Metal in microwave

Discussion in 'General Science & Technology' started by Beer w/Straw, Sep 19, 2018.

  1. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    I'm sure we've all done it by accident and we stop the microwave when we hear/see sparks.

    However, metal can go in the microwave i.e. staple on tea bag label. I once read that it may be just too small to make a difference but what if else?
     
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  3. Jeeves Valued Senior Member

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    I killed a microwave once with aluminum foil that I didn't realize was in the packaging.
    That was many years ago - maybe they've put in safeguards since.
     
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  5. sideshowbob Sorry, wrong number. Valued Senior Member

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    Would that be related to the wavelength?
     
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  7. Gawdzilla Sama Valued Senior Member

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    I saw a guy on TV microwave fowl with metal straps holding its legs together. He said to just put some wax paper over it. Never tested that.
     
  8. Jeeves Valued Senior Member

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    Yes, come to think of it, some microwavable packages are lined with foil on the inside and covered with paper on the outside. I've sometimes wondered whether that was some special kind of foil, special kind of paper, or whether you just need to hide the metal.
    It dosn't come up in my house, since we only ever use the microwave to warm up cups, bowls and plates of comestibles, not to cook.
     
  9. Gawdzilla Sama Valued Senior Member

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    Hot Pockets are like that, IIRC. I'm over them so I may be wrong. I know Marie Callender's pot pies are shiny on the inside. Is it foil? No clue.
     
  10. exchemist Valued Senior Member

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    I wondered that at first but I don't think so. Metals have currents induced in them by microwaves and the reflect the microwaves back, if the surface is flat. As I understand it, the problems are with thin curved or pointed segments of metal foil, in which currents are induced that are too strong for the thickness of the metal and so it heats up and burns. Perhaps a single staple, being quite thick in relation to its length, will easily carry the current without overheating.

    But I'm not an electrical engineer. Billvon will know the answer.

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  11. DaveC426913 Valued Senior Member

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    It's also possible that a staple is just too small to hold enough heat to do any heat damage.

    Analogous to sparks from a grinder: they're glowing red hot, but you can't even feel them.
     
  12. Gawdzilla Sama Valued Senior Member

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    Ever seen a microwave with carousel when it's been run empty? The metal axles melt the rollers. Not much metal there, but it's catching all the microwaves apparently.
     
  13. billvon Valued Senior Member

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    Sort of. Wavelength sets the skin depth; the depth that the energy penetrates. Microwave frequencies give you very shallow skin depths (microns) on metals. This means that currents are set up in a very thin layer of metal, but the layer is so thin that there's not much heating. (And the currents inherently oppose the incoming wave, so they tend to reflect most of the energy right back.) From the perspective of the microwave, too much metal can change the impedance that the magnetron sees (due to reflections) and damage it. But small pieces won't.

    The problem arises when you have very sharp edges or points. Then the entire thickness of the edge (or point) carries that skin current. That leads to high potentials which can cause arcing or sparking. You might notice that a fork causes arcing but a spoon doesn't, for example. The risk there is that it will set things on fire inside the microwave and damage the interior. However, some microwave-ready foods actually have metal in them to reflect some of the energy where they want it; as long as the edges of the metal are very rounded and dull this isn't too dangerous.

    In food, the same issues arise - but since you have a deeper skin depth (centimeters) for other materials with medium dielectrics (like wet foods) you don't get arcing. Note that in rare cases with very conductive foods you can still see arcing, like a hot dog (very salty = conductive) cut into a point.

    BTW, NXP now has something they call a food warmer that uses tens of MHz to heat up food. The lower frequency allows a much deeper skin depth, such that the food is heated far more evenly.
     
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  14. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    I was earlier going to suggest the greatest experiment you can do at home: Stuff your microwave to capacity with only staples then turn that mother on!

    Me? I'm too lazy and timid. Someone with a Type A personality should do it and they'd be my hero.

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  15. exchemist Valued Senior Member

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    What about hot dogs cut to a point? At least you can eat the mothers afterwards.

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  16. Q-reeus Valued Senior Member

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    'Hiding the metal' via paper on outside could not work. Most likely the 'metal foil' is actually metalized plastic film, which presumably has a discontinuous structure (iirc holding such films up to e.g. sunlight, one observes some light gets through) not permitting appreciable currents to flow. Either that or you're supposed to empty the contents into a microwave friendly container before zapping it.
     
  17. Q-reeus Valued Senior Member

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    Not true. Currents actually tend to avoid sharp edges and points. What makes sharp protrusions prone to arcing is the large E fields set up there - corona discharge. Currents are a minimum in such regions.

    And the reason e.g. tea bag staples are not a problem is not due to skin depth but the small electrical size of the staple - effectively a very short parasitic dipole antenna. The impedance of which is way too high to permit appreciable currents thus voltages that could lead to noticeable arcing.
     
  18. Jeeves Valued Senior Member

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    In that case, the guy on tv was all wet. Or his chicken's legs were tied with plastic bands, not metal ones. Coz that would be way bigger than a staple.
    (You could just take the tag off the teabag, as well.)
     
  19. sweetpea Valued Senior Member

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    I got sparks from using a saucer with a gilt patterned rim.
     
  20. sideshowbob Sorry, wrong number. Valued Senior Member

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    I was thinking of measuring the speed of light with a cheese slice
     
  21. iceaura Valued Senior Member

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    The timid can try slicing a grape into two halves, without cutting all the way through - leave a bridge of peel, like a hinge. Hit that for a few seconds, enjoy the show.
     
  22. sweetpea Valued Senior Member

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    Learnt from experience... Don't do frozen brussels sprouts in microwave. If you have to use the micro, then let the sprouts thaw for a few hours, then deep cut them before putting in micro. No explosions. I like to think I'm the only one that knows this, It's my only cooking tip.
     
  23. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    I've done that with a stop watch and cheese in the microwave.
     
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