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A new scorchprint experiment

Politeness Dec 19, 2010 12:46 PM

Questions have been raised about cast iron cookware, whether it can heat evenly or always exhibits hot spots.

Questions have been raised about induction energy sources, whether they are capable of supplying even heat on the insides (the food side) of vessels used on induction cooktops and ranges.

This post will not provide a definitive answer to either question. However, it offers a data point or two that may advance the knowledge base.

The test instruments were our LG LCE30845 induction cooktop and a Nambutetsu (Iwachu brand) pot, a bail-handle cauldron that is about 8" in diameter at the top rim, and has a rounded bottom. We previously have posted photos of the same pot on Chowhound. http://www.chow.com/photos/318806 The rounded bottom of the pot does not actually touch the cooktop, as the pot has three nub feet on which it rests, leaving the bottom of the pot maybe one millimeter above the surface. This pot has worked well as a cooking vessel in the past, not only on induction, but on a ribbon radiant Ceran-top cooking surface. Our sister-in-law in Japan has the same pot, and there she cooks with it on a gas range.

In common with cutipie721 as posted at http://chowhound.chow.com/topics/7418... we have only politically correct, environmentally friendly, unbleached, and therefore natural ecru, parchment paper in our household. So we attempted the "Arnold method" of sprinkling flour in the pot for the scorch test. This introduced some complications to the test, as will be discussed below.

It has been a very long time since we did any baking in which the recipe called for sifted flour, and along the way, our flour sifter has got misplaced. I looked for it, but could not find it. So I attempted manually to sprinkle flour evenly across the bottom of the pot, not very successfully. To even out the distribution of the flour, I used the bottom of a large soup ladle to spread out the flour, which made the thickness of the flour look pretty even in the fairly bright light of our kitchen, but which proved to be the source of the most striking feature of the test.

The four photos below (if they get uploaded properly) show the flour at the bottom of the pot in various stages of heating. In the first photo, taken before the induction burner was turned on, the instantaneous illumination from the camera's flash, combined with the format's inherent contrast, show much more clearly than the naked eye saw with fairly bright ambient illumination the variations in thickness of the flour layer at the bottom of the pot. The dark areas at 3:00 to 7:00, at 9:00 to 10:00, and at 12:00, are nearly vertical sidewalls of the pot that either never were touched with flour at all or from which the flour slid right down toward the floor of the pot. There is a ridge of fairly thick concentration of flour from about 11:00 toward the center of the pot, and a valley of very thin coverage just inside the bare spot on the walls at 9:00 to 10:00. There is a thick concentration around the edge of the floor of the pot from about 11:00 to 3:00, and again, around 7:00, with fair dollops of flour closer to the center at about 2:30 and 7:00.

The second photo shows the pot as the flour just began to scorch; it also shows that I had failed to center the pot on the burner. There is a very even circle of scorch, about 7" in diameter, but an unscorched crescent near the sidewall at the bottom of the photo, where the pot was beyond the burner's circle (on the cooktop); the width of the crescent is slightly exaggerated by the inverted keystoning caused by perspective distortion with close-up photography. Within the browning 7" circle, the degree of browning is completely congruent with, and inversely proportional to, the thickness of the layer of flour. From this, I derive the conclusion that flour is a lousy heat conductor, and it would be unwise to construct cooking vessels entirely of flour.

The third photo shows the pot after the flour had got pretty deeply browned. The 7" evenly browned circle is even more evident in the third photo, as is the fact that the unevenness in the degree of browning on the floor of the pot coincides pretty much exactly with the thickness of the layer of flour being browned: the thicker the layer of flour, the whiter, and he thinner the layer of flour, the deeper brown. The flour piled up along the vertical sides of the pot has barely browned at all, suggesting that there had been little heat conduction up the sides of the pot.

The fourth photo is a closer photo of just the left center of the floor of the pot, in an area where the elevation gradient between a thin layer of flour and an adjacent much thicker layer of flour is steepest. At the upper left of the fourth photo, you can even see a bit of the cast iron surface of the pot itself peeking through the very thin coating of flour at that spot. Were this a photo of the surface of Mars, we might be speculating whether that was snow on the mountain peaks.

There you have it. Another data point.

 
 
 
 
  1. paulj Jan 12, 2011 12:24 PM

    My induction hotplate just died (with a pop and smoke). It's 5 yrs old, so I am not overly heartbroken about it. I took it apart, and found that one of the large capacitors connected to the main coil had blown. I may be able to replace it. Whether I can fix it or not, I am looking at replacements.

