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ArcticMyst Security by Avery

Empty microwave question

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Just a random thought while enjoying a Youtube guy microwaving guns:


I've heard that operating a microwave without food is not recommended due to the microwave radiation not being absorbed by water molecules, but where does the energy go?

Mine is rated at 700W, so the power must heat up something, but I'm not exactly sure where it's being dumped. :thinking:
 





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The energy goes back into the magnetron. This will cause it to overheat and shorten its lifespan. The load doesn't need to be food though. Other materials can absorb the energy including metal in certain shapes which can act as an antenna which will induce a current in the metal.
 
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Yep, Cyparagon hit the nail on the head. The lack of a load causes an impedance mismatch (empty cavity is essentially an infinite impedance (or realistically a very high one)) which causes a high Standing Wave Ratio which reflects power back to source (magnetron). The magnetron then dissipates this as heat.
 
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While this is an interesting video,
I would point out that running a Microwave with the RF shield compromised is a very
dangerous idea.

Even the briefest exposure to uWaves of this magnitude can cause near instant nerve damage from quite some distance.

Cyparagon hit on an interesting point, water isn't the only material capable of absorbing uWaves well. It just so happens that
some kinds of glass and ceramics can absorb enough to actually melt inside a microwave oven. I have bear witness to this effect
myself with a glass container.

The other effect of absorbing uWaves in semi-conductive plastics (C) containing, such as in this pistol result in arcing, which leads to a fire.

Thin coatings of metal on plastics and glass can also do this.


A nice little demo (Dangerous do not attempt)
of a magnetron running without a complete RF shield.
Lucky these guys aren't dead.
 
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Also, to add to the list of hazards; the corneas are particularly sensitive to uWave radiation, it takes a FAR less dose to cause blindness than it does to seriously injure yourself from thermo-dermatological effects.
 

Tmack

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Wow I cringed throughout that entire video. It's started to get annoying how careless he was trying to be because of the camera .
 
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Also, to add to the list of hazards; the corneas are particularly sensitive to uWave radiation, it takes a FAR less dose to cause blindness than it does to seriously injure yourself from thermo-dermatological effects.

Right... Actually that was what I was going to say after watching this video.
Cataract risk is well below the amount needed to produce thermal effects, as you have indicated.

Anyways... :beer:
 
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Someone I know, forgot to put their coffee cup in the microwave one early morning. About 2 minutes in, the standing wave shut it down.

Put the coffee in and turned it on. Nothing.

They assumed it was torched, so I flipped it over and depressed/reset the overcurrent switch and it came back to life. It lived for another ~10 months before it quit.

Poor magnetron.
 
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Thanks for the info guys! Is the magnetron the only component that is going to heat up?

Seoul, I just saw the video you posted and it did seem incredibly dangerous. I'm assuming the clear fluorescent (not really) tube is a UVC bulb used to sterilize dishes? Scary how the guys are holding it with their bare hands. Sorry, would let me rep you.
 
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Russians actually gather in front of glass wall UVB mercury lamps to get cheap vitamin D, so that's probably what it is, not a quartz envelope UVC sterilization lamp.

Fun fact though, welding exposed you to more UVC than most UVC systems. Yeah you can deck out in PPE, but most experienced welders wear only a helm, gloves, and if needed sleeves over their t shirt. Worked for many years that way myself.
 
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Thanks for the info guys! Is the magnetron the only component that is going to heat up?

Seoul, I just saw the video you posted and it did seem incredibly dangerous. I'm assuming the clear fluorescent (not really) tube is a UVC bulb used to sterilize dishes? Scary how the guys are holding it with their bare hands. Sorry, would let me rep you.

Actually, that tube also may be a water sterilization bulb for hydroponics or home use. These bulbs ionize quite easily as they too are Hg filled.

Actually looking back at the video, there are a couple of times that make me shudder. One instance the fellow filming decided to put the arc out with his fingers. Had he completed a circuit with the ground there is more than enough current to provide defibrillation even seeing as the high voltage is Ghz band.

I'm sure looking back at the video at least one person has permanent nerve damage in his hands from holding that test tube so close to the magnetron.

Stupidly dangerous yet entertaining and educational all at the same time.
:thinking: :thinking: :p
 

WizardG

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Yeah, I found out about running your microwave oven empty the hard way a decade or so ago. Thought I had set the timer for etching a PCB but I had actually turned the oven on. My board was done before the `timer` went ding and when I went to shut it off the microwave (the whole box) was almost too hot to touch. It heated the magnets on the magnetron above their curie temp' turning it from a source of microwaves into a big rectifier tube.
 
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I've heard that very high frequencies don't electrocute people, I was wondering why they're weren't dead.

More info:
Humans are able to withstand 10 times more current at DC and at 1000 hertz than at 50 or 60 Hz.. Electro-Surgical equipment operating above 100,000 Hertz pass high currents through the body with no effect on the heart or breathing of a patient.
Electrocution Thresholds for Humans - Bass Associates Inc.

Wow, that's really interesting. Thanks for sharing that WizardG.
 
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Yes, above several dozen Kilohertz nerves stop responding to AC signals.

The danger with the magnetron is that it has LETHAL DC high voltage powering it, and should that get into the output when you draw an arc, you die. HV RF can carry DC current along the ionization path.
 
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Yes, above several dozen Kilohertz nerves stop responding to AC signals.

The danger with the magnetron is that it has LETHAL DC high voltage powering it, and should that get into the output when you draw an arc, you die. HV RF can carry DC current along the ionization path.

While I agree with you about the DC component
on the ionization path, the HVAC at high frequencies is
rather insidious. While your body doesn't respond to it
it still does travell along the nerves doing all sorts of damage.
There are a few very well documented
explanations by 4Hv.org members from the tesla coil builders
society.
There is unfortunately the perception that
there is a "skin effect" occurring on people as
one finds on metal conductors. This is a proven to be a
myth. The current will always follow the path of least resistance,
This being your nerves, blood vessels, etc.
One good reason to not try pulling tesla coil arcs to your fingers.

From HV.org safety wiki.

The dangers of contact with high-frequency electrical current are sometimes perceived as being less than at lower frequencies, because the subject usually does not feel pain or a 'shock'. This is often erroneously attributed to skin effect, a phenomenon that tends to inhibit alternating current from flowing inside conducting media. It was thought that in the body, Tesla currents travelled close to the skin surface, making them safer than lower-frequency electric currents.
 
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I should have elaborated; nerves don't respond but certainly aren't impervious to RF. It'l roast you but you won't know it.
 




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