Potters & Sculptors - Making Rock from Mud
I am working with an Orten controller on my electric kiln. In spite of low use (<30 firings) my thermocouple doesn't appear to read very accurately. The automatic cone fire programs consistently overfire.
I am trying to get the adjustments just right to get the combination of a good soak, slow cool and actually hitting cone 6.
My teacher really recommends I soak for 30 minutes and cool at 90 degrees and hour. If I set my controller to fire cone 6 with these parameters it will flatten a 7 cone flat as a pancake not to mention flattening all of my colors along with it.
I fiddled with a few firings and found a cone 4, slow ramp, with 30 minute soak, Thermocouple offset of -10 degrees and 90 degree cool down would put a cone 5 over and leave my cone 6 at about 2:30. Colors looked pretty good but my teacher is telling me the clay body feels underfired (using a cone 6 stoneware). I would like to hit the cone 6 on the nose but not sure which factor to tweak. My controller manual isn't much help since it pretty much only gives the conversion chart for what looks like a no soak firing and my temp is unreliable.
First, type K thermocouples are not all that accurate, especially as they get hotter. We use them because they are cheap and generally work well enough for the degree of accuracy that potters need. So there will always be some level of inaccuracy in firing with digital kilns. And just when you've got it perfect, your thermocouple will fry out and have to be replaced, and then it might have to be calibrated again. Standard range of error in a Type K is about 0.75%, which at the high end of its recommended use comes out to 17 degrees plus or minus. That's 34 degrees total, which it almost an entire cone. If you want better accuracy you'll have to switch to a Type S thermocouple, which is made to function at higher temperatures, with a range of error as low as 0.1%. They cost a lot more, though. And if you want even firings top to bottom, you have to really watch how the kiln is loaded, or get a kiln with zone control.
Second, no digital controller will be as accurate as cones. This is one of those rare areas where computers do not give of greater accuracy. Cones respond to actual heat work, while computers run a bunch of algorithms to predict heat work based on ramping speed and temperature. If you want 100% accuracy, use cones with every firing and turn the kiln off manually when you see the cone drop. Otherwise, you have to accept some variation. I think 1/2 cone is acceptable. Any well formulated glaze can deal with that much variation.
Third, adding a hold time to a firing increases heat work, which has the same effect as firing hotter. So if you fire to cone 6 then hold for 1/2 hour, you're over firing your cone 6 glazes. Depending on the kiln and temperature range in which you are firing, anywhere from 20 to 40 minutes of hold time equals one cone higher. Personally, I think firing to a lower cone with an added hold time to achieve the desired cone does wonderful things for glazes, and also helps to even out firings. To figure out how much hold time you need, you'll have to do a firing with visual cones in the kiln, set the computer to hold for an hour at the end of the firing, then watch the cones to see how long it actually takes for the cones to drop. When I was firing cone 8, I programmed cone 6 with a 40 minute hold to achieve cone 8.
I don't suppose $240 platinum type-S thermocouples have a particularly long life?
We might be able to swing a purchase if they last longer, but I suppose not.
I've also noticed that firing to a lower cone, with time added to achieve a higher cone heat work, gives glazes more time to out-gas and create a more ^10 look. Not as important as slow-cooling, but valuable.
There will be the odd interaction between a glaze and clay where a prolonged hold can create more pin-holing, but I noticed this primarily on fritted glazes used on a white ^10 fired to only ^6.
Yes, they actually do last a lot longer. They are contained inside a ceramic tube & away from fumes & chemicals, so barring breakage from a shelf or pot, they will last a good long time & be very accurate. I have them in both my kilns & will never go back to Type K. jhp
Jeff, thanks for the advice. I talked with Paragon Kilns technical support and they said the same thing. The accuracy on the K is around 2% but at cone 6 that is 40+/- degrees. The S type is 0.1% which is around 2 degrees /-. With a ceramic protection tube Paragon said the thermocouple should last a long time - the biggest danger is hitting it while loading or unloading.
I ordered an S type and hope this is a cheap $250 investment given the number of refires and disappointments I have had. My wife likes to use a float glaze that is very sensitive to the temperature and when loads vary in results it means a lot of pots get given away. I use a cone in every firing and have noticed the same program shifting from 3 o'clock to 6 o'clock to almost laying down. I am hoping this is the thermocouple.