    But in the process I got a full view of the induction coil. It is a ring more than 2" thick, with and inner diameter less than an 1 1/2", and outer nearly 7". Given the bubble patterns on low conductivity pans I expected a narrower ring (e.g. 4-5" diameter, 1" wide). Apparently the greatest heat is generated near the middle of the coil.

    3 Replies
    1. re: paulj
      k
      kaleokahu Jan 12, 2011 01:06 PM

      paul: RIP, hotplate. Yep, 5 years is what sellers have told me to expect. If you get her fixed, do a "print"?

      1. re: kaleokahu
        paulj Jan 13, 2011 12:20 PM

        The capacitor that blew is similar to ones that often have to be replaced in ceiling fans.

        1. re: paulj
          k
          kaleokahu Jan 13, 2011 12:50 PM

          paulj: Good news. If you decide to replace it yourself, it may be worth it.

    2. c
      cutipie721 Dec 20, 2010 08:02 PM

      Thanks for the experiment! We just need more people to show that induction is the way to cook!

      1. Caroline1 Dec 19, 2010 11:02 PM

        Day late and a dollar short. I didn't get here much yesterday as a mid-morning micro-power failure wiped out my primary computer, which left me totally bummed out the rest of the day, and I HATE this Dell notebook! It's independently quirky. Not qwerty. Quirky! Anyway....

        An interesting test, Politeness. But it does raise one massive question in my mind: Does the test vessel being raised slightly above the induction burner allow for modifying air circulation that evens out the heat with the bottom of the vessel? Seems to me that air gap would allow heat convection currents to work between the glass cook top surface and the bottom of the pan which could modify and/or "level out" heat distribution. Hey, that may not be a bad thing! Maybe you've discovered a way to modify the problems with cast iron on smooth surface heat sources! So that begs the question of how different would the results have been had a flat bottom cast iron pan that made full contact with the induction burner be? And the third and final question in my mind is how similar would the scorch print be if done with your tri-legged cauldron on a ribbon element glass top electric burner? <sigh> Curious minds are a pain in the donkey! Sorry. '-)

        11 Replies
        1. re: Caroline1
          Politeness Dec 20, 2010 03:49 AM

          Caroline1: "But it does raise one massive question in my mind ..."

          As I think you are aware from my postings in another corner of the Internet, I much prefer that you use an adjective other than "massive" to modify the noun "question," which is a concept that can have no mass.

          "Does the test vessel being raised slightly above the induction burner allow for modifying air circulation that evens out the heat with the bottom of the vessel?"

          I doubt that there is any air circulation worth speaking about underneath the pot.

          "So that begs the question of how different would the results have been had a flat bottom cast iron pan that made full contact with the induction burner be?"

          As a matter of arcane interest, I am pretty sure that the #9 Griswold skillet I use -- which looks like this http://cdn3.ioffer.com/img/item/139/1... -- touches the surface of the cooktop at the rim only, and that the bottom of the main part of the skillet sits a fraction of a millimeter above the cooktop, but not in actual contact with it.

          1. re: Politeness
            Caroline1 Dec 20, 2010 01:48 PM

            hmmmm.... Yes. I tried to think of an adjective to quantify my curiosity but something that was Politeness acceptible did not pop to mind. "Huge" curiosity? I think not. "Big" question? Same problem. But then, I do admit my thinking cap was in sad shape yesterday, the result of my primary computer's deep coma. Maybe "intense curiosity" would have been a better choice. I'm still brain numb. But maybe a better response to todays discussion would simply be, "Honey Bunch, you ain't seen MY curiosity! It's humongous!" '-)

            As to the differences between the open air space under your cauldron and the closed air space under your skillet, unless you are using extremely large atoms and molecules in your house, there is still room for convection currents to modify heat diffusion in both pans. Some of my copper pans have warped bottoms as a result of decades of use. If I set one on my glass cooktop to boil a pot of water for pasta, they will spin as a result of the convection currents in the water and the bottom of the pan providing a single point on which to pirouette. When I try to saute of fry something with minimal oil, there is a marked difference in the browning ability of the copper spot in direct contact with the cooktop's surface and the areas that are not in direct contact but must rely on copper's ability to conduct heat to those heat starved segments. Those distances from copper pan bottom to cook top surface are probably less than you get with your cast iron.

            Oh, dear. I wrote this at least seven hours ago and here it sits. It's time to hit "Post My Reply!" It's been a long hard day of baking baklava and getting fatter by osmosis.

