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PotSherder

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  1. What I was trying to say is that since only total watts matter for peak temperature, not watts per element, there is no reason to even try the 2-element test, and I can stick with the current 4-element design for even heating. (And I have to look elsewhere to get the peak heat I need, either plugging the lid leak or adding more insulation.) In the now-abandoned 2-element approach, I would have of course controlled the element voltage to have the same total power input. If I hadn't, halving the load resistance at the same voltage would have doubled the total power (for a few milliseconds until the breaker tripped). Power is voltage squared over resistance, so I'd have had to cut the voltage to about 70.7%. Each element is about 1 ohm and the original voltage was about 100 V, yielding power of 10000 / 4 = 2500 watts. With 70.7 V and 2 ohms it's about 5000 / 2, also 2500 watts..
  2. After a lot more thinking (always dangerous!) I no longer believe the number of elements has any bearing on the ultimate kiln temperature, except regarding temperature distribution. It's all down to watts, as others suggested. My claim that the max temperature couldn't exceed the element temperature was true, but spreading the watts over more elements shouldn't make any particular difference. The reason is that the *total* power is what is heating the chamber, and the temperature of the chamber will keep increasing until the losses balance the power input. The added elements may start off cooler at the early stages when convection is important, but as things heat up and shift toward radiant, the element can't get rid of excess heat until it is hotter than the walls... heat only flows from hotter to colder. So it will keep heating up until its temperature balances the wall losses, no matter how low its individual wattage, since the total watts are heating the walls. The formula I use is Temperature = (Watts * wall thickness) / (wall area * thermal conductance) I use meters and meters^2 for thickness and area, and W/(m * K) for conductance, and since we are talking about temperature change relative to ambient, it's OK to use Celsius instead of Kelvin since the degrees are the same size. The trick is dealing with the fact that some of the thickness is IFB and some is fiber, with different conductances. I've used approximations instead of anything fancier, which may be one factor in my calculation coming up with 1300 C but actually getting 1150 from the kiln, though in the past it has been pretty close. So it looks like I'm back where I started, with fixing lid leaks versus extra insulation. Of course we *know* it has to be insulation, since that's the most work! <g>
  3. No, I don't test with a full load, just a couple of test bars sitting on the floor. I don't have the exact peak watts at hand, but it's around 2500 or so. I called off the test at 1152 C, which was a rise of only 13 C from the prior hour (1139). At turn-off the kiln had been at max for 4 hours , starting at 1039 C. Amps were steady at 24.5 through the elements (all in series). I don't have the tap voltage at hand, but it's somewhere just over 100 V. I'm an old hand at dimmers (electrical engineer since 1971), and actually have a homebuilt job on the output of the transformer. I normally keep it all the way on to prevent RF noise. I haven't played around with PID controllers, though I'm familiar with the theory. If they have some sort of PWM output that could drive an SSR, that would be perfect. But first I have to get the kiln working right! The 4 rods run fore and aft, near the corners of the front face (not too close to the walls, as they don't like that!), with the intention of maximizing unobstructed view. When I cut back to only two elements for my test, it will be the 2 at the bottom since that's easiest to re-wire. If putting the same peak watts through only 2 elements allows me to go above 1200 C, then I can consider changing their placement at my leisure. I chose silicon carbide not only because I was already familiar and had the transformer and stuff, but also because they are simpler to build into a rectangular kiln that uses ordinary rectangular IFB. I was not interested in trying to carve wire element channels into brick faces. Might have been simpler to buy normal electric kiln bricks (or an old kiln and rewire it), and build my own metal jacket big enough to hold the extra insulation. But that would mean a lot of fancy sheet metal bending, compared to rectangular.
  4. Yes, to control the rate of rise. You are correct that the elements could go to full power more quickly without harm to the elements... it's the pots that I'd be concerned about in that case! So I'm manually doing the firing curve for now. What silicon carbide doesn't like is full power until a target temp, full off until below, full on, etc. Electronic controllers for silicon carbide are like lamp dimmers, in that they control the average power by only switching on for a fraction of the AC cycle. My understanding is that for maximum element life it's even better to chop the cycle even finer, so that instead of getting small bursts once or twice per cycle you get multiple narrower pulses. I probably won't go to that much trouble, since I expect the elements to outlast me anyway!
