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Author Topic: Is my microwave safe?  (Read 21561 times)

Jenny

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Is my microwave safe?
« on: 26/02/2009 09:30:01 »
Jenny asked the Naked Scientists:
   
Hello!

A few months ago my microwave oven fell off the shelf it was sitting on and one of the sides got bent up. The microwave works fine except it has an opening on the side where it fell.

Is it safe to use the machine? Is there any risk of the microwaves hurting me?

MUCH THANKS! I love the show!

JENNY
New York, NY

What do you think?


 

Offline Karen W.

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Is my microwave safe?
« Reply #1 on: 26/02/2009 12:09:57 »
Welcome to the forum Jenny...!

What a good question! I would tend to worry and not use it..but I am not the answer lady.. I am sure someone who knows will come and give you a good reliable answer.

Enjoy the forum and thanks for Joining in!
« Last Edit: 28/02/2009 12:18:15 by Karen W. »
 

Offline rosalind dna

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Is my microwave safe?
« Reply #2 on: 26/02/2009 12:31:54 »
Welcome Jenny, I have dropped my microwave a few times in the past
and only bruised myself with no damage to the microwave as far as
I can make out. Anyway no machine lasts for ever. Mine's 14 years old.
Still working fine.
 

Offline Madidus_Scientia

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Is my microwave safe?
« Reply #3 on: 26/02/2009 15:55:26 »
Do you mean there's an opening all the way through into the actual inside?
 

Offline Chemistry4me

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Is my microwave safe?
« Reply #4 on: 27/02/2009 03:25:42 »
I don't think she'll come back (but who knows :)?), but let's assume that there was an opening all the way through.
 

Offline Karsten

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Is my microwave safe?
« Reply #5 on: 27/02/2009 12:48:50 »
 I believe to have learned that when there is an opening to the inside microwaves can "escape" and could cause harm. The oven needs to be encapsulated in metal, or at least metal mesh (as in the window).
 

Offline rosalind dna

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Is my microwave safe?
« Reply #6 on: 27/02/2009 14:01:55 »
yes and it's slightly using radiation but I am not too sure about this fact.
Also that if any metallic item is placed in a microwave then it will explode.
 

Offline daveshorts

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Is my microwave safe?
« Reply #7 on: 27/02/2009 14:54:42 »
If there is a hole or even a crack all the way through the metal then the microwave oven could certainly leak microwaves, and they could cook you!

There was a viral advertising campaign recently which appeared to show phones cooking popcorn, of course this wasn't the case, what actually happened was that the people setting up the video had put a dismantled microwave oven under the table and it was cooking the popcorn through the table as the microwaves were escaping and heating up anything they hit.
 

Offline Chemistry4me

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« Reply #8 on: 27/02/2009 23:14:51 »
So should I NOT open the microwave door until the beeping has stopped? ???
 

Offline daveshorts

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« Reply #9 on: 28/02/2009 11:33:14 »
The microwave designers have thought about that, and there are several interlock switches around the door which disconnect the power as you open the door, so this is perfectly safe (unless you disable the interlocks of course.
 

Offline Chemistry4me

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Is my microwave safe?
« Reply #10 on: 28/02/2009 11:44:29 »
So those beeps after heating are for? ???
Warning you NOT to touch the heated food?
 

Offline daveshorts

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Is my microwave safe?
« Reply #11 on: 28/02/2009 11:48:06 »
Telling you that it has stopped cooking and your 'delicious' TV dinner is ready, so you should go and collect it.
 

Offline Chemistry4me

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« Reply #12 on: 28/02/2009 11:50:45 »
Yes, of course. You're right daveshorts, why didn't I think of that!
 

Offline Karen W.

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Is my microwave safe?
« Reply #13 on: 28/02/2009 12:22:13 »
That is kinda scary... That if it ic cracked that the waves could escape and cook what ever is in range of them... ooooooohhhhh! LOL Remind me not to break my microwave and try to use it anyway!
 

Offline graham.d

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Is my microwave safe?
« Reply #14 on: 28/02/2009 13:18:10 »
Microwave ovens need to be enmeshed electrically (an approximate Faraday cage) to prevent microwave leakage which can be harmful if you are close enough. Dropping a microwave oven is not good! Even a small gap (badly fitting door for example) can leak significant microwaves. The eyes are particularly susceptible and damage is cumulative.
 

lyner

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Is my microwave safe?
« Reply #15 on: 28/02/2009 18:24:59 »
Microwave ovens need to be enmeshed electrically (an approximate Faraday cage) to prevent microwave leakage which can be harmful if you are close enough. Dropping a microwave oven is not good! Even a small gap (badly fitting door for example) can leak significant microwaves. The eyes are particularly susceptible and damage is cumulative.
To stop 'microwaves getting out' you need to suppress currents flowing out of the gap in the door. It is not difficult to do this effectively by the use of a 'slot' all around the door. Transmission line theory tells you that a short circuit in a line is transformed into an open circuit by a quarter wavelength line (this appears in series with the bad contact between the door and case - still producing an open circuit) and back into a short circuit after another quarter wave (producing a good short circuit across the  join of door and case). This technique is frequently used in microwave connectors ('choke flange') and it works well in turning a poor contact into a good contact - suppressing leakage well.
Looking at my microwave oven I think I can identify the structure I'm referring to. The thickness  of the case is about 3cm (λ/4) and there is a slot within the door which could be the remaining (another λ/4 bit).
There are certainly no 'spring fingers' around the door gap, and, without these, a strong spring, keeping a tight join all the way round (impossible) - or some arrangement like I describe, the stuff would be pouring out as much as the smell of cook chill curry!

