enter the cooking chamber. The technique they’re using is similar to that used in phased-array radar; both devices steer their microwave emissions by adjusting the relative phases of their many emitting antennas. If they swirl the emitted wave around rapidly, calling it a “cyclonic wave” would seem appropriate.

By changing that microwave steering rapidly, the Panasonic oven can vary the standing wave pattern inside the cooking chamber so that each region of space has approximately the same average microwave intensity. Food should therefore cook relatively evenly in this oven.

The approach makes sense and should be effective. As to whether the technique is cost effective and how it compares to the other techniques for improving the uniformity of cooking, that’s beyond what I can predict myself.

What happens when you leave a fork in food while microwaving that food?

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What happens when you accidentally microwave a metal fork, and then eat the food it was in? — RH, Seattle

Both the fork and the food are almost certainly safe. While the microwave oven is operating, electric current will flow through the fork and electric charge will accumulate momentarily on the tips of the fork’s tines. However, most forks are thick enough to handle the current without becoming noticeably hot and have tines that are dull enough to accumulate the charge without sparking. The end result is that the fork doesn’t do much while the oven is operating; it reflects the microwaves and therefore alters the cooking pattern slightly, but you probably won’t be able to tell. Once the cooking is over, the fork is just as it was before you put it in the oven and the food is basically just microwaved food.

If a fork has particularly sharp tines, however, then you should be careful not to put in the microwave oven. Sharp metal objects can and do spark in the microwave oven. Those sparks are probably more of a fire hazard than a food safety hazard—they can ignite the food or its container and start a fire.

If a microwave oven door reflects microwaves, why doesn’t everything?

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If microwaves are reflected by the door on a microwave oven, how are they able to pass tens of miles from mobile phone transmitters through solid walls and into our mobile phones? — JW, Belfast, Northern Ireland

The door of a microwave oven is carefully designed to reflect microwaves so that they can’t escape from the oven. That mesh that you see in the door isn’t plastic, it’s metal. Metal surfaces reflect microwaves and, even though the mesh has holes in it to allow you to observe the food, it acts as a perfect mirror for the microwaves. Basically, the holes are so much smaller than the 12.2-cm wavelength of the 2.45-GHz microwave that the microwave cannot propagate through the holes. Electric currents flow through the metal mesh as the microwave hits it and those currents re-radiate the microwave in the reflected direction. Since the holes aren’t big enough to disrupt that current flow, the mesh reflects the microwaves as effectively as a solid metal surface would.

As for how your cell phone and the cell tower can communicate for miles despite all the intervening stuff, it’s actually a challenge. The microwaves from your phone and the tower are partly absorbed and partly reflected each time they encounter something in your environment, so they end up bouncing their way through an urban landscape. That’s why cell towers have multiple antennas and extraordinarily sophisticated transmitting and receiving equipment. They are working like crazy to direct their microwaves at your phone as effectively as possible and to receive the microwaves from your phone even though those waves are very weak and arrive in bits and pieces due to all the scattering events they experience during their passage. Indoor cell phone reception is typically pretty poor unless the building has its own internal repeaters or microcells.

There are times when you don’t get any reception because the microwaves from the cell phone and tower are almost completely absorbed or reflected. For example, if you were to stand in a metalized box, the microwaves from your cell phone would be trapped in the box and would not reach the cell tower. Similarly, the microwaves from the cell tower would not reach you. Moreover, the box doesn’t have to be fully metalized; a metal mesh or a transparent conductor is enough to reflect the microwaves. Transparent conductors are materials that conduct relatively low-frequency currents but don’t conduct currents at the higher frequencies associated with visible light. They’re used in electronic displays (e.g., computer monitors and digital watches) and in energy-conserving low-E windows. I haven’t experimented with cell phone reception near low-E windows, but I’m eager to give it a try. I suspect that a room entirely walled by low-E windows will have lousy cell phone reception.

Are microwave makers responsible to warn people about superheating explosions?

