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  #1  
Old 01-07-2004, 01:32 AM
neoof neoof is offline
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Is it dangerous to turn on an empty microwave?

If so- why?
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  #2  
Old 01-07-2004, 01:51 AM
David Simmons David Simmons is offline
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My microwave warns that it should not be turned on when empty. Why?

When there is food in the oven, a large fraction of the output of the microwave transmitter is absorbed by it. When the oven is empty none, or almost none, is absorbed and the energy reflects around inside the chamber resulting in large standing waves that can damage the transmitter power tube.
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  #3  
Old 01-07-2004, 01:58 AM
Achernar Achernar is offline
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My microwave does not warn that it should not be turned on when empty. Why not?

Seriously, when I got my microwave oven I read the manual. There were some warnings about not running it if the door is broken, but nothing about running it empty. Is it just thought to be common knowledge now or what?
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  #4  
Old 01-07-2004, 02:05 AM
Desmostylus Desmostylus is offline
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We've had this argument before. There's some difference in the construction of the klystrons used in modern microwaves that makes them less susceptible to damage when running empty.
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  #5  
Old 01-07-2004, 02:16 AM
David Simmons David Simmons is offline
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Quote:
Originally posted by Desmostylus
We've had this argument before. There's some difference in the construction of the klystrons used in modern microwaves that makes them less susceptible to damage when running empty.
It is possible to put a unit (whose name escapes me at the moment but it's not a directional coupler) in the output waveguide that passes power out of the klystron and aborbs power coming in. This increases the cost of course.
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  #6  
Old 01-07-2004, 02:24 AM
Desmostylus Desmostylus is offline
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Nah, it was something simpler than that. But it escapes me at the moment, too.
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  #7  
Old 01-07-2004, 02:35 AM
Desmostylus Desmostylus is offline
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I think it's just something really simple, like an attenuator, combined with more heat-resistant construction of the klystron.

I'll look it up later, if no-one else shows up with an answer.
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  #8  
Old 01-07-2004, 02:42 AM
David Simmons David Simmons is offline
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Quote:
Originally posted by Desmostylus
I think it's just something really simple, like an attenuator, combined with more heat-resistant construction of the klystron.

I'll look it up later, if no-one else shows up with an answer.
Are we thinking of something like a ferrite circulator, or some other ferrite waveguide unit?
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  #9  
Old 01-07-2004, 02:50 AM
Desmostylus Desmostylus is offline
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That doesn't ring a bell. I can't really concentrate at the moment. I'll come back to this after the kids are in bed.
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  #10  
Old 01-07-2004, 03:08 AM
trabi trabi is offline
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During a brief period assisting in a workshop that repaired all sorts of kitchen appliances I was taught that it isn't dangerous as such to run the microwave empty, but it can burn out the magnetron, basically rendering the machine useless.

We were told to always at least put a mug of water in the machine before switching it on to test it.
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  #11  
Old 01-07-2004, 03:22 AM
Desmostylus Desmostylus is offline
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Here's what I was thinking of when I said "We've had this argument before":

http://boards.straightdope.com/sdmb/...65#post3113265
Quote:
Originally posted by bbeaty
I've read a couple of articles which say that older microwave ovens were prone to failure when running empty. The older magnetrons had a small glass cover over the antenna rod. This would have a glass/metal seal which could crack when hot, ruining the vacuum. The glass could even melt, producing a "suck-in" failure. Glass isn't immune to dielectric losses, and being so close to the antenna (with its huge e-fields) would let the glass absorb a bit of EM and heat up.

Modern magnetron tubes use a ceramic "window" instead of glass, so they're much more robust.
I don't expect you to take that as authoritative, of course, David Simmons. But it's something I've seen elsewhere, and sounds right. I'll try to source some manufacturer's data.
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  #12  
Old 01-07-2004, 05:36 AM
Desmostylus Desmostylus is offline
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OK, now that things are a bit quieter:

a) I shouldn't have said "klystron". Consumer microwave ovens use magnetrons. Magnetrons are self-tuned oscillator/amplifiers. Klystrons are more general purpose amplifiers.

b) I found the cite (the "something I've seen elsewhere") I was thinking of. It's completely irrelevant to the topic at hand. It was about extremely high-powered kylstrons used in radar work.

If bbeaty comes along, perhaps he can oblige with a reference to one of the stories he's seen about consumer microwave ovens.

