[SI-LIST] Re: Honey, your conductors are ready!

  • From: Ivor Bowden <bowden.ivor@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Sun, 16 Nov 2014 06:47:08 -0800

CDs (those plastic disks with internal metalized layer 
representing digital codes) make a nice little light show in 
the microwave, especially with the lights turned out. I 
usually put it on top of a glass of water to avoid a lot of 
reflected power to the output, though I don't suppose it 
makes much difference for a short duration? "Conventional 
Wisdom" says don't operate microwave oven unloaded, it could 
get damaged, is that so?
~Ivor

On 11/16/2014 1:47 AM, Loyer, Jeff wrote:
> At first glance I thought that susceptors might be some relatively poor 
> conductor from ferromagnetic material, perhaps with a rough surface.  There 
> are articles that suggests otherwise (in fact that aluminum foil might be 
> duplicating a susceptor almost exactly).
> >From http://www.foodprocessing.com/articles/2005/234/, 
> >http://www.chemistry-blog.com/2011/11/17/microwave-grilling-how-does-it-works/:
> "Susceptors are metal, most often aluminum, bonded onto a paperboard 
> substrate."
>
> Reading the articles it appears they actually might not want all the heat 
> (dissipative portion of insertion loss) they can get from the metal, hence no 
> need to go with ferromagnetic or rough.  But, we'd probably have to find a 
> dedicated microwave food engineer (PhD MFE?) to discern all the subtleties.  
> Will know more in a week or so.
>
> On the bright side of all this, once I get home I have a microwave I can 
> dedicate to my shop and science experiments!  And, to tell the truth, I never 
> liked this particular microwave anyway (LED scrolling display to tell me my 
> food's ready - really?).
>
> What I still don't understand is why some metals spark, and some don't (it 
> seems that shape plays as much a part as anything, besides having it touch 
> the walls).  And also, what makes them reflective (or not - maybe we should 
> be asking stealth fighter designers?).  Still lots to learn.
>
> Jeff Loyer
>
> -----Original Message-----
> From: Ray Anderson [mailto:ray.anderson@xxxxxxxxxx]
> Sent: Saturday, November 15, 2014 8:16 AM
> To: Loyer, Jeff; Smith, Larry; si-list (si-list@xxxxxxxxxxxxx)
> Cc: Ray Anderson
> Subject: RE: Honey, your conductors are ready!
>
> Jeff-
>
> It is some sort of resistive material that heats up when the microwaves 
> impinge upon it. It absorbs the energy instead of reflecting it.
>
>   I believe they call the coating a 'supsceptor'.   <spelling??>
>
> -Ray
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Loyer, Jeff
> Sent: Friday, November 14, 2014 3:51 PM
> To: Smith, Larry; si-list (si-list@xxxxxxxxxxxxx)
> Subject: [SI-LIST] Re: Honey, your conductors are ready!
>
> Good stuff Larry - thanks!  What do you suppose they're putting in those 
> chicken pot pie bowls?  Can I try it with that stuff?
>
> Jeff Loyer
>
> -----Original Message-----
> From: Smith, Larry [mailto:larrys@xxxxxxxxxxxxxxxx]
> Sent: Saturday, November 15, 2014 7:45 AM
> To: Loyer, Jeff; si-list (si-list@xxxxxxxxxxxxx)
> Subject: RE: Honey, your conductors are ready!
>
> Hi Jeff - I really enjoyed your write up, it put a big smile on my face.
>
> But just a few comments.  I'm not sure that the foil is going to get hot.  A 
> conductor is going to do a good job of reflecting and re-directing microwaves 
> but it is not going to absorb a lot of energy.  Certainly some eddy currents 
> are going to flow just as they do any metal structure near a radiating source 
> and this is going to cause some IR loss and power absorption.  But mostly the 
> energy is going to bounce around the oven and possibly do damage.  The metal 
> can concentrate fields.  I once had a coffee cup with a gold plated design on 
> it.  The sparks flew!
>
> But I think most of the energy absorption in a microwave oven happens in the 
> dielectric.  Wikipedia makes a point that the polarized molecules in the food 
> rotate and build up the thermal energy for heating.  Water is a good example 
> of that.  Please see https://en.wikipedia.org/wiki/Microwave_oven .  Further 
> down in the article there is a section on metal objects, aluminum foil being 
> one of them.  A quote: "Aluminium foil is thick enough to be used in 
> microwave ovens as a shield against heating parts of food items, if the foil 
> is not badly warped. When wrinkled, aluminium foil is generally unsafe in 
> microwaves, as manipulation of the foil causes sharp bends and gaps that 
> invite sparking."
>
> Anyway, I enjoyed your post.  Experiments are good.  :)
>
> Best regards,
> Larry Smith
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Loyer, Jeff
> Sent: Friday, November 14, 2014 3:03 PM
> To: si-list (si-list@xxxxxxxxxxxxx)
> Subject: [SI-LIST] Re: Honey, your conductors are ready!
>
> Interesting responses.  I'm surprised that no one on this list braved the 
