Running a microwave oven empty

In summary, the limiting factor appears to be the magnetron heating up quickly and it is bad for the cathode.
  • #1
So I know it's generally not a good idea, but I wondered what actually is the limiting factor. Does the magnetron heat up much more quickly? - and if so what's the physics going on that's causing it to heat up - is it increased current in the resonant cavities ? Also is it bad for the cathode ? Is there increased back bombardment etc ?

I've always assumed the glass turntable absorbs some microwaves in case it is run empty.

Is this the usual cause of the antenna cap arcing - and is that just because of the very high voltage it gets to ? I've noticed that high power magnetrons don't have a metal cap at all they are all ceramic.
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  • #2

I don't know if this applies to today's microwaves, but:

anorlunda said:
I once owned one of the original Amana Radar Ranges (version 1.0). Twice, it was mistakenly turned on with no food inside. Both times, it melted a hole in the glass tray leaving a 3cm molten glass marble under the tray. So, twice I had to buy a new tray.

I spoke to Amana engineers about that. They insisted that I was a crackpot who made up the story and that no such thing could happen. But thereafter, I always thought of a microwave oven with no food inside as a potential death ray or at the very least something very hazardous.
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  • #3
Glass has a strange property - it's mostly resistant to microwaves until it gets very hot - once molten it absorbs microwaves readily and a thermal run-away continues melting the glass bright yellow. I suspect a rotating turntable - or a microwave oven with a stir fan should prevent glass sitting in a standing wave. Interesting. :cool:
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  • #4
The other day I was trying to heat up a little metal tray full of pins and things did not go well.

I should think the effects of operating a microwave empty would depend quite strongly on the specific kind of oven, and probably the condition. Maybe if there are scratches and such these may do whacky things to the energy absorbing nature of various components.

Then there's my oven which, due to the detritus of the previous 100 uses, is not technically empty anyway.

Then there's this.

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  • #5
Thanks for this - and really interesting video! I can see some more experiments coming.
  • #6
Well, I'm not sure we can be of much more help on this, other than to recommend that you keep a fire extinguisher handy (and know how to use it), and set up a way to kill power to the oven if anything bad happens. Stay safe please.
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