            1. re: Caroline1
              Politeness Dec 20, 2010 03:07 PM

              Caroline1: "As to the differences between the open air space under your cauldron and the closed air space under your skillet, unless you are using extremely large atoms and molecules in your house, there is still room for convection currents to modify heat diffusion in both pans."

              Yehbbut ... the air space between the Ceran surface of our induction cooktop and our little cauldron is nothing compared tp the air space between any old gas burner and the bottom of a cast iron pot being heated by that burner, and in the latter circumstance the gas flame already has set up a mighty convection current in the space, to boot. But kaleokahu has discerned hotspots on cast iron pots heated with gas.

              Moreover, the specific heat of air at room pressure at sea level is nothing compared to the specific heat of a cast iron cooking vessel, so there would need to be a large (ought I call it "massive"?) volume of air passing between the surfaces of the cooktop and the bottom of the pot to have any discernible effect on heat distribution in the pot, certainly enough volume of air to make an audible whoosh, which I have yet to hear.

              1. re: Politeness
                Caroline1 Dec 21, 2010 04:05 AM

                LOL! "An audible whoosh?" Politeness, you've been hanging around those steel mill smelting facilities again, haven't you!

                I think we're comparing apples to oranges to peaches here, i.e. gas, electric resistive, and convection. They ain't the same critter! A pan on a gas burner -- ANY gas burner -- is subject to the quirks and spurts of the whole kitchen, or whatever location the gas stove or hot plate may be. It's why kitchen designers who know what they are doing put larger range hoods over gas stoves than they do over electric, and most importantly over island locations. People just walking by have a passing impact on the moment to moment convection patterns between a gas hob and the pot sitting over it, as does an air conditioner or forced air heater kicking in. A gas burner is part of a very large open system and how it behaves under any given pot will be modified by that environment to a greater or lesser degree, depending on room activity. With both glass surface electric resistive and induction cooking, the picture is changed, especially with flat bottom pans that make direct and full contact with the smooth surface.

                My point is that with both your footed cauldron and enclosed but elevated bottomed griddle, convection currents do set up, but on a more protected and reduced scale, and therefore will not interact with the kitchen's total environment to the degree that the same pans over a gas flame would

                Just sayin'.... To my way of thinking, we're talking undifferentiated "fruit" here, not apples, oranges, and peachy keen induction! '-)

          2. re: Caroline1
            k
            kaleokahu Dec 20, 2010 08:45 AM

            Caroline: Re: Shape of vessel: I initially thought the results might have been more even in Politeness' cauldron than in a flat skillet by virtue of a cauldron's more-hemispherical bottom. This shape, centered on a ring-shaped coil, would tend to put the very bottom further into the ring-shaped field, and the regions directly over the ring further FROM it, thereby tending to lessen cold-spotting in the center and to moderate translation-through scorching directly above the ring. But it sounds to me like Politeness' pot does have a sizeable (7") flat area in it's bottom. If that's not the case, then issues remain.

            1. re: kaleokahu
              Politeness Dec 20, 2010 09:28 AM

              kaleokahu: "... cauldron's more-hemispherical bottom. This shape, centered on a single-ring coil, would tend to put the very bottom further into the ring-shaped field, and the regions directly over the ring further FROM it, thereby tending to lessen cold-spotting in the center and translation-through scorching directly above the ring."

              I think that you are attempting to equate the distance from the inverter wires to distance from the coil in a resistive electric burner. However, the distribution of magnetic lines of force (magnetic lines of force are a virtual representation and do not exist in the real world, but aid in comprehending how magnetism operates) is much more complex than the straightforward radiation of heat from a coil of heated metal. The barest rudiments of an explanation may be found here: http://www.tpub.com/neets/book1/chapt... Of course, in the inverter of an induction cooktop, the magnet that provides the energy is not a bar magnet with poles at the opposite ends of a straight azis, and fixed polarity (orientation); rather the field is created by an electrical current in a wire that is wrapped spirally around a core that most commonly is, in turn, a toroid; and the current reverses rapidly, reversing the polarity every time the current reverses. The magnetic field that such geometry creates is initially complex in and of itself, but the presence of a magnetic object (the pot on the burner) within the magnetic field alters the shape of the field; the shape of the magnetic field is the result of an interactive process between the induction coil and the pot.

              So, where heat radiated from a resistive coil on a conventional electric burner falls off straightforwardly as a function of distance from the source, and the center of a resistive coil, having no heating within it, is cooler than the ring of the coil, those considerations do not carry over to a magnetic induction inverter, despite the visual similarity of a toroidally shaped element through which electricity passes.

