If so- why?
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.
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?
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.
Nah, it was something simpler than that. But it escapes me at the moment, too. :smack:
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?
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.
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.
Here’s what I was thinking of when I said “We’ve had this argument before”:
http://boards.straightdope.com/sdmb/showthread.php?s=&postid=3113265#post3113265
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.
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.
Yeah, but on the other hand, Klystrons have much cooler names.
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.
I believe the device you’re think of is “flux capacitor.”
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.
Thanks 
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:______________________
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.