Sorry 'bout that.
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  #13  
Old 01-07-2004, 10:11 AM
CurtC CurtC is offline
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Yeah, but on the other hand, Klystrons have much cooler names.
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  #14  
Old 01-07-2004, 12:03 PM
Ca3799 Ca3799 is offline
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Mt B-I-L left his empty microwave on for an hour. He meant to use the kitchen timer while baking something in the oven, but used the microwave intead. It melted the glass tray and ruined the microwave (which was brand new and had been used about 3 times). He said the glass tray actually had small flames on it.

So don't use it empty for an hour.
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  #15  
Old 01-07-2004, 12:32 PM
aerodave aerodave is offline
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I believe the device you're think of is "flux capacitor."
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  #16  
Old 01-07-2004, 12:32 PM
bbeaty bbeaty is offline
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Oh boy, kitchen physics!


The stuff about older magnetrons was in an old book (70s?) on microwave oven repair I had from the library long ago. The failures involved overheating of the magnetron tube (melting the glass parts, or cracking the metal/insulator bonds.)

I've been seriously abusing my own microwave oven for years with little harm. Generating fierce arcs and plasmoids. Shattering pyrex cups. Melting beer bottles. Making molten lava (from pumice or obsidian.)

Running an empty microwave oven for a long time will heat up everything including the metal walls, glass dish, front glass, and magnetron parts. Also, the electrostatic fields in the standing waves become immense and can cause outbreaks of high-voltage arcing which behave like blow torch flames. Sometimes these arcs happen right at the magnetron tip, and I doubt that even a modern magnetron could survive many minutes of such blowtorch temperatures.

A weird effect: if any glass part ever heats to a dull red glow, the glass becomes a resistor. It then absorbs microwave energy like mad, and you get an outbreak of glowing orange "lava" that propagates through the glass. I'd predict that this is a fairly rare occurrence, but if a piece of glass happened to have some red hot charred food laying against it, that might be enough to trigger an outbreak of melting.

Also, if your oven door has a leak, the escaping energy might be hundreds of times higher with nothing in the oven (and with huge standing waves built up inside.)


Here's some speculation: with hundreds of thousands of ovens sold, hundreds of customers accidentally run their ovens for long periods with nothing inside. Some ovens die, and customers complain or demand refunds. Appliance repair shops tell customers which ovens fail easily, and word gets around. Manufacturers spend just enough so that most modern ovens can take such rare abuse. Yet ovens coming off the manufacturing line vary, so some magnetron tubes may be more delicate than others.
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  #17  
Old 01-08-2004, 12:23 PM
neoof neoof is offline
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Thanks
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  #18  
Old 10-01-2013, 04:30 AM
SlackerInc SlackerInc is offline
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Fyi I just ran my microwave for about 10 seconds accidentally with nothing in it and a lot of sparks flew and scary sounds and then I stopped it and it smelled smoky. I found this thread and figured I was probably okay to go ahead and put the TV dinner in it and try again. But the result was more sparks and bad sounds so I stopped.

Fuck. I can't believe my microwave got nuked, no pun intended, from something so simple and brief. BTW in my case I was intending to microwave something but absentmindedly still had it in my hand.
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  #19  
Old 10-01-2013, 07:45 AM
lost4life lost4life is offline
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Quote:
Originally Posted by SlackerInc View Post
Fyi I just ran my microwave for about 10 seconds accidentally with nothing in it and a lot of sparks flew and scary sounds and then I stopped it and it smelled smoky. I found this thread and figured I was probably okay to go ahead and put the TV dinner in it and try again. But the result was more sparks and bad sounds so I stopped.

Fuck. I can't believe my microwave got nuked, no pun intended, from something so simple and brief. BTW in my case I was intending to microwave something but absentmindedly still had it in my hand.
I don't know shit about microwaves, but I doubt running it empty for 10 seconds caused this. I'd look really good for any piece of hidden metal that got left inside. Maybe some foil or something?

Insert zombie joke here:______________________
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  #20  
Old 10-01-2013, 11:11 AM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
Originally Posted by SlackerInc View Post
Fyi I just ran my microwave for about 10 seconds accidentally with nothing in it and a lot of sparks flew and scary sounds and then I stopped it and it smelled smoky. I found this thread and figured I was probably okay to go ahead and put the TV dinner in it and try again. But the result was more sparks and bad sounds so I stopped.
That's pretty rare. Most microwaves can run empty for a while without anything going kablooey. Running it empty is a bit harsh on the magnetron, but it usually takes several minutes of running it empty to break it.

Dying that quickly isn't completely unheard of, but I suspect that your microwave was already on its way out. The extra stress of running it empty probably just put the final nail in the coffin.