              1. re: Politeness
                k
                kaleokahu Dec 20, 2010 09:50 AM

                Politeness: Thank you for taking the time to explain the differences. I knew there were differences, but not salient ones.

                Would you also take the time to explain why the "Arnold" photos seem to be showing scorches corresponding exactly with the location of the underlying coil? Or do you not believe that there is any correspondence at all between the shape of the coil and heat differentials within the pan?

                1. re: kaleokahu
                  Politeness Dec 20, 2010 09:27 PM

                  kaleokahu: "Would you also take the time to explain why the 'Arnold' photos seem to be showing scorches corresponding exactly with the location of the underlying coil? Or do you not believe that there is any correspondence at all between the shape of the coil and heat differentials within the pan?"

                  Our first induction cooktop was a Jenn-Air hybrid, which had two induction cooking areas on the left and two ribbon radiant burners on the right. We did not purchase our Griswold skillet until after we had replaced the Jenn-Air with our current all-induction LG cooktop, but we did use a same-size Lodge cast iron skillet -- already nearly twenty years old when we purchased the Jenn-Air -- during the time we owned the Jenn-Air, and we continued to use that Lodge for a few months after we purchased the LG, until we replaced the Lodge with the (very much superior) Griswold.

                  As we do with the Griswold, we never washed the Lodge thoroughly with soap and water, but just wiped it down after use, and occasionally scrubbed it with dry kosher salt. As a result, the Lodge skillet retained a thin film of grease on its working surface when cold, and that grease could be seen liquifying as the skillet warmed up on the burner.

                  With our former Jenn-Air, we did see a nonuniform, not quite circular, pattern on the surface of the griddle as the cooktop started to heat up, before the entire surface became uniformly shiny. With the LG cooktop, and with the Griswold griddle as well on the LG cooktop, the pattern starts as a fairly large round disk in the center of the griddle, which then expands rapidly to the edges of the griddle. There is never a time when the disk is just a point or a small circle; like Minerva, it emerges fully armed, and only grows stronger from there.

                  Because there were differences between the patterns in the liquifying grease film on the same Lodge griddle when the only procedural difference was the use of different ( Jenn-Air vs. LG) induction cooktops, I conclude that the differences most likely are due to the design and execution of the respective induction inverters on the two cooktops.

                  Arnold conducted his one scorchprint test over an induction energy source using what he described as a "cheap" portable induction burner, and one that he noted in his reporting of the results was too small for the size of the pan he was using for testing.

                  1. re: Politeness
                    paulj Dec 20, 2010 10:15 PM

                    With my induction hotplate it is obvious that there is one induction coil, about 5" in diameter. More expensive induction stoves could very well have multiple concentric coils, and operate the ones that fit the pan. I would expect such a burner to produce a fairly uniform disk of heat, not just a ring.

                    1. re: Politeness
                      k
                      kaleokahu Dec 22, 2010 12:00 PM

                      Politeness: "With the LG cooktop... the pattern starts as a fairly large round disk in the center of the griddle, which then expands rapidly to the edges of the griddle."

                      Yes, you're talking about the pattern in the grease as it liquifies. I understand how even a small gas hob would do this. It would be more useful to know the surface temperatures--a la Athanasius' data--to see if there is as large a differential at the edges as he found (100-200F).

                      I think we are getting to the point where we would agree that a smart consumer would want to know the "design and execution of the respective induction inverters", rather than stare at painted circles on the opaque glass.

                2. re: kaleokahu
                  Caroline1 Dec 20, 2010 01:55 PM

                  With convection, the most critical thing is the material the pan is made of rather than it's shape. One of the things most people love about convection is that it doesn't give a damn if the bottoms of your pans are warped or not, it heats them just the same. See my very late posting I just got off but wrote in response to Politeness some seven hours ago. It covers why I am curious whether convection currents -- mini to micro -- may be at play in the evenness of the cast iron cauldron on peg legs... Politeness thinks not, and Politeness is a very experienced and observant user of convection. But I am always looking for loopholes... '-)

              2. k
                kaleokahu Dec 19, 2010 08:46 PM

                What just happened to the thread? Politeness, please read supra.

                1 Reply
                1. re: kaleokahu
                  Politeness Dec 20, 2010 04:56 AM

                  kaleokahu: "What just happened to the thread? Politeness, please read supra."

                  I very substantially edited my post of Dec 20 4:32 a.m. in light of reading the manual that you linked and to which I referred in my post of 5:29 a.m. Now, the 4:32 a.m. post appears on my computer with the edited changes when I am logged in, but when I log out, it still appears as originally posted, without the edits.