  5. Almost as soon as I had sent my last post, I had a 'duh' head-smack moment. As noted in my original post, "in theory, with enough insulation you could reach any cone if you wanted to wait long enough, even with a wimpy heat source, as long as the element temperature is above the cone temperature". It (finally) dawned on me that using 4 elements (for more-even heating) meant that the 2000 watts was being emitted from twice the surface area of my old 2-element designs. That *has* to mean a lower element temperature. (Or just consider that each element is getting only half the watts.) I'll have to crack open my dusty old undergrad "Heat Transfer" book and root around for the proper equation, or do some Googling, to get a computed temperature value. But it's pretty simple to just move a couple of cables to omit two elements. I'm betting that will solve my problem without any lid caulk nonsense. Just FYI, the elements are driven by a massive old transformer with multiple taps, along with a control panel with an ammeter and big Coarse and Fine contactor knobs, all stripped from an ancient Burrell industrial test furnace circa 1940s vintage. For those not familiar with silicon carbide elements, they don't like the full-on, full-off of the standard "infinity" controls typically used with wire elements. You have to bring the power up more smoothly... hence the manual control knobs. I watch the thermocouple readout and adjust as needed every hour... a colossal pain. When I get everything working, the plan is to buy or build an electronic controller (essentially a giant lamp-dimmer) that can be automated. So everyone who smelled something fishy about my whole project, your nose was right on! Again, many thanks to all who responded.
  6. Thanks to all that have responded. To be clear, this caulk idea is for a one-shot test to see if it gives a higher peak temperature. (Yes, the 1150 C was measured. My calculations showed around 1300 C.) I am familiar with the 0.6 cu.ft. test kilns, but notice that's about the biggest to be found. That indicates to me that maybe it's not as easy to go bigger as I (and some of you) assumed. My kiln now has 4 layers of 1" 8# fiber packed into 3.5" between the IFB and the outer sheet-metal shell. The IFB is edgewise so only 2.5" thick. Heating elements are 4 silicon carbide rods custom ordered for this job. What I'm trying to determine by my quick-and-dirty caulk idea is just where my losses are actually coming from. If it's not the lid, then I need to enlarge the shell to handle more fiber... not so easy! I considered trying to use fiber for this test, but although it does have magical insulation properties, I'm not convinced I could get a good seal... which is a slightly different matter. Not to mention that it would require the "closed" position of the lid to be increased to allow for the fiber thickness, a cumbersome adjustment with my simple hinge system that I am hoping to avoid with caulk that just squishes out to fit. The sort of wadding used to seal wood kilns was my inspiration for the caulk idea, but that (as far as I know) is applied to the outside of the sealing joint, where it is easy to remove later. Dunno how it would be when compressed between two surfaces and fired into place... considering the surface area involved, it wouldn't take very much tensile strength to bond the lid to the body so well that I couldn't lift it without damage to something. I guess I'll have to run some tests with various compositions of refractory materials plus organic burn-out binders. I'll report back here. (Might take a while.) Thanks again!
  7. Hello everyone! New to the forum and not at all sure this is the right group to post to, but here goes: I'm building a one-off electric kiln, the goal of which is to get about 1 cubic foot of interior, reach at least cone 6, but still run on household 120VAC. I took this on not only because of my personal need, but also as an engineering challenge since it seems to be in the "can't be done" category. The idea was to use *lots* of insulation, since in theory with enough insulation you could reach any cone if you wanted to wait long enough, even with a wimpy heat source, as long as the element temperature is above the cone temperature (say, 1200 C for cone 6). So far, I can only get to about 1150 C (about cone 3). My calculations could be off, of course, but they've been pretty good on prior projects. One possible problem is that the calculations are based on zero leakage, and the most likely culprit there is the lid seal. I know this isn't as good as I'd like it to be, but fixing it is going to take a lot of work due to the design. I'd like to be certain that lid leakage really *is* the problem before tackling that, so my idea is to run a test where the lid is "caulked" to get a perfect seal. Due to the nature of the extra lid/body insulation, I can't apply this caulk around the edge after the lid is closed; I will need to run a bead around the top edge of the kiln body, and have it squish into place when the lid comes down onto it. So the question is what I can use that will be squishy enough to easily extrude and fill irregularities a the lid/body interface, yet not stick the lid and body together, so I can still open the lid after the test. Since it will take a couple of minutes to run the bead around the edge, the caulk has to resist drying out from having its moisture sucked out due to contact with the surface (IFB with kiln wash over it), so it can still squish properly when the lid comes down. However, my sense it that the goopier I make the caulk, to keep it squishy long enough, the worse it will stick after being fired. I'm thinking of something that will pretty much turn to crumbly 'sand' after firing, like maybe flint or grog or alumina or ? mixed up with an organic binder that will make it squishy to apply, but burn out later. Library paste? Flour and water? (I will of course run tests on IFB scraps before risking the kiln getting stuck shut.) Many thanks for any help, including where to ask if this isn't the right place!
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