Talk:Microwave oven/Archive 2

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Food heating

Can anyone explain why some microwaved foods heat mostly on the outside (and inner parts stay cold), while other foods get heated evenly? Thanks, --Abdull 10:37, 7 April 2006 (UTC)

From this web page:

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.

- Bloodshedder 17:11, 7 April 2006 (UTC)

Heating mechanisms

The "proof" offered on the main page that the heating is mainly conductive due to that a drop of water is less heated than a glass takes not into account the difference of surface area. The proof should also be supported by calculations, and not only by anectodical stories. I will check the theories by calculations, and change the text accordingly. —Preceding unsigned comment added by Mossig (talk • contribs) 12:48, 9 April 2006

After having had a look at the curves at http://www.lsbu.ac.uk/water/microwave.html I can not say that the new statement in the article is correct. I will remove it. Before reinsering it, plese argue with references to the mentioned curves as to way this interpretation is correct. —Preceding unsigned comment added by Mossig (talk • contribs) 13:04, 9 April 2006

I was one of the people who did the little "anecdotal" home experiments back in April allegedly proving that electrical conduction was the dominant factor in heating of microwave ovens. I'm going to include our original comments below for reference:

It is interesting to note that while most references refer to the oscillation of polar molecules as being the primary heating mechanism of water in a microwave oven a few simple home experiments illustrate this is in fact not the case. Dielectric heating can be caused either by electrical conduction or molecular rotation. The dominant heating mechanism in a microwave oven is actually electrical conduction. This is illustrated by two facts. First, a drop of water will heat poorly if at all in a microwave while a glass of water adjacent to it will heat rapidly. This demonstrates that the heating mechanism is not molecular which would be independent of scale but rather conductive which requires the food item to be on the same scale as the wavelength for efficient coupling. Second, distilled water will heat much more slowly than regular tap water. This demonstrates that the heating is dependant on the conductivity of the food item rather than the presence of polar water molecules.

Thank you for pointing me to the interesting article cited above. We ran our little experiments in Tucson, which has anywhere between 160 to 760 ppm (see http://www.ci.tucson.az.us/water/clearwater_faqs.htm). So if I'm interpreting the data from the lsbu.ac.uk article above (I'm referring to the un-labelled plot about two-thirds the way down the article, where they plot the loss coefficient against temperature and salt content), even the minimum level of 160 ppm would produce many times greater heating than pure deionized water. I don't know how pure the off-the-shelf DI water we were using was, but assuming it was, say 10 ppm, even 35 ppm would produce twice the rate of heating as the deionized water. At low ppm it appears essentially linear with ion concentration, so 160 would yield a "dielectric loss" of approx 140 (I just eye-balled it),

compared to the 20 of my hypothetical deionized water -- that means tap water would heat 7 times as fast as deionized water! Those poor souls with 760 ppm tap water might see their tap water heat up as much as 30 times faster than deionized water. To be a little more clear about what's actually going on here: the molecules in deionized water are simply spinning around really really fast, but not actually jostling and bumping into each other, thus not producing heat. On the other hand, a much larger proportion of the input power of the microwaves gets turned into heat in hard water. Thus I conclude that salt contet is more than just important -- it seems critical. Yet, this is not even mentioned in the main article.

Secondly, the lsbu.ac.uk article you cite only considers dipole rotation. Have you seen literature showing that current induced directly in the salt ions (presumably due to gradient in the EM field) doesn't contribute significantly? (I can't find any data supporting either theory.) Our experiment showing that tiny objects don't heat as readily as large objects was designed to show that this effect was at least important if not outright dominant. However, thank you for pointing out that we neglected to consider the greater relative cooling of the small drop of water due to its much greater surface-to-volume ratio. I'll have to try to come up with a more sophisticated experiment! Until then I have to concede that our experiment, while suggestive, is inconclusive.