Lou Bloomfield Leave a comment

I read the letter on your website about being burned from water exploding from heating in the microwave. This recently happened to me with a cup of coffee. I thought it was because of the material of the coffee cup. I too felt ridiculous trying to explain my injuries to anyone because it seemed impossible. My question to you is: “are microwave makers responsible for advisement of this kind of hazard”? — JB

When you use a microwave oven to heat water in a glass or glazed container, the water will have difficulty boiling properly. That’s because boiling is an accelerated version of evaporation in which water vapor evaporates not only from the water’s upper surface, but also through the surface of any water vapor bubbles the water happens to contain. I use the phrase “happens to contain” because that is where all the trouble lies.

Below water’s boiling temperature, bubbles of water vapor are unstable—they are quickly crushed by atmospheric pressure and vanish into the liquid. At or above water’s boiling temperature, those water vapor bubbles are finally dense enough to withstand atmospheric pressure and they grow via evaporation, rise to the surface, and pop. At that point, I’d probably call the water vapor by its other name: steam. But where do those steam or water vapor bubbles come from in the first place?

Forming water vapor bubbles in the midst of liquid water, a process called nucleation, is surprisingly difficult and it typically happens at hot spots or non-wetted defects (places where the water doesn’t completely coat the surface and there is trapped air). When you boil water in a metal pot on the stove, there are hot spots and defects galore and nucleating the bubbles is not a problem. When you boil water in a glass or glazed container using a microwave oven, however, there are no significant hot spots and few non-wetted defects. The water boils fitfully or not at all. The “not at all” possibility can lead to disaster.

Water that’s being heated in a metal pot on the stove boils so vigorously that the stove is unable to heat it more than tiny bit above its boiling temperature. All the heat that’s flowing into the water is consumed by the process of transforming liquid water into gaseous water, so the water temperature doesn’t rise. Water that’s being heated in a glass container in a microwave oven boils so fitfully that you can heat it above its boiling temperature. It’s simply not able to use up all the thermal energy it receives via the microwaves and its temperature keeps rising. The water becomes superheated.

Most of the time, there are enough defects around to keep the water boiling a bit and it superheats only a small amount. When you remove the container of water from the microwave oven and toss in some coffee powder or a teabag, thus dragging air bubbles below the surface, the superheated water boils into those air bubbles. A stream of bubbles suddenly appears on the surface of the water. Most people would assume that those bubbles had something to do with the powder or teabag, not with the water itself. Make no mistake, however, the water was responsible and those bubbles are mostly steam, not air.

Occasionally, though, the water fails to boil at all or stops boiling after it manages to wet the last of the defects on the glass or glazed surface. I’ve made this happen deliberately many times and it’s simply not that hard to do. It can easily happen by accident. With no bubbles to assist evaporation, the water’s only way to get rid of heat is via evaporation from its top surface. If the microwave oven continues to add thermal energy to the water while it is having such difficulty getting rid of that energy, the water’s temperature will skyrocket and it will superheat severely.

Highly superheated water is explosive. If something causes nucleation in that water, a significant fraction of the water will flash to steam in the blink of an eye and blast the remaining liquid water everywhere. That boiling-hot water and steam are a major burn hazard and the blast can break the container or blow it across the room. I’ve heard from a good number of people who have been seriously hurt by exploding superheated water produced accidentally in microwave ovens. It’s a hazard people should take seriously.

After that long introduction, it’s time to answer your question. Yes, I believe that the microwave makers are responsible for advising people of this hazard. Moreover, they know that they are responsible for doing it. If you look at any modern microwave oven user manual, you will find a discussion of superheating or overheating. Look at your manual, I’ll bet it’s in there.

But that discussion will almost certainly be buried in the middle of an long list of warnings. For example, in one manual, the discussion of overheated water appears as item 17 of 22, after such entries as “4. Install or locate this appliance only in accordance with the provided installation instructions” and “12. Do not immerse cord or plug in water”. To be fair to the manufacturer, warning 17 is longest of the bunch and it suggests mostly reasonable precautions (although I’m not so happy with recommendation 17a: “Do not overheat the liquid.”). No Duh.

