Top of electric kiln too hot

[NoArtist]

My daughter has a skutt KM 1027 electric kiln,  and the top shelf fires too hot. We installed a downdraft vent, but still get significant temperature difference.  All of the eliments measure well within expected range for resistance and current, and the relays were recently replaced.  She is also trying to leave space for convection between shelves,  and it is only modestly packed. This is a used kiln, but seems to have been well cared for.  Is there anything else we should be looking for? 

[liambesaw]

Maybe you can do a Thermocouple offset on the top zone if it's computer controlled.

 

[Bill Kielb]

First comment, convection only plays a part early in the firing. After red heat ( prox. 1000f) it’s radiation, conduction, then a very small part convection. So to keep things as even as practical especially later in the firing, staggered half shelves are generally helpful. Since radiation is the major component of heating, then line of site to the elements will have the largest effect.

The 1027 is a popular kiln so I need to ask some questions which may prompt more questions depending upon you answers.

• Is this a single zone kiln or three zone kiln?
• About how many firings are on the elements?
• Exactly what did the elements meter out to be? 10% change is usually excessive, A 3000 watt element group is probably worn when it goes from 19.2 ohms to 21 ohms. Worn elements generally will cause over firing.
• what type  of glaze Fire did you do? Slow, Medium, Fast? How much time did it take to fire to what cone?

There are ways to offset and balance zones in the control as well as offset overall cones but, experience says if the elements are worn then these offsets will not likely be a long term fix. If the kiln is single zone then thermocouple offsets will not apply to your situation.

Answering the questions above will be helpful

 

[neilestrick]

It's not at all uncommon for single zone kilns to fire unevenly. If find a section is firing too hot, then pack that section tighter with pots so there's more mass there. If you have a cold area, pack that section lighter so there's less mass.

[Hulk]

Top shelf runs cool in my KM1027; staggering the shelves (half shelves) and arranging so there's a tall shelf on at least one side at the top is working for me. 

Hence, shorter gap for your top shelf may help, however, my guess is it could run too cold if the pieces there don't "see" enough element.

Assuming you are gauging heat work with cones, yes?

[Bill Kielb]

12 hours ago, NoArtist said:

My daughter has a skutt KM 1027 electric kiln,  and the top shelf fires too hot. We installed a downdraft vent, but still get significant temperature difference.  All of the eliments measure well within expected range for resistance and current, and the relays were recently replaced.  She is also trying to leave space for convection between shelves,  and it is only modestly packed. This is a used kiln, but seems to have been well cared for.  Is there anything else we should be looking for? 

Well not sure if you have been back to the site but after looking at the Skutt circuit diagrams I finally found their element specs. See below : 

**Note their table is arranged as if you are measuring each set still connected so elements 1 & 2 in parallel measure in your case 13.6 ohms. Also interesting to note,  the top and bottom element sets output more power (watts of heating) than the middle set.

 

 

[Denice]

My old KS1027  Skutt has always had a cool bottom and a hot at the top.  I would just adjust my load with glazes that could handle a little lower temp and put them on a lower shelf and the glazes that could use a little extra heat on the top shelf.     Denice

[neilestrick]

People generally load their kilns opposite of the way they should, with smaller pieces at the bottom and tall pieces at the top. It's simply easier to load the kiln that way. But the bottom is naturally going to run cooler, so you should be packing it looser down there- tall pieces and bowls, things that take up a lot of space but with low mass. Save mugs and cups and things that pack tightly for the top.

[NoArtist]

Ok. Here are some answers to a gew questions.  Yes, this is a single zone kiln. It has about 23 firings on the elements to cone 6 each time. The element resistances are 15.2 ohms for both the top and bottom elements, and 17.5 ohms for tje center element. This is over  a 10 percent change since new, so perhaps that is part of the problem. This is a single phase system, and local power runs around 247 volts measured at the kiln receptacle. The over temperature diagnosis was based both on glaze appearance and witness cones.  I will need to check with my daughter about more details on loading, but I am on the road for the next two days, so I may not be able to reply very reliably.

