Fun with cookware

[wilheldp_PE]

So in my line of work, I have run into 5 pieces of clad cookware (with a slab of aluminum captured in between different combinations of copper and stainless steel cladding layers on the base of the cooking vessel) that have separated and caused burns or fires. The cookware manufacturer's acknowledge that that this is a possibility, but only if you allow the pot to "boil dry." All of the witness statements accompanying the evidence I have investigated have stated that they did not allow the pot to boil dry. I have also found some other cases online where this phenomenon has occurred with liquid in the vessel.

Yesterday, I stumbled across the following article: http://www.home-ec101.com/the-curious-case-of-the-meltingall-clad-stainless-steel-pan/

The source doesn't exactly sound like the most reliable, but the engineer's explanation seems to make good, logical sense.

The basic premise of electric range operation is to pass current through a resistive conductor thereby transforming electrical energy to heat energy. Another effect produced by this is a rapidly expanding and collapsing magnetic field. This is the source of the magnetic lines of flux, which interact with the non-ferrous metal layer in the pan through the Lorenz effect, to produce Eddy Currents (Note: You may be thinking that Hysteresis losses would play a big part in this, but you’d be wrong). It may be this centralized inductive build up of heat due to the excessive eddy currents in the aluminium layer that is causing temperaturess to exceed 1220 degrees fahrenheit and the eventual catostrophic failure of the cooking vessel.

I am trying to devise a test plan where I can recreate this failure while collecting data that will prove/disprove the theory above. I can monitor the temperature of the heating element, temperature of the cookware, and magnetic field (with a Gaussmeter), but I don't know how to prove that eddy currents are being generated inside of the aluminum slab. I suppose the Lorenze force equation or Faraday's law of induction would prove that it is happening, but I would like some empirical data to prove it.

Any thoughts?

 

[roadwreck]

I thought for sure this topic was going to be about alternative uses of a turkey baster.

 

[Wolverine]

I would go all Mythbusters and set up an experiment that recreates the normal heating element, but then adds a ridiculously large magnetic field, maybe up to 10X or 100X variable (or 1000X!) normal measurements, and go till it blows (kind if like the time they connected a high power motor to stainless-steel ceiling fan blades and tried to decapitate Buster the Dummy) [failed]

First idea is to stack a stove element and an EMF generating coil or transformer. Might have to switch it on and off to get the effect. Better yet, switch it on/off with reversed polarity. Sounds like a good senior project challenge for a local college student.

Sorry I can't answer your real question which is how do you measure eddy currents. Sounds complicated.

 

[mudpuppy]

Given aluminum's relatively low resistivty, I would expect the stainless layer to give out due to eddy currents before the aluminum layer did. This is a well-known effect in large generators, where the iron parts overheat and melt due to eddy currents long before the copper windings do.

To answer your question--a current requires a voltage drop, right? So you could try measuring the drop over a bunch of different points across the cookware. Sounds pretty tedious, though, since you'd have to check a whole lot of point pairs. And you'd also have to measure the resistance between each point pair to calculate the current.

I'm doubting the whole idea though. I got a piece of steel rebar to melt in the lab, but it took several minutes of 200 A to do it. I don't think you're going to anywhere close to that since the primary winding (the heating element) current is much lower. On the other hand, the layer of steel in the cookware is much smaller than a rod of rebar.

Interesting theory.

 

[mudpuppy]

If only there was a way to see electricity it would be easy.

 

[Supe]

I would talk to Olympus, QMI, or any one of the NDE firms that does ECT. Eddy current testing is a pretty common practice in the welding world (typically for checking tube bundles in vessels), so I'm sure they could offer some advice or provide equipment recommendations.

 

[wilheldp_PE]

Given aluminum's relatively low resistivty, I would expect the stainless layer to give out due to eddy currents before the aluminum layer did. This is a well-known effect in large generators, where the iron parts overheat and melt due to eddy currents long before the copper windings do.