                2. paulj Dec 19, 2010 08:13 PM

                  Something that might improve the flour distribution is to use the baking idea of coating the pan with a solid fat, then sprinkle on the flour, and shake off the excess.

                  1 Reply
                  1. re: paulj
                    Caroline1 Dec 19, 2010 11:08 PM

                    The solid fat would be an uneven layer under the flour and could modify the results. A coating of Pam would be optimal though.

                  2. k
                    kaleokahu Dec 19, 2010 03:23 PM

                    Politeness: Thank you for your work. The browning looks pretty even in the central area of the cauldron. Under enlargement, photo #3 seems to show a slightly cooler area dead center in what I gather is the lowest "slung" area (nearest the cooktop). Out past the browning, it is difficult to discern the vessel's walls. How wide is the unbrowned ring at the walls?

                    Can you post photos showing the size and configuration of the actual coil (beneath the glass) you ran this rest on?

                    It would also be interesting to see what data you'd get if you used a flat-bottomed CI skillet, as most everyone else has used in these tests, rather than a footed, round-bottomed cauldron.

                    Edit: Here is a link to the service manual for this appliance: http://www.appliancefactoryparts.com/.... That manual shows that the burner elements are sealed units, which means that one cannot--without destroying them--see the actual configuration of the induction coil(s) inside. There are also some other interesting things in there concrerning the sensors, the prohibition about aluminum foil, cross-burner problems with helper handles, hot surfaces, flat-bottomed pan use only, stepped (not infinite adjustability) controls, etc.

                    17 Replies
                    1. re: kaleokahu
                      Politeness Dec 19, 2010 04:57 PM

                      keleokahu: " ... a slightly cooler area dead center in what I gather is the lowest "slung" area (nearest the cooktop)."

                      That is the area where the flour layer is the thickest; it is the area in the upper right of the enlarged detail of photo 4.

                      "Can you post photos showing the size and configuration of the actual coil (beneath the glass) you ran this rest on?"

                      In the first post in this thread, I linked to a photo I had uploaded some time back: http://www.chow.com/photos/318806 in which (by coincidence) the same pot is seated on the same burner. (Note: if you click on the "other" photo thumbnailed beneath the featured photo when you visit the link above, the burner in that OTHER photo is a different burner.)

                      "Out past the browning, it is difficult to discern the vessel's walls. How wide is the unbrowned ring at the walls?"

                      As noted, the total width of the pot at its rim is very close to 8". The diameter of the brown scorched circle in the photos is right at 7". If you look at the photo of the pot at the link above, you will see that there is a flare from the bottom of the pot to the rim, or a narrowing of the pot from the rim to the floor of the pot. Because the transition from wall to floor is curved, the point of transition from horizontal to vertical or vice versa can be no better than a guesstimate.

                      "It would also be interesting to see what data you'd get if you used a flat-bottomed CI skillet ... rather than a footed, round-bottomed cauldron."

                      I have a flat-bottomed Griswold skillet on which I fry a couple of eggs almost every morning. It is seasoned to near-perfection (maybe to perfection, but the only way to know that is to overshoot the mark). It is slicker than a greased frog. No way -- even in the service of science -- that am I going to expose the seasoning to trial by flour.

                      1. re: Politeness
                        k
                        kaleokahu Dec 19, 2010 05:30 PM

                        Politeness: Please, spare the Griswold, that's OK.

                        It's just hard to see any inside walls in the photos to scale size. The other thread's pics don't show the inside of the pot.

                        If the cauldron is 8" across at the rim, Pic #3 makes it look like there's about 1.5" of unscorched flour all around the periphery.

                        Which burner of your LCE30845 was this test run on?

                        1. re: kaleokahu
                          Politeness Dec 19, 2010 06:25 PM

                          kaleokahu: "Which burner of your LCE30845 was this test run on?"

                          Left front, one half of the siamesed pair of burners that (with an extra step on turning it on) can be activated (with a third, center, hour-glass-shape element) to make a long oval griddle-ready burner.

                          1. re: Politeness
                            k
                            kaleokahu Dec 19, 2010 07:17 PM

                            Politeness: OK, thanks, that's the 1800W (2400W boost) burner, right? What heat setting did you use, and how long did it take to the point of the scorch in Pic #3?

                            Edit: Now what happened? Another disappeared thread? Hard to respond to something that's not there; my response disappeared before I even posted! And now the link to the service manual is suddenly bad.

