Mouth of the dragon's lair (the secret revealed)

[Planapo]

Here's a picture of the new burner and the pot in question, slid sideways so you can see how it all fits together.

Aha, now I see! Cultural differences, always interesting to explore!
I've never come across such an electric burner like yours before, we don't have those over here. From its looks alone, I would have mistaken it for a gas burner. Well, and I had thought 'cookstove' was my middle name ...

Thanks Rik, for the additional picture.

--Betty

[JW]

Rik

Don't forget that there is also present iron, from the shell and wire, and probably chromium and nickel - this material didn't just disappear.

A commonly formed substance in this circumstance could be a Mg-spinel, such as the analogue of natural magnesioferrite, MgFe2O4, with Cr and Ni the formula will be (Mg,Ni)(Fe,Cr)2O4 - when pure could be colorless to pale yellow. Related is the gem spinel (ruby), MgAl2O4. But both MgO, periclase, and MgFe2O4 spinel will form octahedral crystals and have similar hardness, barely hard enough to scratch glass, so maybe difficult to distinguish.

[DQE]

Here's a picture of the new burner and the pot in question, slid sideways so you can see how it all fits together.

Aha, now I see! Cultural differences, always interesting to explore!
I've never come across such an electric burner like yours before, we don't have those over here. From its looks alone, I would have mistaken it for a gas burner. Well, and I had thought 'cookstove' was my middle name ...

Thanks Rik, for the additional picture.

--Betty

My curiosity was tweaked by your post. Can you describe (or provide a simple photo) of a typical stove in your part of the world?

In the USA, the stove Rik posts is probably the most common by far, and the electric heating coil element simply unplugs from a loosely fitting socket in the top of the stove. One other electric designs I can recall seeing in stores used quartz-halogen heating elements (each stove top "burner" sits underneath a ceramic/glass sheet that makes up the outer surface of the top of the stove. I believe there are also electromagnetic induction heating elements available now: Here's a link to an example of this (expensive ) design:

http://www.bestbuy.com/site/Electrolux+ ... &cp=1&lp=7

and

http://www.bestbuy.com/site/Electrolux+ ... cp=3&lp=12

"Automatically detect magnetic cookware and activate the induction field and adjust it to the exact size of the pan for excellent results."

"Cooktop Style - Electric Induction"

=================
It is quite amazing how wide the price range is for a kitchen oven and 4-element stove top..
.

[Planapo]

Phil, you seem to enjoy remote exploration of foreign cultures too.

The most common and cheapest stove burner type over here is one that has cast iron plates on top. The heating element is attached to the underside of the plates but the plates are firmly fixed to the stove top and in normal operation can't be taken off. Maybe they can be disassembled just for a repair, but with today's zeitgeist (nice word, innit?! ) to throw away and buy new instead of repairing, I'm not sure.
http://de.wikipedia.org/w/index.php?tit ... 0509141327
http://www.preissuchmaschine.de/in-Herd ... 20020.html

The other a bit more expensive type is the glass ceramics tops that you have mentioned and linked. Commonly known over here by its Schott brand name Ceran and hence often called Ceranfeld.
http://de.wikipedia.org/wiki/Ceranfeld
http://www.preissuchmaschine.de/in-Herd ... 22120.html

And then the induction type you've mentioned.
http://de.wikipedia.org/wiki/Induktionskochfeld
http://www.preissuchmaschine.de/in-Herd ... 48450.html

And at last, and what I like best, the ones with gas burners. (They can also be had with a glass ceramics top)
http://de.wikipedia.org/wiki/Gasherd
http://www.preissuchmaschine.de/in-Herd ... 509-W.html

I hope Rik doesn't mind our intercultural stove studies in his thread.

--Betty

[DQE]

Betty,

Interesting stove technologies.

I had no idea that stoves differed so much from country to country.

We also have open flame gas stoves, too, probably the most dangerous since they use an open flame.

Apparently your electric heating elements are much better shielded from the user than the typical USA designs. The design Rik shows in his photograph is very common in the USA and is similar to the one I now use. The burner's heating element is exposed but is easily replaced when the inevitable happens and one of the elements burns out. I didn't know they could burn out in such a spectacular fashion, though!