I think the issue is this: most product warnings are provided not out of any sincere concern for the consumer, but out of fear of litigation. A manufacturer’s goal when providing those warnings is therefore to be absolutely comprehensive so that they can point to a line in a user manual in court and claim to have fulfilled their responsibility. The number and order of the warnings makes no difference; they just have to be in there somewhere.

So all those warnings you ignore in product literature aren’t really about consumer safety, they’re about product liability. You ignore them because everything now comes with a thousand of them, ranging from the reasonable to the ridiculous. For my research, I ordered 99.999% pure sodium chloride (i.e., ultrapure table salt). It came with a 6-page Material Safety Data Sheet that identifies it as an “Xi Irritant”, noting that it is “Irritating to eyes, respiratory system and skin” and recommending first aid measures that include:

“After inhalation: supply fresh air. If required, provide artificial respiration. Keep patient warm. Seek immediate medical advice.
After skin contact: Immediately wash with water and soap and rinse thoroughly. Seek immediate medical advice.
After eye contact: rinse opened eye for several minutes under running water. Then consult a doctor.”

So much for swimming in the ocean…

By design and by accident, our society has lost the ability to distinguish real risk from imaginary risk. We treat all risks as equal and spend way too much time worrying about the wrong ones. If you want to be safer around your cell phone, for example, you should worry more about driving with it in your hand than about the microwave radiation it emits. The current evidence is that your risk of injury or death due to a cell-phone related accident far outweighs your risk from cell-phone microwave exposure. Even if further research proves that cell phone microwave exposure is injurious, we should be acting according to our best current assessments of risk, not according to fears and beliefs.

That said, I’d like to see product literature rank their warnings according to risk and put the real risks in a separate place where they can’t be overlooked or ignored. Put the real consumer safety stuff where the consumers will see it and put the product liability stuff somewhere else where the lawyers can find it. For a microwave oven, there are probably about half a dozen real risks that people should know about. Several of them are relatively obvious (e.g., don’t heat sealed containers) and some are not obvious (e.g., liquids heated in the microwave can become superheated and explode).

Maybe we’ll get a handle on risk someday. In the meantime, inform your friends and children that they should be careful about heating liquids in the microwave, particularly in glass or glazed containers. Just knowing that superheating is possible would probably halve the number of burns and other injuries that result from superheating accidents.

Can you break a microwave oven by running it with nothing inside?

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My mother used the time cook setting as a timer and after a while of running it completely shut off. There was nothing in the microwave to cook. So I guess what I would like to know is, did she burn up the microwave? — JR, Hanford, California

When you run a microwave oven without any food inside, there is nothing to absorb the microwaves and they build up inside the cooking chamber. Eventually, something has to absorb them and that something is the oven’s microwave source—its magnetron. The magnetron isn’t good at handling excessive power that returns to it from the cooking chamber and it can be damaged as a result.

In all my years of experimenting with microwave ovens, I’ve only killed a magnetron once. But then again, I haven’t run a microwave oven for more than a minute or two without anything inside it. If the oven works again after cooling down, then you’re probably OK. The oven may have thermal interlocks in its microwave source to prevent that source from overheating and becoming a fire hazard. If the oven fails to work after an hour of cooling off, then you’re probably out of luck. The magnetron and/or its power supply are likely to be fried and in need of replacement.

Is it safe to operate a microwave oven with a broken window?

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The outside window on my microwave oven broke today (by a baseball) can I have it replaced or just still use it? — R

The glass window itself isn’t important to the microwave oven’s operation, but the metal grid associated with that window certainly is. The grid forms the sixth side of the metal box that traps the microwaves so they cook food effectively. In principle, you can remove all the glass and still cook food, but I think that would be a bad idea. The grid isn’t very sturdy on its own and if it develops cuts or holes, it will allow microwaves to leak out of the oven. You want those microwaves to stay inside the box to cook the food and not to escape to cook you.