It's amazing how such a sophisticated bit of wave theory can be at work in something so mundane as an oven door. And be ignored by everyone!
Has anyone else any knowledge of this being used for microwave ovens?
http://www.freepatentsonline.com/4700034.html I found this to prove I'm not just raving mad!
« Last Edit: 28/02/2009 18:57:29 by sophiecentaur »
 

Offline techmind

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Is my microwave safe?
« Reply #16 on: 03/03/2009 21:44:37 »
Jenny asked the Naked Scientists:
A few months ago my microwave oven fell off the shelf it was sitting on and one of the sides got bent up. The microwave works fine except it has an opening on the side where it fell.

Is it safe to use the machine? Is there any risk of the microwaves hurting me?

What do you think?

If there's any evidence of physical deformation (which it sounds like there is) you should get it checked out by someone who is qualified to do so, and has a device to check for leakage.

Without seeing a photo of the damage I can't give any opinion (not that I'm strictly qualified to anyway) but from what you've said I would be 'concerned'.
 

Offline techmind

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Is my microwave safe?
« Reply #17 on: 03/03/2009 21:52:19 »
...

It's amazing how such a sophisticated bit of wave theory can be at work in something so mundane as an oven door. And be ignored by everyone!
Has anyone else any knowledge of this being used for microwave ovens?
http://www.freepatentsonline.com/4700034.html I found this to prove I'm not just raving mad!

I'd came to a similar conclusion a few years back, having considered how RF-proof test-chambers are built with the fine gold-fingers etc around the doors. And how putting a mobile phone inside a metal biscuit tin (with the lid on) doesn't stop it working (unless reception is very marginal anyway).

So how frequency-selective is the slot system? Does that explain why a mobile phone (on 1800MHz or 900MHz) can still make a call from inside a microwave oven cavity?
(Warning: if trying this, DO NOT switch on the microwave oven with the phone inside else you will destroy the phone in an instant)
 

lyner

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Is my microwave safe?
« Reply #18 on: 03/03/2009 22:28:47 »
I found a figure of 20% bandwidth for choke flanges - whatever that may mean in terms of effective screening. This would mean that 1800MHz would not be very well screened and 900MHz would pour through like a dose of salts.
The magnetron frequency is very well defined so it would be operating with the optimum effective screening.

The space between the door and the case won't be particularly critical (it can't be or too many people would be fried).

RF proof test chambers have a much harder job to do. They have to ensure that there is a 'real' short circuit between the door and frame because it has to operate overr many frequencies. I was once actually GIVEN one, which was surplus to requirements. Unfortunately, there was nowhere to assemble it so we had to pass it on before we could make use of it.
 

Offline techmind

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Is my microwave safe?
« Reply #19 on: 03/03/2009 22:31:06 »
Here's a photo showing my (Orange / 1800MHz) mobile receiving a call while inside the oven cavity. I put a torch in there too!




Reception seemed a bit marginal with the 1800MHz signal ("1-bar" or thereabouts).

I tried with an old Vodafone (900MHz) and still had 4-bars (maximum strength for that phone).


There's both Orange and Vodafone base stations not more than 100metres away, so plenty of signal strength outside the microwave oven!


I don't have anything 3G-mobile (2.1GHz) or WiFi/Bluetooth 2.45GHz (same as oven) which I can conveniently try :(
« Last Edit: 03/03/2009 22:57:08 by techmind »
 

Offline DrN

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Is my microwave safe?
« Reply #20 on: 03/03/2009 22:48:00 »
I'm a bit dubious about microwaves. Ours at work are tested annually, and last year we were told one was leaking and we should bin it. It didn't appear to have any cracks or any damage, and wasn't especially old (although I guess it had had a lot of use). I must admit I never stand in front of a microwave when its on, whether its old or new.
 

Offline techmind

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Is my microwave safe?
« Reply #21 on: 03/03/2009 23:49:05 »
I'm a bit dubious about microwaves. Ours at work are tested annually, and last year we were told one was leaking and we should bin it. It didn't appear to have any cracks or any damage, and wasn't especially old (although I guess it had had a lot of use). I must admit I never stand in front of a microwave when its on, whether its old or new.

We had a microwave in the canteen area at my previous work and one day I noticed that it kept on going even when one of my collegues opened the door.  :o Eeek! I told him to turn it off before opening the door, then prompty sent the oven to our Instrumentation department for test/repair.
« Last Edit: 03/03/2009 23:52:24 by techmind »
 

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