This does happen to illustrate quite well why I always put a "don't try this at home" warning whenever we talk about stupid microwave tricks around here though. Things like this may be rare but they do happen on occasion.
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  #21  
Old 10-02-2013, 04:37 AM
SlackerInc SlackerInc is offline
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There was no metal in it, and it had always run fine until this happened (and was less than a year old, and our "auxiliary" microwave, only used about three or four times a week). It was however a small microwave that was the absolute cheapest model available at Wal-Mart, so there's that.
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  #22  
Old 10-02-2013, 04:16 PM
t-bonham@scc.net t-bonham@scc.net is offline
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Originally Posted by Achernar View Post
My microwave does not warn that it should not be turned on when empty. Why not?
Because if you do ruin it by running it empty, you will buy a new one -- more profits for the manufacturer!
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  #23  
Old 10-02-2013, 05:59 PM
TBG TBG is offline
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Originally Posted by t-bonham@scc.net View Post
Because if you do ruin it by running it empty, you will buy a new one -- more profits for the manufacturer!
Not if you do that while it's still under warranty!

Anyway, just got a new microwave earlier this year, and it definitely had the warning in the manual.
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  #24  
Old 10-02-2013, 06:33 PM
t-bonham@scc.net t-bonham@scc.net is offline
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Not if you do that while it's still under warranty!
No, that would be misuse of the product, and would not be covered by the warranty.
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  #25  
Old 10-02-2013, 07:25 PM
picunurse picunurse is offline
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Check the mica cover that isolates the magnatron. If it has a black spot or a hole in it, it needs to be replaced.
Don't go to a small appliance repair shop, go to Lowes or Home Depot and get a sheet of mica, then, using the old one as a template, cut a new one and replace it.

The small appliance repair shop will charge upwards of $40, whereas a 10"X10" sheet of mica will be less than $10.

Last edited by picunurse; 10-02-2013 at 07:26 PM..
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  #26  
Old 10-03-2013, 04:45 PM
SlackerInc SlackerInc is offline
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Too late, threw the microwave out already.
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  #27  
Old 10-05-2015, 02:55 PM
EmptyMicrowave EmptyMicrowave is offline
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Hey, made this account (as you can tell by the name lol) just to respond to this, for some reason.

I left my microwave on empty for about 1 minute a couple years back. It was an old crappy microwave, not sure which brand or model/make or year etc.

Anyway, it was set to run for about 3 minutes, but about 1 minute in I heard weird static noises and saw light flashes from around the corner.

I go in and it's straight out of a sci-fi movie lol long blue electricity bolts flying out of it all over the place. I was freaked out to get close, but I just kind of bolted in and hit Stop.

This is 100% true btw no exaggeration.

I have a question though, I've been wondering about it since it happened - Would I have been electrocuted slightly, or inflicted with slight radiation or just lightly shocked if I had made physical contact with these crazy electricity bolts?

Thanks!
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  #28  
Old 10-05-2015, 03:01 PM
Thudlow Boink Thudlow Boink is offline
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Has Mythbusters ever done anything with microwave ovens?
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  #29  
Old 10-05-2015, 03:06 PM
SlackerInc SlackerInc is offline
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I have only watched about ten episodes of the show, but I remember them doing one on the myth that microwaves cook from the inside out. They put a big hunk of meat in one and found that the outside was more cooked than the inside. (Still, I think calling this a "myth" is extreme, as it is clearly less "outside-in" than a conventional oven.)
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  #30  
Old 10-05-2015, 03:51 PM
engineer_comp_geek engineer_comp_geek is offline
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BRRRAAAAIIIIINNNNSSSSS!!!!!!!

(It's the second time this zombie thread has been raised)

Quote:
Originally Posted by EmptyMicrowave View Post
I have a question though, I've been wondering about it since it happened - Would I have been electrocuted slightly, or inflicted with slight radiation or just lightly shocked if I had made physical contact with these crazy electricity bolts?
There's no "radiation" in a microwave, at least not in the way that the term is commonly used. Microwaves use radio waves to cook your food. So you don't have to worry about any kind of radiation risk.

The way most people use the term "radiation", they are referring to ionizing radiation, and microwave radio waves are too low in frequency to be ionizing. The spectrum goes long waves, radio waves, microwaves, infra-red light, visible light, ultra-violet light, x-rays, gamma rays, and cosmic rays. Part way through the ultraviolet part of that is where it starts to get ionizing, which means that the electromagnetic radiation will strip the electrons off of atoms and create ions. This causes cell damage, cancers, makes your hair fall out and you die, that sort of thing. So, high enough frequency ultraviolet, x-rays, gamma rays, cosmic rays, all ionizing. Visible light, infra-red, microwaves, radio waves, long waves, all non-ionizing, so no Godzilla-creating type of radiation here.