63.194.18.133 23:40, 16 September 2006 (UTC)

The reason that your waterdroplets dont heat up is due to their small size. As they are much smaller than the wavelength (12 cm) they will be in effect invicible for the electromagnetic waves, and thus not absorb any power. This is the same effect that is responsible for the power being contained in the oven by the metallic mesh in the window. A first order approximation of a microwve oven gives that all power is absored by the food in the oven. Thus heating times for homogenous substances in the oven will not vary much. (But for different substances in teh same oven there will be differences.) Your resoning about water molecules spinning around themselves without causing any heating has no basis in physics. The link above do take ionic conduction, and the corresponding loss, into account. Mossig 18:34, 17 September 2006 (UTC)

It's certainly not true that particles smaller than a wavelength are somehow invisible. If that were so, atoms could not interact with visible light which is thousands of times their size, wavelength-wise. Yet of course they do. They even undergo elastic scattering, which is why the sky is blue. And droplets scatter by Rayleigh and Mie type mechanisms in the radar range as well, which is why clouds and rain are certainly not invisible to radar with wavelengths many times the size of the droplets being seen. Effects analogous to Raman scattering cause absorption to actually predominate over scattering for very small droplets, and the mechanism is the very sort of Debye relaxation we're talking about (where water molecules rotate in an E field and cause friction while doing it). That runs best at about 30 GHz.

I don't know where anybody got the idea that decreasing absorption with thickness means a non-molecular mechanism. Think of a manifestly molecular mechanism-- a thin layer of dye won't absorb. So lose this notion.

A final comment: while it may be true that conductive solutions absorb microwaves several times better due to solvent drag effects (ions beging dragged through solutions, causing heating), that doesn't mean foods with salt in them heat that much faster. All microwaves introduced into an oven bounce around until they're absorbed, whether there's saltwater in there or distilled water. The only difference is the SKIN thickness the absorption happens in. SBHarris 01:37, 18 September 2006 (UTC)

About microwave oven safety

One of the claims of the people who question microwave oven dangers is that there have been no government studies to prove its safety.

They also claim that microwave ovens were forbidden in Russia in 1976 due to its health risks, and that there was a woman killed by a simple blood transfusion when a nurse "warmed the blood for the transfusion in a microwave oven". This wouldn't make sense unless microwave oven did something else than heating. And then there are the testimonies of people like the swiss scientist Hans Hertel.

As with any new procedure or invention, the burden of the proof should be on those who propose its use, not on those who question its safety. In other words, we should not expect studies proving microwave oven dangers, but just the opposite: convincing scientific proof of its safety before even considering its use.

If it is true what they say, that there have not been a large scale study to prove microwave oven safety, I think we should be very careful about its use. --200.63.240.128 22:06, 23 April 2006 (UTC) Nelson Cevallos Estarellas

It's impossible to prove that something is safe, it's only possible to prove that something is unsafe. I've not been able to find the original source of the Urban Legend about the nurse heating blood in a microwave oven. If it did happen, the explanation is fairly obvious. As explained in the article, microwave ovens generate heat spots. It would be very hard to heat blood without causing solids to appear. Injecting solids in to the bloodstream is not the smartest thing to do.

There's plenty of people about on the Internet making bizarre claims regarding microwave ovens. They don't understand Science and as a result fear anything they don't understand. In the past people used to burn people as witches, now they just create websites.

Microwaves are non ionising electromagnetic waves. People realise that some forms of electromagnetic radiation cause cancer and they apply their limited knowledge and deduce that microwave ovens must be dangerous. However to this date, no peer-reviewed study can validate these fears.

If people believe that Microwave Ovens are dangerous, then they have the choice not to use them. However I'd question how these people manage to live their lives. As mentioned above, there is no way to prove that anything is safe. Science isn't perfect, but it does give people a logic to decision making rather than random assumptions based on gut instinct. 82.209.178.74 03:35, 19 August 2006 (UTC)

I beg to differ. Microwave Ovens are not be required to "live" ones life. Ha. Thats just silly;)

Perhaps someone more interested in types of technology and how it operates might instead, ask, WHY there hasn't been more research into what microwaving does to our food, and in turn, to ourselves. I don't think its unfounded to desire more awareness on this topic.

FYI, safety and what that constitutes, is a POV, at least when it comes to something MILLIONS have and use EVERYDAY in their homes. What we are referencing is the health effects and what those might be, no matter what increment of effect.

—The preceding unsigned comment was added by 71.246.208.226 (talk • contribs) .

There's been relatively little research because there are essentially no known mechanisms by which microwave ovens can do anything much different to food than ordinary thermal ovens already do, and in all the years that we've been using them, there's esentially no evidence that anything is going wrong in microwave cooking (other than the known risks of undercooking foods). Atlant 00:06, 18 September 2006 (UTC)

Perhaps we should just revert back to fire...Safe, safe fire. Oh, wait, what's the score?