[rjlittlefield]

Betty, no problem with the intercultural stove study in this thread. I confess, part of the reason I post these things is to see what unexpected direction the discussion will go!

JW, thanks for the further information about likely identities for the crystals.

Phil, open flame gas stoves may well be the most dangerous, but they are also much loved by cooks because they provide more even heat and instantaneous control of cooking speed when used with a thin pan. Exposed coil electric elements make the pan very hot where it contacts the element and relatively cool where it doesn't. To compensate for the unevenness, one buys cookware with thick bottoms to diffuse the heat, and then thermal inertia kicks in to introduce a time lag between control and effect. Induction and IR are more uniform so they can work with thinner cookware, but there's a price to be paid for that improvement too. Tradeoffs, tradeoffs...

This is the most spectacular failure that I've seen, producing a sustained jet of blue-white stuff with significant melting of the shell. In previous years I've had other units develop only small holes when they failed.

By the way, I checked my other elements before going to the store. The two smaller elements seemed to be fine so I kept those, but the other large element had obvious cracking along its seam on the underside. I replaced that one too, and sacrificed it to produce the cross-sections seen earlier in this thread.

--Rik

[ChrisR]

Our electric hobs (UK) are like the more expensive end of Betty's depicted range, on the whole. The spiral type are thought of as old-fashioned though, the solid metal topped ones are slightly less unpopular. Spirals live in student flats and bed-sits.
Very few would choose a straightforward electric metal type though. If someone prefers or has to have an electric hob, either induction or halogen lamp styles are much preferred.
The majority of home cooks prefers gas (mains methane). Some use bottled propane just for the hob because they won’t use electricity.
No self-respecting restaurant chef would not have some gas rings. For a wok burner
there's no easy alternative. This one's 145000 Btu/hr, = 42kW. What does the world use for those where there’s no mains gas?

Nobody worries much about ordinary domestic hob burners being dangerous. They've only recently needed to have monitoring to turn the gas off if the flame goes out. (Thermocouple or flame rectification.) We have a few incidents from gas grills, because they fall apart etc and produce CO when the flame's bad.

Interesting that JW also suggested spinel - I can't remember why it's in my brain - something from way back.

I'm still unwilling to think of this as an "arc" situation. If you short the two conductors you get a spark, that's not an arc to me. A rather short one if you must! Parallels with welding are tempting but all that I can think of are flawed.
Yes you can weld at low voltage, yes you can weld at low current, but neither is simple.
For low current you need a high frequency. (As in razor blades).
For low voltage you fundamentally need low source resistance, certainly not a length of resistance wire in the way. You can weld things with about 3-4 car batteries, because the source impedance is really low. You can strike the arc and get a couple of hundred amps, no problem. But like half the forum members might have found when trying to get the prop stand on their motorbike back in one piece, you need GOOD car batteries. The one your dad thought was getting a bit tired but kept for no good reason, is no good to you, because its resistance is too high. You touch the metal, get a fat shower of sparks and some melted metal, a puff of something, the rod sticks to the bike stand and everything gets hot in the wrong places!
Something similar would likely happen in the hob element. As soon as current flows the voltage drop in the resistance wire to the scene of the accident, would kill the spark. For those not used to such considerations, if you have 50 Amps flowing through 5 ohms ( about a quarter of the heating element) you have a 250 volt drop. So the arc would have trouble being maintained, but it would certainly be hot. If some metal melted off, then you might lose continuity and have your voltage back, and hot ionised air/gas which conducts, so the spark happens again, and so on.
I still favour the (water-in-a-?) crack (unseen, underneath) as initiating aetiology. If the heating had occurred within the closed sheath and it had "blown" a hole through the sheath I think there would have been a bang and noticeable swelling of the softened sheath before it "went".
Unless there was a “bang”, Rik? I’ve only changed a couple of dozen of these things and I’ve never looked hard, but I’m pretty sure I’ve seen cracks. Some of the pictures show suspiciously uneven surfaces. As explained previously, we know that elements can survive with the core exposed to water.