> experiment (though I still can't recommend it, since I don't know all the 
> things that can go wrong, as my wife will attest).  I'm a huge fan of 
> Feynman, and would wager that he'd risk his microwave to try it.
> Placing conductors in a microwave is not necessarily a bad thing.  If you buy 
> a chicken pot pie that promises "even browning" you'll probably see it comes 
> in a rather shiny bowl.  Put an ohmmeter to that bowl and you'll find that 
> it's a conductor!  (or at least parts of it are)  The folks who make 
> microwaveable food understand what conductors do when exposed to microwave 
> energy and take advantage of it, we (signal integrity engineers who transmit 
> microwave-frequency data) should also, I think.  And, we should understand 
> the difference between the fundamental microwave mechanisms at play in a 
> dielectric (chocolate bar) and a conductor.
> I also don't think a simulation accomplishes the goal - I devised this 
> experiment for a class I'm putting together, and I'm trying to remove a layer 
> of abstraction for the audience.  Showing them the results of a simulation 
> won't be nearly as effective as demonstrating that one piece of foil is 
> hotter than heck, while the other is relatively cool, all with items everyone 
> has access to.  I think a physical demonstration will allow people to "grok" 
> why a rough surface (with more surface area) is a bad thing for a PCB trace.  
> And, as someone pointed out off-line, how would you prove correlation between 
> your simulation and test results? :-) I wonder what percentage of EM PHd's 
> would predict correctly the results of this test? (I'm guessing Dr. Huray's 
> students would fare well) I guess we'll have to wait a bit for the results of 
> the test, when I get back from China (unless my brother pulls through for me, 
> he has an old microwave he was tossing and might be willing to try it).
> Jeff Loyer
> From: Loyer, Jeff
> Sent: Friday, November 14, 2014 5:21 AM
> To: si-list (si-list@xxxxxxxxxxxxx)
> Subject: Honey, your conductors are ready!
>
> I thought you all might enjoy hearing the results from an experiment I 
> recently concocted (but haven't yet completed).  Here's how it was crafted:
> Cut 2 identical squares of aluminum foil.  Shape one of those into a ball, 
> really tight and compressed (sphere = minimum surface area).  Leave the other 
> one flat (maximum surface area).  Put them both in the microwave and heat 
> them for 15 seconds (not sure of exact time).  See if there's a significant 
> difference in the temperature between them.  The flat sheet should be much 
> hotter - more surface area for the microwaves to impinge upon.
>
> Here's what I've learned from this exercise so far:
>
> 1)      Don't do this unless you're prepared to buy a new microwave, or can 
> live with lots of singe marks.
>
> 2)      If you blow the microwave circuit breaker, reset it quickly if you 
> have a freezer on the same circuit.
>
> 3)      Don't have your spouse try this while you're in business in China - 
> wait until you can do it properly.
>
> 4)      The foil has to be small enough not to contact the walls.
>
> As you can guess, I asked my spouse to try it.  I'm in China but was anxious 
> to hear the results.  I should have waited! :-)   Here's the text from her 
> synopsis:
>
> "Well.  Our microwave looks like he[ck] - burned the door seal, and put black 
> streaks on the back wall.  My flat piece of foil was not a square - it was a 
> rectangle, so the edges grazed the sides of the box insides as the tray 
> rotated.  There was lots of arcing, sparking, and color.  But as far as Kath 
> and I could tell, neither piece got warm!!!!   Go figure!
> I could try again.  [note from Jeff: how did I find this woman?!?!]
> I began with a rectangular piece approximately 4 inches by 6 inches.    
> Should have tried a square.
> Had difficulty making a ball out of the one piece, after a certain point of 
> compressing it.  It was a little more of a prism than circular.  But close.
> We weren't clear if you wanted both pieces in the microwave at the same time, 
> or if the test should be run separately for 15 seconds.  We (Kath and I) 
> decided to put them both in, beside each other, but not touching.  All the 
> arcing activity caused the ball to roll under the rectangle, though.  At 
> least so it seemed.  The pieces were lightly stuck together when we were 
> done."
>
> Anyway, I still think it's a cool demonstration of the fundamental principle 
> that a layman can try (if they're brave or dumb).  There's a nice limit that, 
> if the sphere is small enough to equal the skin depth, the two pieces will 
> heat evenly (I think).  I'm anxious to try it when I get home.
>
> If you choose to try this in the next week at the risk of your microwave and 
> food in your freezer, please let me know what you find.
>
> Cheers,
> Jeff Loyer
>
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