Microwave oven: 1 (due to botched transfusion)

Cooking with fire, ovens, stoves: <1000 deaths per year (due to residential fires)!

I think its obvious where "more awareness" is needed.

Mikeeg555 13:36, 2 April 2007 (UTC)

External links

It appears that a link to an actual enterprise has been inserted at the very bottom of this article. Should it be there? —Preceding unsigned comment added by Zalzalahbuttsaab (talk • contribs) 11:28, 23 May 2006

What belongs in "External links" is always a judgement call; on that link, I'd tend to say "no". Atlant 13:04, 23 May 2006 (UTC)

I don't understand why you can't link to pages where you can actually find what is being talked about like speedcook? On Service Dogs you can link to a place that actually provides service dogs, on famous people sites you can link to web pages that talk about that person. Why can you not link to the actual product or products that the discussion is talking about? Havy31 13:29, 15 August 2006 (UTC)

Wiki is not intended to provide free advertising for any company that happens to come along and notice a relevant article. Because of this, most links to commercial enterprises are routinely removed unless the enterprise is the direct and sole subject of the article. So, for example, the article about General Electric could contain a link to GE, but an article about microwave ovens probably won't contain links to Amana, GE, Samsung, Sharp, etc.; there are just too many companies that make microwave ovens. And links to accessory vendors are clearly right out.

As I said above, there's no cabsolutely clear-cut guideline, but most WIkipedians have a pretty good sense of what's OK and what's not for external links. Atlant 13:53, 15 August 2006 (UTC)

Reverse effect: cooling device

I've heard on television that there must be some kind of machine instead of warms up your food quickly, it kind of cools your food quickly. Myth or truth? --Shandris^{the azylean cat} 12:15, 16 June 2006 (UTC)

Basically, myth. Because of the irreversability of frictional heating, you can't beam some sort of ray at a large mass and cause it to cool down. (At the level of individual atoms or molecules, lasers can actually do this; see laser cooling, but the effect isn't yet available on the large scale of, say, your no-longer-cool iced tea.)

At the macro scale, we're limited to things like ordinary mechanical refrigerators, Peltier coolers, and, for the brave/foolhardy, blasts of liquid nitrogen. Atlant 13:13, 16 June 2006 (UTC)

Portable Peltier coolers are sometimes advertised and marketed as Shandris describes. However their cooling ability is modest when powered by the typical 12 VDC automotive power system. --Blainster 15:50, 16 June 2006 (UTC)

Hmm, guess we'll have to wait a few years until we get something like this. I hope there will be developed a 2 in one: microwave heatener/cooler, would be very handy. --Shandris^{the azylean cat} 16:19, 16 June 2006 (UTC)

You missed my point: There is no physical principle which would support such a device as a "reverse microwave oven". Atlant 16:27, 16 June 2006 (UTC)

Of course Alant is correct that microwaves cannot be directly used as a cooling device, but all refrigeration devices create a temperature differential that can be used to cool (example: propane powered absorptive refrigeration), so such a device is not out of the question. Also, Pelltier devices are commonly used to either heat or cool (mildly) by reversing the plate. --Blainster 16:47, 16 June 2006 (UTC)

What I'm really trying to say is that aside from laser cooling, we have no mechanism other than good old conduction or convection with which to remove heat from the object you'd like to cool. This is different than the situation for the microwave oven where the microwaves directly cause the object to heat itself through friction; essentially no heat is conducted or convected into the object to be heated. Atlant 19:21, 16 June 2006 (UTC)

Or to put it another way, microwaves heat objects by EM radiation, and there's no limit to how fast you can do that. But objects are very limited in how fast they can COOL by radiation, because the heat radiation must be *generated* by the object's own temperature, and is greatly limited by that. Basically, you drive heat in by radiation as fast as you like, but there's no way to suck it OUT by radiation, any faster than the object itself generates it. SBHarris 17:30, 15 August 2006 (UTC)

Thank you -- you stated it quite a bit more clearly than I did. ;-) Atlant 17:46, 15 August 2006 (UTC)

Resonance

The second paragraph of 'Description' describes the mechanism by which water (etc) are heated in a microwave, and also that is incorrectly explained by resonance of watermolecules. However, the difference between the incorrect and correct mechanism is not clear to me, since the correct mechanism seems to 'use' resonance as well (that of the molecular dipoles). Perhaps someone can clarify this. NoobX 20:17, 19 June 2006 (UTC)