Oh – flux stabilises an arc and, primary, keeps oxygen out. They’re quite complex mixtures now, though I’m aware they used to be much simpler, such as plain borax. It could be, fortuitously, that the insulator is something active like borosilicate glass, but I don’t think it needs to be. As above, we don’t need a sustained “arc” for it to get hot, just for something to melt. I have no idea what the “something” would be though again, the melting points of NiChrome and Fe aren’t far off, neither are various non metallic compounds.. The white powder and the crystals certainly don’t look like metal, so that only leaves melted insulator. Some metal/insulator could have oxidised, burned, even evaporated. Being blown around by expanding gases would make it look as though it had evaporated.
I don’t think MgO is a contender – it has a very high temperature melting point, and used because it’s fireproof. If some had melted, wouldn’t there have been a lot of melting of lower melting point materials close by?

This has been a trip down memory lane to a point, I used to know more. It’s always easy to shoot at theories, not so easy to be sure without doing what I used to do – cut it up and get it under a SEM, look for evidence of melting and do an analysis. I wonder what those cost on ebay. Would they mark them as “gift”?

(hadn't seen Rik's post when I hit the "submit" button")

[rjlittlefield]

Unless there was a “bang”, Rik?

Nope. Red glow, increasing in brightness and changing color to blue-white, persisting long enough for me to think "Oh, something in the drip tray is on fire...whoa, that's odd, never seen that color in a drip tray fire before...slide pot so I can see what's happening...whoa! that's coming from the burner...looks like an arc welder...need to turn off the power...oh good, that worked OK, whew!". Certainly the thing was blowing blue-white for several seconds, 'cuz I don't catch on super fast to something completely unexpected.

Now, here's some further evidence that seems relevant.

First off, there are holes melted in the element in two places far apart. I've marked their locations in this photo:



Then I had the idea that I might go probing for connectivity using an ohmmeter and a steel drill. That worked great on the other sacrificed element as a test, so I applied it to the Dragon. Here's what I found. Arrows mark the blown-out holes, boxes are probe spots -- red connected to the red contact, green to the green contact, and black to neither:



So now we have connectivity data consistent with some destructive process that wiped out 12 inches of nichrome around 1-1/4 turns of the element, and a personal observation that the process went on apparently uninterrupted for several seconds. That's rather longer in both senses than anything that I'd like to call a "spark".

Before I hack the thing completely apart so we can see more of what that disconnected section looks like inside, would you care to predict what we'll find? (Hint: I've now spent some time peering into those through-holes, and I'm really confused! )

--Rik

[ChrisR]

A series of erratic spark-heat-melt-spark cycles is what I was suggesting.

Curious.
Is one end of the ring near "ground" potential and the other "live" or "line" or "hot" or whatever the US conventional name would be? Would that be the inner end?
In the UK, Red would tend to mean Live and green Earth, but I know Ireland is very different! (Black is Live I hear
Is this 220VAC 60Hz? I'm not sure what the US uses. Canadians use two voltages I believe.
Seeing the whole ring, that looks like what, about a seventh of the whole length from the green end of the curve to the "mouth". Depends whether the wire goes up the radial tube.
2.5kW (8500Btu/hr) ring? So you could have perhaps in the order of 80 Amps, while the breaker was having a think. (Assuming the live end is the short distance one.)
( Our breakers can be quite slow for such relatively small overloads - though we'd only use 30A for a cooker, 50 at 200+V is really too high!)
Interesting that there's more than one point of incident - probably some hidden in the tube too? Suggests time for heating - if one "short" had established it would have considerably reduced the voltage at other sites because of the resistance drops, and the spark/arc hopped back up the line towards the voltage source. That doesn't predict where the gasses would have shot out though - path of least resistance.
Is there another set of wobbles in the sheath here?


Inside the tube I expect there's a bit of a mess. The heating wire might have melted through, in sections. Perhaps only one of the two would be conducting at a particular time. Thinking of what happens when a welder strikes up on dirty metal, there's a minor explosion "splat" of dirt and metal. Every time I use wire cutters to cut through a live/neutral/earth mains cable I see (after I've recovered) evidence of melted and burned copper and steel. There's always stuff "gone" which would be a spray of metal droplets. We don't need the metal to vaporise to get expansion and distribution, heating air would do that.