                            Anyway, I was going to say that I don't disbelieve your pan worked on your hob, Politeness. I take you at your word. The cribbed manual is kinda a stretch though, don't you think? I actually stumbled onto the manual accidentally, and the .pdf had "Internal Use Only" stamped all over it. Why would they plagiarize for themselves only?

                            Sorry, I'm still not understanding about the photos. I'm REALLY not trying to pick another fight here, but your Pic #3 looks as if it could be of a 5-inch... "warmer area", surrounded by about 1.5 inches of unscorched flour. Maybe it's that it was taken from so close in and the pan is so black. The good news is that the browned circle is pretty uniform in the center, and that's an improvement over Arnold's tests.

                            If you repeat the test, maybe lay a $1 bill in the pan for a shot after it cools down?

                            1. re: kaleokahu
                              Politeness Dec 20, 2010 03:32 AM

                              keleokahu: " ... that's the 1800W (2400W boost) burner, right? What heat setting did you use, and how long did it take to the point of the scorch in Pic #3?"

                              I cannot remember the wattage of each of the burners. The outer ring of that burner printed on the Ceran top has a diameter of 6-7/8", which is an appropriate size for the pot I was using.

                              The burner was set to 4. (four-dot or four and a half) on a 16-setting scale of L-1-2-3-3.-4-4.-5-5.-6-6.-7-7.8-9-P. At that setting, it took 15 to 20 minutes to get from cold through the thickness of the flour layer to the last visible scorch level in the third photograph. If I had been cooking the flour, I should have been stirring the pot, I think, to even out the browning, as there were no convection currents within the flour.

                              "The cribbed manual is kinda a stretch though, don't you think?"

                              No.

                              LG needed a manual to contain some Underwriters Laboratory warnings and other warnings required by law [see note] and disclaimers of warranties. The cheapest way for a Korean company to produce a manual in English on a short deadline is to copy somebody else's manual. There were not many manuals for induction cooktops out when LG introduced the LCE30845 (and still are not many), so (it appears) a manual for a noninduction "glass" top cooktop is the next best thing to copy. The owners manual that accompanied our cooktop -- which differs significantly from the manual that you linked, see EDIT below -- is written in perfect English, but the substantive content of the manual is rife with errors.

                              EDIT: I followed your link to the on-line manual. It is v-e-r-y different from the black-and-white (no color) manual that came with our cooktop. Many of the errors in our manual have been removed, and more relevant information has been substituted and added. See further discussion in another message below.

                              [note] When we go hiking in the Cascades and leave our car at the trailhead, we put a sun shield inside the car's windshield to reflect sunshine back out and keep the temperatures inside the car from going too high. The sun shield, which is totally opaque and covers the entire inside of the windshield, contains a warning, "Remove before driving vehicle."

                              1. re: Politeness
                                k
                                kaleokahu Dec 20, 2010 09:17 AM

                                Politeness: "The outer ring of that burner printed on the Ceran top has a diameter of 6-7/8", which is an appropriate size for the pot I was using."

                                It's your stove and I'm sure you're used to it. The manual says, at p. 8, that the minimum pan size for that burner is 5.75 inches, and that the outer ring is the maximum size (without giving a dimension). If the outer ring is 6.875", that's a pretty narrow range, isn't it?

                                Since you saw the photos of the burner element assemblies themselves in the service manual, do you agree that they are sealed units, and that you can't see the coils themselves (To me it looked like the leftside assembly is racetrack shaped, but it apparently contains at least THREE coils, one for each round and a third for the bridge)?

                                Somewhere I saw a photo of an actual exposed coil--I think THAT'S what we need to know about any given induction burner to tell whether or not it is likely to cause hotspots in use. Without cracking yours open, it sounds like yours is a good one.

                                1. re: kaleokahu
                                  Politeness Dec 20, 2010 09:47 AM

                                  kaleokahu: "Since you saw the photos of the burner element assemblies themselves in the service manual, do you agree that they are sealed units, and that you can't see the coils themselves"

                                  You can see the (grey metal) units on page 37, and the (copper color) coils on page 42.

                                  See my post elsewhere in this thread about the shapes of magnetic fields. If you can discern the shape of the magnetic field from the shape of the inverter, then as they say, you're a better man that I am, Charlie Brown.

                                  1. re: Politeness
                                    k
                                    kaleokahu Dec 20, 2010 10:39 AM

                                    Politeness: "You can see the (grey metal) units on page 37, and the (copper color) coils on page 42."

                                    I think the coils shown on p. 42 are two small transformers that are part of one of many printed circuit boards.

                                    LOL, I'm not saying I can see the field, but I think I can see the effects.