It could be described as a damped resonance, which is not what people commonly think of as a resonance. It is not a sharp peak at a certain frequency band that is usable for heating water, frequencies from 500 MHz to 10 GHz would work. —The preceding unsigned comment was added by Mossig (talk • contribs) 13:28, June 19, 2006 (UTC)

What I mean is that the difference between the correct and incorrect mechanism is unclear, what exactly is resonating in the correct and incorrect explanations? Both explanations mentions 'molecules' resonating. NoobX 22:02, 19 June 2006 (UTC)

The explanation, if I understand it, is that "resonance" means the resonant frequencies of O–H bond stretching (3600 cm^-1) or bending, which do not occur in the 2450 MHz region. In this case the dipolar H₂O molecule is generating heat from friction against other molecules by trying to flip the O–H axis to align itself in the (E field/B field?) generated by the magnetron. Is that close? --Blainster 23:55, 19 June 2006 (UTC)

Seems to be correct (my understanding of atom physics is a bit old, so the bond stretching I am unsure about). Should be "E-field". The heating effect is due to repolarisation, ie. that the water molecules try to align themselves with the incoming E-field and thus rub against each other. As this effect is more or less pronounced depending on the frequency, it could be said to have a "resonance". (The word "Resonance" has no standard technical meaning - it is used slightly different in different fields of engineering.) For a look at these absorption curves, see http://www.lsbu.ac.uk/water/microwave.html Mossig 11:53, 20 June 2006 (UTC)

Wireless Disruption

I'm a little confused, it says earlier that the casing of the microwave and the mesh on the door stop any microwaves from escaping, then later the article says that microwaves can disrupt wireless networks. If the microwaves can't escape the microwave oven how are they able to disrupt wireless networks? Kniesten 18:56, 5 September 2006 (UTC)

Well, sometimes if the oven is badly constructed, enough microwave radiation gets out to disrupt a radio network, but still far less than would be biologically dangerous. No screen stops EVERYTHING. SBHarris 19:34, 5 September 2006 (UTC)

Dangers: Toxic Ozone?

The "Dangers" section mentions dielectric breakdowns in air forms ozone and nitrogen oxides, then claims both are toxic. Whie nitrogen oxides are toxic, I don't belive ozone poses any heath hazards in low exposures such as those one would experiance from the dielectric breakdown of air inside a microwave. Does anyone know enough about ozone health hazards to confirm this is a mistake?--LaughingMan42 06:57, 8 September 2006 (UTC)

Ozone production in a microwave shouldn't occur unless something is causing arcing. Absent the arcing, there shouldn't be any detectable ozone so no health risk there. Atlant 12:45, 8 September 2006 (UTC)

Staring at a Microwave Dangerous?

One of the biggest things I hear with microwave safety is that one should not stare at them while on. I've heard from elsewhere that this is a myth. Anyone heard about this from a reliable source? It would be good to add this to the Radiation section. KingOfAfrica 07:30, 16 September 2006 (UTC)

No they have a faraday shield (that mesh you see between the glass/plastic door) meaning the 2.4ghz wavelength is too large to penetrate the small holes of the faraday shield. —Preceding unsigned comment added by 124.177.151.101 (talk • contribs) 07:03, 27 January 2007

Sorry about adding that the first test was on a human (which exploded in the experimenters face, someone told me that anyone can change wikipedia and I didn't believe him so I had to see firsthand, I thought you would at least have to sign up first, there must be so many silly vandals. —The preceding unsigned comment was added by 124.177.151.101 (talk • contribs).

It penetrates as an evanescent field to a distance on the order of the size of the viewing holes and falling exponentially. Potatoswatter 07:59, 17 February 2007 (UTC)

Microwave oven doors are usually (if not always) designed such that the outer window is 2cm or more away from the inner mesh of the door. The exponentially decreasing field probably becomes negligible if even existent at this distance. So might it be safe to press your face against the door? :) Mikeeg555 12:38, 2 April 2007 (UTC)