I did slap an ohm meter on the coils to verify their integrity - your kiln manufacturer can give you the target ohm readings for each size coil, simple matter to drop the cover box and test them. Doesn't require any electrical knowledge, just buy a $12 ohm/voltage meter, set the switch on ohms and (with the power unplugged) read the two terminals where the coil attaches. Took longer to watch the how to video on Paragons web site than to make the actual readings.
I have inserted three K type thermocouples into my test kiln and measured the output at about 1900F at the end leads with a voltmeter. The difference in voltage between the three amounted to around 200F! Theoretically it should have been far less than this.
So I believe the best you can hope for is to reproduce the ramp that worked for you and never mind the temperature reading. As the thermocouple ages, even this will begin to drift. Watch the witness cones. They probably tell the tale better than the thermocouple.
Remember, all the chips, thermocouples and electronics in the controller are probably made in China.
Follow up - after replacing my K type thermocouple I have run several test firings using 5/6/7 cone packs on every shelf. I can say my new S type thermocouple appears to be providing the controller with very accurate readings all the way up to cone 6. I have even been able to watch cones 5 and 6 bend during firing right on schedule with their rated temperatures.
With this level of reliability I have been able to adjust my temperature + soak time to get a very fine tuned exact cone 6 with a perfect bend on what I am hoping will be a very long term repeatable basis. I can certainly tell the difference in my glazes already.
Messing with the K type thermocouple was a false economy when I consider the number of overfired or underfired rejects I ended up with. I don't think my thermocouple just drifted over time, I now believe it read inconsistently from one firing to the next. anyway, never going back.
I have gotten relatively consistent firings with an Oten controller and a K thermocouple. I never pay much attention to the temperature reading, I go for a consistent firing based on behavior of cones. Once I have the ramp that gives me what I want as far as the witness cones, I leave it there and don't mess with it. The placement of the thermocouple is all important. Measure from the wall and keep it within 1/8 of an inch of where you had it for the good ramp. If you move it, the temperature reading will change because it is always measuring both kiln temperature and radiant heat from the elements.
Sorry you had to go to an "$" (get it?) type. I have never used them but I believe the same issues apply.
I have thermocouple envy.
Type K's are down hill from the first firing. The metal immediately starts to deform, oxidize and corrode from the get-go. They are not accurate at all in the upper temperature ranges. S-type, being made from platinum & rhodium are extremely stable at very high temperatures & having a protection tube are kept from harmful kiln fumes. I'll never go back after after seeing the accuracy of an "S". About the only thing K's would be good for is if you did nothing but bisque in a kiln.. jhp
I fire hundreds of pots every month to cone 6 with my type K thermocouples, and get very consistent, reliable results. Once you get them dialed in, they work very well. Each of my kilns took one small adjustment to get them just right, and I haven't made a change in years, even when I have to switch them out. And type S aren't the only ones with protection tubes. You can put a protection tube on any thermocouple. They're standard equipment on L&L kilns. My type K's last about as long as my elements, which means they last 14 to 24 months, depending on how often I fire that kiln. And at 1/18 the price of type S, I think they're a worthwhile investment. I see far more problems from using single-zone systems than I do from using type K thermocouples.
Let me clarify. I won't use a type k for anything but bisque. Yes, you can put a protection tube on any thermocouple, be it ceramic or pyrocil, but the TC will still deform & NOT be accurate the closer you get to ^10. Add on any long holds and it will quickly fail. The $15 cheapies on e-bay are even worse. Had to go through several to find one that was somewhat accurate & I had one that never did work. BTW, the last S I bought to retro-fit my Skutt 818 was $175. Unless I do something really stupid, it will outlast the kiln. All I'm saying is that for glazes that need accuracy(crystalline, etc.) the price of an S - TC is a small price to pay compared to useless doorstops. jhp
Tidings of great joy! Our 30 month-old Type K thermocouple has failed.
Bartlett says we can use a Type S or Type R in addition to the Type K and Type N.
Jeff and Brent, where did you purchase your Type-S thermocouples?
Is Type S more accurate than Type R?
Is there anything special we need to specify?
I could not have been more happy this morning to see a ^6 firing fail with an ERR-02, followed by FAIL and random temperature readings.