                                    1. re: kaleokahu
                                      Politeness Dec 20, 2010 03:23 PM

                                      kaleokahu: "I think the coils shown on p. 42 are two small transformers that are part of one of many printed circuit boards."

                                      Perhaps. But: (1) they look just like the induction inverters that were in our previous Jenn-Air (which I did see); and (2) no transformers are visible in the exploded view on page 47.

                                  2. re: kaleokahu
                                    Politeness Dec 20, 2010 10:34 AM

                                    kaleokahu: "The manual says, at p. 8, that the minimum pan size for that burner is 5.75 inches, and that the outer ring is the maximum size (without giving a dimension). If the outer ring is 6.875", that's a pretty narrow range, isn't it?"

                                    I never have regarded it that way, but I guess others may come to that conclusion. First, I may have mis-measured that outer ring. On the two right side burners, the outer ring is a double line, the outer line thicker than the inner. On the left side, the thicker outer line is part of the oval that encircles the wntire simesed burner griddle area; my 6-7/8" measurement was of the diameter of the inner outer ring, the outer outer line is a quarter inch further out.

                                    Whenever there is a thread here on recommendations for starter pieces of cookware, somebody is always suggesting a 24-quart stockpot, or saying that you need at LEAST an 8 quart pot to cook pasta for two. (I suspect that these are the same posters who appear on the Gardenweb laundry forum complaining about the number of king-size comforters that can be washed in one load in an automatic washing machine.) We usually cook OUR pasta (for up to four) in a 3.2 quart Demeyere Mussel Pot, which is a hair under 8" in diameter, and thus a tad smaller in diameter than the width of the griddle that LG itself supplies for use on the left side burners. I simply do not grok the need to heat more water than that for that quantity of pasta.

                                    Among the non-frypan, non-griddle, pots and pans that we use on a daily basis, two take 16 cm (6.29") lids and two take 18 cm (7.09") lids, and we use the three smaller burners pretty much interchangably for all four. We use the 11" Griswold skillet only on the largest burner, and its outer ring just coincides with the outer ring printed on the cooktop. We have an even larger Descoware enameled frypan, but its maximum size is at the rim, and the base that sits on the cooktop is less than 11".

                                2. re: kaleokahu
                                  Politeness Dec 20, 2010 04:07 AM

                                  kaleokahu: " ... your Pic #3 looks as if it could be of a 5-inch... "warmer area", surrounded by about 1.5 inches of unscorched flour. Maybe it's that it was taken from so close in and the pan is so black."

                                  Around the rim of the pot you will see a number of "piano key" striations from about 11:00 to about 3:00. The ends toward the pot's center of those "piano keys" define the curved transition from the bottom of the pot to the sidewalls; similarly, the depression with the black dot at about 5:00. The full bottom of the pot is close to 7" in diameter. In the lower left corner, the rim of the pot near 7:00 and 8:00 appears to be flattened (not perfectly circular) -- that is the perspective distortion effect.

                                  1. re: kaleokahu
                                    Politeness Dec 20, 2010 04:29 AM

                                    kaleokahu: "And now the link to the service manual is suddenly bad."

                                    The link is fine. In your post, you placed a period at the end of the link, and if you click on it, the browser tries to resolve the link including the period. Remove the period and reload.

                                    The manual you found is much more comprehensive than the manual that came with our cooktop, and unlike our manual, it contains color illustrations. Photos of the induction units themselves (inside the cooktop) are shown at pages 37 and 42 of the linked manual. Viewing the photos this morning was the first time I have seen what they actually look like, as the Ceran plate never has been removed from our cooktop.

                                    I note, also, that the illustrations that are in our owners manual of using a ruler to check the flatness of the bottoms of pots for suitability for use on the cooktop are missing from the linked manual, although there remains in the linked manual a lot of silliness, such as the admonition to use only LG's supplied griddle on the bridged left side burners.

                                    1. re: Politeness
                                      k
                                      kaleokahu Dec 20, 2010 09:25 AM

                                      Politeness: "...illustrations... of using a ruler to check the flatness... are missing from the linked manual.." Well, I apparently don't have what you have, but the linked manual does have such an illustration, with instructions, at p. 18.

                                      1. re: kaleokahu
                                        Politeness Dec 20, 2010 09:52 AM

                                        You're right; I had gone to the table of contents on page 8, which stated that the operating instructions started on page 23, so skipped right past page 18. The flat-bottom instruction and illustration are still there, and they remain flat-out (pun intended) wrong, wrong, wrong.