Your sci-fi light show was high voltage arcing. Basically, they were miniature lightning bolts. There was enough energy in those mini-lightning bolts to give you a nasty burn, so "electrocuted slightly" is probably the closest thing on your list.

Quote:
Originally Posted by SlackerInc View Post
but I remember them doing one on the myth that microwaves cook from the inside out.
What I like to say is that microwaves cook from the inside out from the outside in, which may sound confusing, so let me explain it. Microwaves generate heat inside the food by exciting molecules of water, certain sugars and fats, and a few other things. Since the heat is generated inside the food, this is the "inside out" part, and it is noticeably different than the heat from an oven, which cooks from the outside in.

However, microwaves also penetrate into food from the outside, and they only go so far, and get weaker the deeper they go. So this is the "outside in" part. If you have a really thick piece of meat, it's going to cook all around the outside and the inside will remain raw in a microwave (it also won't brown, so it will be disgusting to eat in more than one way).

So you put together the fact that the heat is generated inside the food, and yet the radio waves only penetrate so far, you get microwaves cooking from the inside out from the outside in.

ETA:
Quote:
Originally Posted by Thudlow Boink View Post
Has Mythbusters ever done anything with microwave ovens?
They also did an episode where they tested whether a microwave with the door off could block a radar detector (spoiler - no, it couldn't, and it's dangerous to try it).

Last edited by engineer_comp_geek; 10-05-2015 at 03:54 PM..
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  #31  
Old 10-05-2015, 03:55 PM
SlackerInc SlackerInc is offline
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Nice explanation.
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  #32  
Old 10-06-2015, 11:34 AM
Kevbo Kevbo is offline
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Originally Posted by bbeaty View Post
Oh boy, kitchen physics!

I've been seriously abusing my own microwave oven for years with little harm. Generating fierce arcs and plasmoids. Shattering pyrex cups. Melting beer bottles. Making molten lava (from pumice or obsidian.)
Hey there Bill! I don't think I ever thanked you for pointing me toward the Dope. I have had many years of enjoyment both from this site and yours.

In general it is not a good idea to run any sort of radio transmitter (microwaves are radio waves) without some sort of load. The power generated is going to end up somewhere, and if not into a matched antenna, or a meatloaf, it will end up heating something that wasn't intended to withstand that. Often this will be the output amplifier or matching/coupling circuits. Without a load to dissipate power, it is possible for some of the energy to be stored in resonant circuits or cavities, and build up to destructive levels of current and/or voltage.

As components heat, resistance increases and insulators may become partially conductive (glass, to repeat Bill's example).
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  #33  
Old 10-06-2015, 12:04 PM
bob++ bob++ is offline
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I was once told that the door of a microwave oven had to be sufficiently strong to withstand an exploding egg. Whether they actually put an egg in a sample oven to test it, I have no idea.
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  #34  
Old 10-06-2015, 12:37 PM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
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I was once told that the door of a microwave oven had to be sufficiently strong to withstand an exploding egg. Whether they actually put an egg in a sample oven to test it, I have no idea.
I guess this one failed (fast forward to about the 50 second mark if you are impatient).
https://www.youtube.com/watch?v=6CqIZtMP98Q
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  #35  
Old 10-06-2015, 05:23 PM
davidm davidm is offline
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Quote:
Originally Posted by engineer_comp_geek View Post
...
Your sci-fi light show was high voltage arcing. Basically, they were miniature lightning bolts. There was enough energy in those mini-lightning bolts to give you a nasty burn, so "electrocuted slightly" is probably the closest thing on your list.
...
Is it to be expected that an empty microwave will throw damaging electrical arcs outside of it? That doesn't sound like something that would be considered safe to have in a home. Surely there was something seriously wrong with this particular microwave, beyond being run empty, right?
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  #36  
Old 10-06-2015, 06:23 PM
engineer_comp_geek engineer_comp_geek is offline
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Oh yeah, that microwave was seriously futzed if it was arcing.

Generally speaking, you can run a microwave for a short period of time and nothing exciting will happen. This is one of those "don't try this at home" kind of things though, because it's very harsh on the magnetron. Run it empty long enough and the magnetron will probably fail, and then you'll probably get a really good light show, though it might be very brief.

The arcing will also usually be contained inside the box.

Last edited by engineer_comp_geek; 10-06-2015 at 06:24 PM..
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