Even if the oven door has multiple layers of glass, those layers are there for your protection. If you touch the outside of the metal grid while the oven is on or get close enough to it through the last layer of glass, you’ll be able to absorb some microwave power and it’ll probably hurt. That’s because while the holes in the grid are too small to allow the microwaves to propagate through them and truly escape from the oven, they do allow an “evanescent wave” to exist just outside each hole in the grid. That evanescent wave dies off exponentially with distance beyond the hole, so it won’t travel around the room. But you don’t want to put your finger in it.

For inexpensive microwave ovens, you’re probably best off simply recycling the oven. I’m not happy about the modern everything-is-disposable state of appliances and equipment, but I can’t say that it’s cost effective to repair an oven that costs less than about $100. For more expensive microwave ovens, you can usually replace the window or the door. We have had a GE combination microwave and convection oven over our stove top for about 10 years and the door started to come apart about 18 months ago. I purchased a replacement microwave oven door over the web for $140 and installed it myself. It works beautifully. If you’re not handy or are concerned about microwave leaks, you should probably have it replaced professionally. But you can look up the parts themselves online at a number of web sites and get an idea of what the cost will be.

My microwave oven is interfering with my radio reception. Is it safe?

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Our microwave oven is only 2 years old. Recently, we have noticed that when the microwave oven is being used and our kitchen radio is on, the radio has a lot of static. Is this an indication of a leak? Other than interfering with our radio, the oven appears to be working fine. — RN, Bloomington, Illinois

Because the oven’s microwave frequency is more than 20 times higher than anything a normal radio receives, I’d be surprised if the radio would notice even a pretty severe microwave leak. What you describe doesn’t sound like it’s caused by the microwaves. It sounds like it’s caused by an electrical problem in the oven’s high-voltage power supply.

An older oven would have used a heavy transformer, a capacitor, and a diode to convert ordinary household AC power to high-voltage DC power for its magnetron microwave tube. But since your oven was made recently, it probably uses a switching power supply to produce that high voltage. That supply contains a much more sophisticated electronic switching system to convert household AC power to high-voltage DC power. The new approach is cheaper and lighter, so it’s taking over in microwave ovens. Just because it’s more sophisticated, however, doesn’t mean it’s more reliable.

My guess is that the unit in your oven has a problem. If it has an intermittent contact in it or if there is a conducting path that is sparking somewhere in the power supply or in the unit as whole, they’ll be randomly fluctuating currents present in the oven and those current fluctuations will produce radio waves. A sparking wire or carbonized patch on the power supply will start and stop the flow of current erratically and that can easily cause interference on the AM band. Ordinary AM radio is very susceptible to radio-frequency interference at around 1 MHz and sparking stuff tends to produce such radio waves. A car with a bad ignition system, a lawn mower, and a thunderstorm all interfere beautifully with AM reception. And I suspect that you’ve got a similar electrical problem in your oven. I doubt that your oven is a microwave hazard, but you should probably have a repair person to take a look at it. It shouldn’t have anything sparking inside it.

Can you pop popcorn on a tabletop using a disassembled microwave oven?

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I want to trick my friends into thinking that my cell phone can pop popcorn. Here is my plan: take the magnetron out of my microwave and mount it under a table. Then, put some popcorn kernels on the table right above the magnetron. Finally, place my cell phone near the popcorn and point it at the kernels. Then secretly turn on the magnetron until a couple kernels pop. Will this work and is it safe? — MS, Charlottesville, Virginia

It probably won’t work and it’s definitely not safe. Instead of tricking your friends, you risk cooking them. Here is why I think you’d better leave your plan as a thought experiment only.

Those YouTube videos were complete fakes; they didn’t pop any popcorn while the camera was rolling. To make it appear that the cell phones were popping the corn, the people who produced the videos dropped already prepared popcorn into the frame and then photoshopped away the unpopped kernels. When you watch the video, it looks like the kernels are popping, but they’re really just disappearing via video editing as precooked popcorn is sprinkle onto the set from above.