Yes, it should be safe to press your face against a microwave door. However, in the case where the mesh or door is damaged, more microwaves than usual might escape. In an extreme sitation this might lead to heating of the eye. Unlike most other places in the body where heat is transported away from the affected area by the bloodstream, the eye will loose heat very slowly. Additionally there are no heat temperature sensors in the middle of the eye. So an unsuspecting victim pressing their eye against a damaged door might receive damage to the eye. Though this would be a very unlikely and unlucky situation. --Tunheim 10:54, 3 April 2007 (UTC)

History: Percy Spencer

Again the story of Percy Spencer and his melting chocolate bar. This sounds like a tall story to me (though I’ve seen it repeated several times); I believe the cooking effect of microwaves was known in the 1930’s. I have a children’s book (Boys Annual 1938 ) which mentions an experiment which involved cooking a sausage between two radio emitters; amazingly, the sausage cooked from the inside! Of course, the book didn’t call them microwaves; the article was called “Death Rays Across The Ether”, and featured a futuristic picture of blokes in space suits using a ray-gun to shoot down a plane resembling a DC-3. The account also said the effect was noticed when it was found the mice at Alexandra Palace were getting fried by the radio transmitters there (this may also be a tall story; it was written for 12 year olds) Also, I can think of a number of reasons why a chocolate bar in someone’s pocket might melt, but not a way for microwaves to do this without also cooking any adjacent anatomy.[[62.6.139.11 15:54, 12 October 2006 (UTC)]]

Uhh... high power microwave emitters did not exist in the 1930s. You're confusing microwave heating with simple RF heating which was known since the late 1890s. High-power microwave dates from the klystron and magnetron which were a military secret invention from the early 1940s. A few seconds of research shows that Percy Spencer was an uneducated tech at Raytheon whose problem-solving skills won Raytheon the WWII contract to build the first military radar tubes, got Spencer the job as department head, and later led to Spencer's inventing of the microwave oven. The candy bar story and the popcorn story are "repeated several times" since they are a well-known part of that history, see [raytheon site http://www.raytheon.com/about/history/leadership/index.html]. Also: microwave radar leakage that gently warms a candy bar will gently warm the adjacent person. --Wjbeaty 20:33, 9 November 2006 (UTC)

Thanks...you're probably right about the confusion, I don't know a huge amount about RF heating; so you could get the same cooking effect that way? I know the history, and I wasn't trying to dispute Spencer's invention of the oven, I was just querying whether he was the first to notice the effect. And I know the story is a well-known part of the history, I'm just wondering if it's actually true (...and that children is where microwaves came from ) Richard 62.6.139.11 16:19, 15 November 2006 (UTC)]

RF heating is a similar principle (radio waves induce losses in the object-to-be-heated), but I think the exact principle varies somewhat. Whereas microwaves tend to heat most food via a pretty specific interaction between the electrical field and water molecules, RF heating in general tends to heat by inducing electrical currents into the object leading to Joule heating. Where some confusion arises is that microwaves (being a specific class of radio waves) also do this (so not all microwave heating is caused by water-molecule friction). RF heating is widely used in industrial processes. At the other end of the EM spectrum is yet another related process called induction cooking, but here the energy is transfered mostly as a relatively-low-frequency magnetic field. Atlant 16:40, 15 November 2006 (UTC)

See [1]. RF heating via ion-drag (fluid conductivity) from radio frequency capacitors, is actually how some diathermy machines work to heat tissue, if I'm not mistaken (this is not the surgical diathermy which uses a high frequency A.C. arc to cut tissue, but rather an external device used to warm tissue in physical therapy). These things have been known since the 30's, I think. Such things don't need to touch the patient, so they certainly work via EM fields and not contact conduction. But unlike microwaves, they don't work on pure water, or non-conductive stuff like the fat in a chocolate bar. Microwaves do heat fat (not as well as water, but they do heat them) because fat molecules have an electric dipole moment (you can demo this with a little cooking oil in your own oven). Also, chocolate bars aren't completely dry and water free. I can well imagine that a microwave beam from a magnetron in 1945 might have melted a candy bar in Percy's pocket (hard to do with RF or diathermy) while still not depositing enough energy in the underlying tissue to feel warm, simply due to blood circulation (a type of forced convective cooling) carrying away such heat (a luxury not available to the candybar). BTW, the same "more heating in the thing with no circulation" thing happens in the lenses of the eyes of a person exposed to microwaves at moderate power, which gave cataracts to some of the early people who alligned working waveguides by sight). SBHarris 20:32, 26 November 2006 (UTC)