[neilestrick]

Your elements should not have that much wear from 23 firings. Did you put new elements in when you got the kiln, or are there some firings on them from the previous owner?

[NoArtist]

The 23 firings was a combination of what my daughter has done, combined with what the previous owner reported. But it certainly could be wrong. However,  the interior of the kiln and the elements look pristine.  And the kiln does seem to heat up quickly. The last cone 6 firing completed in just under 9 hours, not including cool down time. I would expect worn elements to take a longer time to get up to speed. 

My daughter is fine with replacing the elements if that is what the problem is. But she really wants to understand the kiln better,  and not just try things that she does not understand. And right now , some of the information seems to be contradictory to her.  

[Bill Kielb]

3 hours ago, NoArtist said:

The 23 firings was a combination of what my daughter has done, combined with what the previous owner reported. But it certainly could be wrong. However,  the interior of the kiln and the elements look pristine.  And the kiln does seem to heat up quickly. The last cone 6 firing completed in just under 9 hours, not including cool down time. I would expect worn elements to take a longer time to get up to speed. 

My daughter is fine with replacing the elements if that is what the problem is. But she really wants to understand the kiln better,  and not just try things that she does not understand. And right now , some of the information seems to be contradictory to her.  

I would agree, 23 firings, especially to cone six would be too soon. Cone ten on the other hand is really tough on electric kilns. Still your measured results are significantly out of spec. Your kiln is rated at 11520 Watts (39283 Btuh) so ordinarily when this figure drops to 90% of its design the rate that it can heat, the losses through the shell vs. the program in the control is too great and over firing begins to occur just because it takes too long. So we would expect at 10368 watts / 90% (35354 Btuh)  or less this kiln can no longer perform as designed  and its usually time for element replacement. Your element measurements equate to 10870 watts (37066 Btuh) which is quite close to our  90% limit so depending upon the accuracy of the meter it would seem to be at or near time for replacement. I would however double check your meter as the measured values are pretty small and verifying this would be prudent. Having looked at one of my most economical meters though, they should be good to the tenth of an ohm.

Nine hours is definitely not excessive ……………….. for a Medium speed cone fire (built in) glaze schedule that is. If you ran the fast glaze cone fire built in program which ends in five hours then this kiln needs elements. You are absolutely correct though that as one learns their normal firing times for normal loads it becomes obvious as firings begin to extend one to two hours longer.  Intuitively knowing this becomes obvious once the user has a good understanding of their typical firing durations in their kiln and a known experienced good starting point to work from. Given some time and more experience she will likely notice things are taking longer and longer very readily.

Over firing - The fact is that during mid and high fire, convection plays a minimal roll in energy distribution. Radiation is king, then conduction through the shelves and losses through the shell then convection. At 2000 degrees the atmosphere is so thin it simply can not contain much heat. Instead, the wares looking at the elements and any radiant body (Shelf / Element) will absorb heat by line of sight, just like walking out into the sun. Hey the earth is heated almost entirely by radiation, go figure!

Air at 80 degrees, 50% RH contains about a little over 31 Btu per pound of air (This is why I added the Btuh above) and take almost 13.9 cu ft. to make one single pound. With respect to  Kilns, radiation is king and losses through the shell are significant  issue. Sections that over fire are often the result of wares that can absorb this radiation longer than in other parts of the kiln and quite often because the elements act too slowly to offset the losses through the shell which means they run longer than anticipated by the controller. This affects the average temperature of the kiln which is pretty much how your controller is trying to determine how much energy has been put into your pots as long as the heating rates remain reasonably as expected. Matters get tougher at the high ending temperatures as maximum shell losses are occurring.

When you consider a mere loss of 1100 watts probably dooms this kiln at cone six it is marginally powered. Now there are reasons power is limited so blaming kiln companies is convenient but they need to work within common power constraints of available electric.

So now that I over complicated this, moving more load to an area of overfiring will place a larger absorption load on the elements (that can be seen by the ware) and can make the hot section heat up slower solving your over fired problem, maybe. Elements that become too out of spec though will need to be replaced as they will eventually not be able to maintain a sufficient rate to offset shell losses that the controller can deal with. 