To answer your question--a current requires a voltage drop, right? So you could try measuring the drop over a bunch of different points across the cookware. Sounds pretty tedious, though, since you'd have to check a whole lot of point pairs. And you'd also have to measure the resistance between each point pair to calculate the current.

I'm doubting the whole idea though. I got a piece of steel rebar to melt in the lab, but it took several minutes of 200 A to do it. I don't think you're going to anywhere close to that since the primary winding (the heating element) current is much lower. On the other hand, the layer of steel in the cookware is much smaller than a rod of rebar.

Interesting theory.

It's doubly difficult, because I need to have the pot/pan resting on the heating element while monitoring the voltage. This also negates Supe's recommendation of the ECT firms because they require the test surface to be exposed. Also, the pot needs to have water in it or the test will just prove the manufacturer's warnings correct.

I think the theory is that the resistive heating element is putting a lot of heat into the pot (usually maxes out around 500 to 700 degrees F), then the eddy currents add enough extra heat to melt the aluminum (1,220 degrees F or lower, depending on the alloy). I'm not completely sold on the theory, but there is a ton of anecdotal evidence that this phenomenon occurs.

I do plan on using an induction cooktop as the worst case scenario since it operates on the eddy current principle to produce all of its heat (I think). There are cookware restrictions on induction cooktops, and this type of failure may be one reason for those restrictions.

 

[pbrme]

In recreating the scenario, I wouldn't use just the induction cooktop, but add resistive and gas for your analysis (to cover the full consumer spectrum and prove a baseline comparison). The restrictions on domestic induction cooktops stem from their frequency limitations which effect the output efficiencies on cookware material. The instruction with most domestic stoves tell you to only use magnetic (ferrous content) pans, in fact some even have a magnet sensor as a safety feature and will only turn on with a steel/iron pan present due to material inefficiencies.

Items I would consider: look at the skin effect and it's correlations on cladding capacitance. Capacitance is greatest when there is a narrow separation between large areas of conduction (the exact setting you have with this pan). I would also consider the effects of pan movement through the induction field (as in High heat stir fry action) by the cook, which could be creating an extra induced current.

However, you've got quite the challenge on your hands. I'm mechanical, but ohm's law comes to mind. There could be a potential difference (voltage) across the conductors (cladding) this could be measure as stated above, then you have resistance. This could be measured separately by dissecting a sacrificial pan, but in mechanical terms, comparing thermal conductivity (over measured distance) with electrical resistance might prove useful. As supe indicated, Olympus makes an Eddy Current cladding thickness meter, and thermal cond. is in tables.

 

[Master slacker]

Use a high speed camera. I've heard that you can detect Eddy currents if you look at the experiment through a one-inch cube of red Jello, too.

 

[wilheldp_PE]

I plan to run the test on 3 identical pots on gas, electric, and induction cooktops. I was also going to have one with water in it and one without on each type of cooktop, but I think it's been proven that the aluminum will melt consistently when boiled dry and given enough time.

This is making my brain hurt. Emag waves and fields were the parts of Physics III that I sucked the most at.

 

[envirotex]

What about just a good oldfashioned defect in the pan? Maybe there could just be impurities in the alloy for a particular pan that causes it to overheat...Or even a mechanical defect in the cladding introduced in the manufacturing process or by the end user...Out of 10,000,000 pans produced there couldn't be 10 that are defective?

 

[Dexman PE PMP]

I still fail to see the "fun" part of this conversation...

I was expecting something more like this:

or this:

 

[wilheldp_PE]

Spraying molten aluminum around the lab isn't fun for you? It is for me.

 

[Dexman PE PMP]

Reading about someone spraying molten aluminum is not fun. Doing it is another story...

 

[pbrme]

he's a civil eng. what do you expect.

Just don't get any on ya... and if you do, make sure to follow the scrub down procedures.