The reason they had to use video trickery is pretty clear: to pop popcorn with microwaves, those microwaves have to be extremely intense. Each kernel contains only a tiny amount of water and it’s the water that heats up when the kernel is exposed to microwaves. If the microwaves aren’t intense enough, the heat they deposit in the kernel’s water will flow out to the rest of the kernel and into the environment too quickly for the kernel’s water to superheat and then flash to steam.

Even when you put popcorn kernels in a closed microwave oven, it takes a minute or two for the kernels to accumulate enough thermal energy to pop. In that closed microwave oven, the microwaves bounce around inside the metal cooking chamber and their intensity increases dramatically. It’s like sending the beam from a laser pointer into a totally mirrored room—the light energy in that room will build up until it is extremely bright in there. In the closed cooking chamber of the oven, the microwave energy also builds up until the microwave intensity is enough to pop the corn. How intense? Well, a typical microwave oven produces 700 watts of microwave power. Since the cooking chamber is nearly empty when you’re popping popcorn, the cooking chamber accumulates a circulating power of very roughly 50,000 watts.

Although that power is spread out over the cross section of the oven, the microwaves are still seriously intense — thousands of watts per square inch. To put that in perspective, a cell phone transmits a maximum of 2 watts and that power is spread out over at least 5 square inches so the intensity is less than 1 watt per square inch. When I saw those videos in Summer 2008, I realized that there was no way cell phones were ever going to pop popcorn. They certainly wouldn’t do it while they are ringing, because that’s when they are primarily receiving microwaves, not when they’re transmitting them. It’s when you’re talking that your cell phone is regularly producing microwaves. It was all obviously just fun and games.

So what about your disassembled microwave oven? Since there is no metal box to trap the microwaves and accumulate energy, they’ll only have one shot at popping the corn kernels. The microwaves will emerge from the magnetron’s waveguide at high intensity, but they’ll spread out quickly once there is nothing to guide them. You could probably pop kernel right at the mouth of the magnetron but not a few inches away. Unless you use microwave optics to focus those microwaves, they’ll have spread too much by the time they get through the table and reach the kernels of popcorn and the kernels will probably never pop.

If that were the whole story, the worst that would happen with your experiment would be that it wouldn’t cook popcorn. But there is a real hazard here. Sending about 700 watts of microwaves into the room isn’t exactly safe. It’s something like having a red hot coal emitting 700 watts of infrared light, except that you won’t see anything with your eyes and this microwave “light” is coherent (i.e., laser-like) so it can focus really tightly. You’d hate to have some metal structure in the room or even inside the walls of the room focus the microwaves onto you. You absorb microwave much better than the corn kernels and you’ll “pop” long before they do. Actually, your eyes are particularly sensitive to microwave heating and you might not notice the damage until too late. Without instruments to observe the pattern of microwaves in the room when the magnetron is on, I wouldn’t want to be in the room.

Can microwaves be used to heat air?

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Could a magnetron be tuned to heat air, oxygen, or nitrogen? Is it a specific frequency or a range? What is the frequency? — VM, Martinsville, Indiana

No, those gases don’t absorb microwaves significantly regardless of frequency. Diatomic molecules are nearly oblivious to long wavelength electromagnetic waves. In fact, that’s why they don’t contribute to the “greenhouse effect.” Oxygen does have an unusual absorption band in the near infrared, but that’s about it.

What should you do about uneven cooking in a microwave oven?

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My microwave oven seems to mostly heat things on the periphery of the plate
and the part in the center is significantly cooler. Is this considered
faulty operation and should I get something replaced? — MD

It’s quite possible that the pattern of microwaves inside your oven is more intense at some places than in others — that’s why most microwaves have carousels in them to move the food around. I don’t think that the pattern will change much with age, but it’s possible that your oven isn’t producing as much microwave power as it once did and you notice the low-intensity regions more than before. It’s not a true “fault”, but it is a nuisance. If you get tired of putting up with it, you should probably replace the oven. It used to be that you could purchase carousel inserts for the ovens, but I don’t see them for sale anymore.