So my suggestion: Try the loading, maybe that cures you for a bit,  verify elements are worn and replace with those in spec and at that point take note of how long firings take for a fresh new kiln as well as general evenness of firing with ordinary loading. One other thought occurred in the interim, some potters will fire one cone less, so let’s say cone 5 instead of 6 and add a hołd at the end of their firing. So a common one is fire to cone 5 with a 15 minute hold at top temperature usually gets a cone 6  firing or at least a safer cone six firing. You may be able to use this to even things out a bit and vary the hold to get you spot on to your cone or desired glaze look.

One final thought for perspective: your kiln is 7 cubic feet so that's about a half a pound of air - clear space - inside which at 80 degrees gives you about 16 Btuh (4.69 Watts) from the air. The shell losses of your 7 cubic foot kiln at about 2300 F will be about 18000 Btu/hr or about 5300 watts. You still need to heat up all that ware which is the real load. Its easy to see how we quickly run out of spare capacity. And just to consider, heavy 3/4" shelves almost consume as much energy as the wares that are upon them. Lighter shelves save energy and help your elements last longer. Better shell insulation, even better still.

Hopefully this will provide some energy and engineering introspect to this, it really is not a simple issue and in fact is a complicated physics issue. One thing for sure, convection is ok in your oven, but in a kiln, not so significant.

[neilestrick]

9 hours to cone 6 on medium speed is acceptable. We usually see the element coils start to lay over a bit when when they reach replacement age. Double check your ohm readings. Also make sure your thermocouple is still good. It should be after only 23 firings.

Also do a full power test to see if any of the sections seem to heat faster or hotter than the others. Do a Ramp/Hold program with 1 segment, rate of 9999 (full on), to 1000F. Turn it on, and it will send full power to the elements. Dim the lights and crack the lid after a couple of minutes and see how the elements look. The very top and very bottom elements should glow first, and glow hotter than the middle 4 elements. I suppose it's possible that the previous owner installed the elements incorrectly and pout both the hotter elements in the top ring, or installed the wrong elements for that kiln or something. Ideally the top and bottom should glow the same, and the middle 4 should glow the same.

[Bill Kielb]

2 minutes ago, neilestrick said:

9 hours to cone 6 on medium speed is acceptable. We usually see the element coils start to lay over a bit when when they reach replacement age. Double check your ohm readings. Also make sure your thermocouple is still good. It should be after only 23 firings.

Also do a full power test to see if any of the sections seem to heat faster or hotter than the others. Do a Ramp/Hold program with 1 segment, rate of 9999 (full on), to 1000F. Turn it on, and it will send full power to the elements. Dim the lights and crack the lid after a couple of minutes and see how the elements look. The very top and very bottom elements should glow first, and glow hotter than the middle 4 elements. I suppose it's possible that the previous owner installed the elements incorrectly and pout both the hotter elements in the top ring, or installed the wrong elements for that kiln or something. Ideally the top and bottom should glow the same, and the middle 4 should glow the same.

His measurements appear to  confirm they are installed in the correct order

[neilestrick]

1 minute ago, Bill Kielb said:

His measurements appear to  confirm they are installed in the correct order

Good point, however it would still be good to do a visual, to see if they're actually running evenly. If the top section is clearly running hotter then an element change would be in order. The numbers just don't make sense with the firing speed. If they were that far off they shouldn't get to cone 6 that quickly.

[Bill Kielb]

Just now, neilestrick said:

Good point, however it would still be good to do a visual, to see if they're actually running evenly. If the top section is clearly running hotter then an element change would be in order. The numbers just don't make sense with the firing speed. If they were that far off they shouldn't get to cone 6 that quickly.

Yeah, I really don’t know and actually we really don’t know the speed they were trying to fire and we really don’t know how much over ..... He seems determined to sort it out though.

[neilestrick]

4 minutes ago, Bill Kielb said:

Yeah, I really don’t know and actually we really don’t know the speed they were trying to fire and we really don’t know how much over ..... He seems determined to sort it out though.