                                        1. re: Politeness
                                          k
                                          kaleokahu Dec 20, 2010 01:58 PM

                                          Politeness: Re: rulers and wrong... I can think of 3 reasons (besides plagiarism) why LG might insist on flat-bottomed pans. (1) The detection circuits might interrupt/disable cooking with a pan that rocks on the glass. (2) The physical and heat stress put on the glass from a rocking pan could be such that it would crack. Or (3) A rounded or footed pan could scratch the glass. Obviously only the first would strike the consumer as a "my pan won't work with induction" issue, but if the detection circuits balk, it WON'T work. Have you tried your cauldron on a different cooktop?

                                          I'm interested now in how far "off the deck" the magnetic field will actually cook effectively. Have you ever (besides your floppy experiment) tried to see? I'm imagining stacking sheets of cardboard 'twixt glass and pan and seeing how many it takes before the pan decouples or the heat tails off.

                                          Here's a related impertinent question: If the field is powerful enough to effectively heat a SS liner on one thick pan that is, say, 1cm above the glass, why wouldn't it heat the tines of the fork you use to scramble eggs in another, slightly thinner pan, also at 1cm?

                                          1. re: kaleokahu
                                            Politeness Dec 20, 2010 03:38 PM

                                            kaleokahu: "If the field is powerful enough to effectively heat a SS liner on one thick pan that is, say, 1cm above the glass, why wouldn't it heat the tines of the fork you use to scramble eggs in another, slightly thinner pan, also at 1cm?"

                                            I am not sure whether the induction units would be very effective that high above the top of the Ceran surface of the appliance. However, with several of our thicker disk bottom pots, it certainly has worked at a distances of about one cm above the induction inverter, which is below the glass -- and it is the distance from the inverter, not the Ceran, that affects the strength of the magnetic field.

                                            And the direct answer to your question about the induction cooktop heating magnetic stirring instruments inside the pot is that it does. It has not been a problem with us, because very few of our implements are made of magnetic materials, and those that are do not stay in the pot or pan for more than a few seconds. (We use a silicone spatula for stirring our scrambled eggs.)

                                            1. re: kaleokahu
                                              Politeness Dec 22, 2010 02:27 AM

                                              kaleokahu: "I can think of 3 reasons (besides plagiarism) why LG might insist on flat-bottomed pans. (1) The detection circuits might interrupt/disable cooking with a pan that rocks on the glass."

                                              But the ruler test is not a ROCKING test. The ruler test would rule out (no pun intended) flat-bottom skillets like our #9 Griswold shaped like this http://cdn3.ioffer.com/img/item/139/1... because the ruler would touch only the rim of the skillet and not the central area; but the Griswold sits stably flat and does not rock.

                                              "(2) The physical and heat stress put on the glass from a rocking pan could be such that it would crack."

                                              To make that test, a ruler and the illustration are otiose. To test whether the pot rocks, one need simply set it onto the cooktop, no ruler needed. If the intent is to ensure that the pot does not rock, LG need merely say exactly that.

                                              "Or (3) A rounded or footed pan could scratch the glass."

                                              Any pot or pan has the potential to scratch the surface of the Ceran if it is slid across the surface, as LG warns elsewhere in the manual.. In fact. a ruler-flat pot or pan might increase the risk of scratching because, whereas a very smoothly polished round-bottom pot by itself is unlikely to scratch, any sand or grit (from less than perfectly washed spinach, for instance) that gets trapped between the pot and the surface is more likely to be dragged across the surface by a perfectly flat-bottom pot or pan than by a skillet like the Griswold that mostly sits a fraction of a millimeter above the surface.

                                              IOW, the ruler test is (1) wrong, (2) wrong, and (3) wrong.

                            2. i
                              iyc_nyc Dec 19, 2010 12:57 PM

                              Thanks, Politness. For us non-scientists - or those who are just slow on the uptake (like me..), what are the main takeaways from this? Thanks!

                              2 Replies
                              1. re: iyc_nyc
                                Politeness Dec 19, 2010 02:54 PM

                                iyc_nyc: "... what are the main takeaways from this?"

                                1. King Arthur All-Purpose Flour is a very poor conductor of heat. I seriously doubt that any other brand or kind of flour would show significantly different results under the same test conditions.

                                2. A properly sized induction cooktop burner is capable of inducing very even heat across the entire bottom interior (food side) surface of an appropriately sized cast iron cooking vessel, despite the relatively poor heat conductivity of cast iron compared to silver.

                                1. re: Politeness
                                  i
                                  iyc_nyc Dec 19, 2010 03:53 PM

                                  Thanks- makes total sense.

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