The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 01-07-2004, 01:32 AM
neoof neoof is offline
Guest
 
Join Date: Jan 2004
Is it dangerous to turn on an empty microwave?

If so- why?
Reply With Quote
Advertisements  
  #2  
Old 01-07-2004, 01:51 AM
David Simmons David Simmons is offline
Charter Member
 
Join Date: Nov 2001
Posts: 12,684
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.
Reply With Quote
  #3  
Old 01-07-2004, 01:58 AM
Achernar Achernar is offline
Guest
 
Join Date: Aug 1999
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?
Reply With Quote
  #4  
Old 01-07-2004, 02:05 AM
Desmostylus Desmostylus is offline
BANNED
 
Join Date: Nov 2002
Location: Sydney
Posts: 5,539
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.
Reply With Quote
  #5  
Old 01-07-2004, 02:16 AM
David Simmons David Simmons is offline
Charter Member
 
Join Date: Nov 2001
Posts: 12,684
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.
Reply With Quote
  #6  
Old 01-07-2004, 02:24 AM
Desmostylus Desmostylus is offline
BANNED
 
Join Date: Nov 2002
Location: Sydney
Posts: 5,539
Nah, it was something simpler than that. But it escapes me at the moment, too.
Reply With Quote
  #7  
Old 01-07-2004, 02:35 AM
Desmostylus Desmostylus is offline
BANNED
 
Join Date: Nov 2002
Location: Sydney
Posts: 5,539
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.
Reply With Quote
  #8  
Old 01-07-2004, 02:42 AM
David Simmons David Simmons is offline
Charter Member
 
Join Date: Nov 2001
Posts: 12,684
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?
Reply With Quote
  #9  
Old 01-07-2004, 02:50 AM
Desmostylus Desmostylus is offline
BANNED
 
Join Date: Nov 2002
Location: Sydney
Posts: 5,539
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.
Reply With Quote
  #10  
Old 01-07-2004, 03:08 AM
trabi trabi is offline
Guest
 
Join Date: Apr 2003
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.
Reply With Quote
  #11  
Old 01-07-2004, 03:22 AM
Desmostylus Desmostylus is offline
BANNED
 
Join Date: Nov 2002
Location: Sydney
Posts: 5,539
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.
Reply With Quote
  #12  
Old 01-07-2004, 05:36 AM
Desmostylus Desmostylus is offline
BANNED
 
Join Date: Nov 2002
Location: Sydney
Posts: 5,539
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.
Reply With Quote
  #13  
Old 01-07-2004, 10:11 AM
CurtC CurtC is offline
Guest
 
Join Date: Dec 1999
Yeah, but on the other hand, Klystrons have much cooler names.
Reply With Quote
  #14  
Old 01-07-2004, 12:03 PM
Ca3799 Ca3799 is offline
Guest
 
Join Date: Feb 2003
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.
Reply With Quote
  #15  
Old 01-07-2004, 12:32 PM
aerodave aerodave is offline
Charter Member
 
Join Date: Jul 2000
Location: Ohio
Posts: 1,619
I believe the device you're think of is "flux capacitor."
Reply With Quote
  #16  
Old 01-07-2004, 12:32 PM
bbeaty bbeaty is offline
Guest
 
Join Date: Aug 1999
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.
Reply With Quote
  #17  
Old 01-08-2004, 12:23 PM
neoof neoof is offline
Guest
 
Join Date: Jan 2004
Thanks
Reply With Quote
  #18  
Old 10-01-2013, 04:30 AM
SlackerInc SlackerInc is offline
Guest
 
Join Date: Feb 2003
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.
Reply With Quote
  #19  
Old 10-01-2013, 07:45 AM
lost4life lost4life is offline
Guest
 
Join Date: Oct 2002
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:______________________
Reply With Quote
  #20  
Old 10-01-2013, 11:11 AM
engineer_comp_geek engineer_comp_geek is offline
Robot Mod in Beta Testing
Moderator
 
Join Date: Mar 2001
Location: Pennsylvania
Posts: 8,822
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.
Reply With Quote
  #21  
Old 10-02-2013, 04:37 AM
SlackerInc SlackerInc is offline
Guest
 
Join Date: Feb 2003
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.
Reply With Quote
  #22  
Old 10-02-2013, 04:16 PM
t-bonham@scc.net t-bonham@scc.net is offline
Guest
 
Join Date: Mar 2003
Quote:
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!
Reply With Quote
  #23  
Old 10-02-2013, 05:59 PM
TBG TBG is offline
Guest
 
Join Date: May 2003
Quote:
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.
Reply With Quote
  #24  
Old 10-02-2013, 06:33 PM
t-bonham@scc.net t-bonham@scc.net is offline
Guest
 
Join Date: Mar 2003
Quote:
Originally Posted by TBG View Post
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.
Reply With Quote
  #25  
Old 10-02-2013, 07:25 PM
picunurse picunurse is offline
Member
 
Join Date: Mar 2003
Location: Seattle
Posts: 11,508
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..
Reply With Quote
  #26  
Old 10-03-2013, 04:45 PM
SlackerInc SlackerInc is offline
Guest
 
Join Date: Feb 2003
Too late, threw the microwave out already.
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 12:59 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.