So how did this start? I doubt the heater wires suddenly connected with the sheath for no reason. Local heating then seems likely. Crack leading to water ingress (almost inevitable with Rik’s cleaning routine) if there were one, or just oxidation of the Nichrome due to age, leading to thinning. Both the perforations are at the roughest part of the sheath. It may have been folded into a tube, containing its vital components, then induction welded. It looks like a potentially weak line though. No swelling prior to the hole. Prior cracks found on other ring. A good chance it’s a crack here then.

The appearance of those crystals is not something I’m familiar with. I’ve seen fumaroles and sulphurous steam-vents which look compellingly similar, but that’s misleading really because I didn’t look this close of course. Those I suppose made me think of crystallization from the vapour or water-droplet phase. Accepting that Rik was not errant in his hygiene duties, these formed quickly though. So – solidification from liquid, all over in a couple of seconds? I don’t know any reason why not. I don’t often melt stuff like that...

Whether one is happy with the word "arc" depends I think on a number of factors, but the term does suggest a continuity and stability.
A spark is a breakdown in a dielectric (eg lightning is referred to as a spark, not an arc).
Arc is perhaps rather flattering for the series of chaotic splats and pauses which I envisage in this event. Perhaps I'll allow a little!

[rjlittlefield]

A series of erratic spark-heat-melt-spark cycles is what I was suggesting.

Could be. I didn't have any high speed instrumentation hooked up. The actual current flow would have been start/stop anyway, this being an AC supply, so I suppose we're haggling over the regularity and frequency, which I really can't speak to. All I can say is that it gave me the same sense of continuity that I used to get when arc welding, back several decades ago when I did that as a youngster.

Is one end of the ring near "ground" potential and the other "live" or "line" or "hot" or whatever the US conventional name would be? Would that be the inner end?

I don't know why they bother to color-code the ends. The system is balanced two phase, "220-240V" means two lines with opposite phase, each 110-120V to ground. Yes, 60 Hz. Our wall outlets are single phase, 110-120V.

Depends whether the wire goes up the radial tube.

Those sections don't get hot.

2.5kW (8500Btu/hr) ring?

Rated as 2600W at 240V, which unexpectedly is what my voltmeter actually reads.

Is there another set of wobbles in the sheath here?

Yes, those correspond to the second hole, shown in my previous post.

Inside the tube I expect there's a bit of a mess. The heating wire might have melted through, in sections. Perhaps only one of the two would be conducting at a particular time.

That's pretty close, I suspect. Peering into the through holes -- the places where there was no conductivity to either end, I see two different conditions. Near each melted hole, the bored holes have rough sides and I can't see nichrome. Farther away from each melted hole, the bored holes have smooth sides and I can see shiny metal where the nichrome should be. So now I'm thinking that the element failed first at the small hole shown in previous post, resulting in a short term overload that caused the element to fail in a second place at the Mouth of the Lair, and at that second place for some unknown reason there sprang up an arc or series of sparks lasting long enough for me to observe and react.

I'm now expecting that when I slice the thing open, I'll find two regions that have been disrupted by electricity, separated by a long section of element that's completely intact except for holes bored through it.

Local heating then seems likely. Crack leading to water ingress (almost inevitable with Rik’s cleaning routine) if there were one, or just oxidation of the Nichrome due to age, leading to thinning.

I agree.

Both the perforations are at the roughest part of the sheath.

In this case I think the roughness is a result, not a cause.

It may have been folded into a tube, containing its vital components, then induction welded. It looks like a potentially weak line though. No swelling prior to the hole. Prior cracks found on other ring. A good chance it’s a crack here then.

Yes, I think that's the construction method, and cracks would be quite likely.

So – solidification from liquid, all over in a couple of seconds? I don’t know any reason why not. I don’t often melt stuff like that...

Could be. Even if some of the stuff came out as a dense gas, it could easily condense to liquid and then freeze, rather than forming crystals directly from gas phase. That would make everything go fast.

--Rik

[DQE]

It would have sure made an interesting close-up video, using for example a DSLR movie mode and perhaps an 100mm macro lens. Probably no video lighting would have been needed?

One has to think and act very fast to do such things in the midst of a nominal crisis, though!