Even if it was lagging behind on a Fast firing and didn't error out, it still shouldn't get to cone 6 in 9 hours with those numbers. I will say, I've run into elements that read goofy even though they're working fine.

[Bill Kielb]

@neilestrick
Yeah, unknown. I just stick to the physics. His measured elements calc out to about 9% loss so if his no load and fully loaded measured voltage match at 247 volts or close to  he gets more energy yet, which probably lands him into 5% degradation.  It’s too hard from a distance to know all these things.

[NoArtist]

Some very good advice. I am between flights,  so i will need to wait till i have more time to discuss this all with my daughter.  She was firing a preprogramed medium speed cone 6 from the controller.  Also, she was using a high quality calibrated ohm meter, but that does not mean much. If you measure innside the kiln, which she did, it can be difficult to get good connections to the oxidized elements. Next time i will suggest that she ppens the controller box and measure at the relay connections.  With the kiln unplugged probably. 

I am now expecting loading to play the biggest  part in her results. The elements probably are due for replacement,  but there is also more going on here. She will probably need to learn more about how this particular kiln behaves, relative to how she wants to use it.

[NoArtist]

!In Denver now, but still traveling.  Is there any document that describes how to get the most out of an electric kiln?  Particularly how to propery load a single zone kiln to reduce temperature variation? I have looked around a bit, but did not see anything that seems to address that area.

[neilestrick]

24 minutes ago, NoArtist said:

!In Denver now, but still traveling.  Is there any document that describes how to get the most out of an electric kiln?  Particularly how to propery load a single zone kiln to reduce temperature variation? I have looked around a bit, but did not see anything that seems to address that area.

Typically, single zone kilns run hottest in the middle, cooler at the top and bottom. But in general, pack less dense in areas that run cold, more dense in areas that run hot.

[Bill Kielb]

17 hours ago, NoArtist said:

!In Denver now, but still traveling.  Is there any document that describes how to get the most out of an electric kiln?  Particularly how to propery load a single zone kiln to reduce temperature variation? I have looked around a bit, but did not see anything that seems to address that area.

I am not aware of a document as such, other than some generic rules. Sometimes  kiln manufactures will suggest special loading for their particular kilns beyond  shelf spacing and ware spacing from elements, etc....

With respect to single zone kilns, it’s interesting  that not all of them have super hot spots but loading in response to how your particular kiln fires is the wise  and knowledgeable potter thing.

I do have to point out though, in the studio, with brand new elements our three zone mid sized kilns fire pretty darn even regardless of loading. (Reasonable loading)  As the elements wear the top begins to fire hotter than the rest of the kiln. I can actually minimize this by lowering  the allowable relay cycle time which simply allows the relays to turn on faster but also has the effect of bringing the firing speed closer to what it was when elements were new. There is a limit to this as 10 seconds is the minimum cycle time and doing this wears the relays out faster.

These kilns have well over 1000 firings on them each, and we get about 150 firings out of a set of elements. So at the end of element life we need to change our loading, offset our thermocouples or even fire to a lesser cone with a hold. Skilled potters become aware of how their kilns work with their glazes and their wares. Unfortunately this takes a great deal of experience, which of course takes time to learn.

I still believe if your elements are that far out of spec, change them when you can and then learn how that kiln fires new as designed.

Everyone will have good advice and their experience. My experience is in industry, energy transfer,  combustion processes, incineration,  etc... so for years I have struggled with the heat rises / convection idea that exists within the pottery community. It’s a very real concern ...................... in a tunnel kiln, not necessarily in a tiny electric pottery kiln.

My suggestion: There are a lot of good articles to read and all contain some valid points. Apply them as they are appropriate. There are even many complicated articles on industrial process / kilns with lots of research. Most are truly more complicated than potters really need. Change your loading, fire  to one cone less with a hold, experience is your best guide realizing with new  / old elements this could change. Figure out the trend if there is one.

[Chilly]

What they said, 

And get to know your kiln, by using cones on all the shelves, for at least the first few (5?) firings.

[NoArtist]

Thanks again.  All very good advice. We need to spend more time with this kiln. And will probably replace the elements before the next firing.