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pyrometer old?


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#1 Val

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Posted 30 December 2012 - 10:50 AM

I bought a used kiln (skutt) and it seems to consistently fire 1cone below what I program in. Does that mean the pyrometer is getting old? Any other advise?

#2 atanzey

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Posted 30 December 2012 - 03:16 PM

You'll probably get some more educated input than mine, but my L&L has a couple of places where you can tweak how it's computer interprets the pyrometer versus cone values. If you're seeing consistant results, it probably just needs some programming to make it consistantly what you WANT to see.

Alice

#3 bciskepottery

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Posted 30 December 2012 - 04:39 PM

Are you using witness cones to check on the cone/temperature being programmed? Or just using a pyrometer?

Unless the previous owner provided you with his/her firing log (that would tell you number and type of firings, etc), you might want to use witness cones on three different shelves (bottom, middle, top) to get a sense of how hot the kiln is actually firing.

You might want to check the offsets programmed for the thermocouplars . . . they may need adjustment. As the elements of a kiln age with firings, the elements become less efficient and the offset for thermocouplars needs to be adjusted accordingly to compensate.

At worse, if you know the kiln is firing one cone below desired temperture, you can always program one cone higher to compensate.

#4 Val

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Posted 30 December 2012 - 06:00 PM

Yes I have been using witness cones. That's how i know it's firing about 1 cone under program. I will proceed with adjustments, just didn't want to do that if it was obvious I needed a new pyrometer :)

#5 weeble

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Posted 30 December 2012 - 07:44 PM

Have you checked your elements? I've had the same problem, firing one cone under program, twice now. When the kiln was firing on the proper schedule (not taking longer than expected) the problem was fixed by replacing the pyrometer. I admit, I skirted the fix for a while by just programming the kiln to fire one cone hotter with no real problems, but the pyrometer replacement fixed it. When the kiln was taking too long to reach temperature and (eventually) I started getting error messages, it was the elements that were in need of replacing. There was no skirting that one, as glazes were fluxing way too long and running, so I bit the bullet pretty quickly there. Of the two, replacing the pyrometer is a whole lot easier and less expensive.

It does sound like your pyrometer is worn. Look at the tip and see how the metal looks. If its pitted and worn looking, or bent (a previous user with no clue used our pyrometer as a shelf support. ONCE) it should probably be replaced.
Maryjane Carlson

Whistling Fish Pottery

#6 Lucille Oka

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Posted 30 December 2012 - 11:52 PM

You are using three different heat measuring devices: cones, programmer and a pyrometer. What temperature are your aiming for and what is your program? Please state it in degrees F. before you spend money or tinker; you said you wanted to fire to cone 1, which cone 1?
John 3:16
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#7 DAY

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Posted 31 December 2012 - 09:48 AM

Remember, the difference between cones is only a "few" degrees. (^5=2201 ^6=2266)

#8 Val

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Posted 01 January 2013 - 10:43 AM

To answer Lucille, In my latest firing,my target was 04,( bisque) since I knew it was about a cone off I set the program to 03 medium speed, no holds. I planned to keep my eye to the peep hole so to speak. about 7 1/2 hours later I tried to see the target cone thru the peep ( unfortunately I was delayed in retuning home or would have checked sooner) and the pyrometer showed the temp. as 1,080 Celsius (1,976 F I think) AS I could not see the cone I assumed it was down, and the temp was so high, I shut off the kiln. When opening, the 04 was down and the 03 cone ever so slightly bent. I am assuming the temp. might not be accurate as that temp should be for a cone 03. Not sure how much longer it would have fired if I had not shut it off.

I previously spoke with Skutt and they walked me thru a temp.adjustment for cone 06.( which is what I test fired to when I bought the kiln) Unfortunately he said the adjustment must be made for each cone . He said there was no overall adjustment .

#9 JBaymore

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Posted 01 January 2013 - 11:19 AM

To answer Lucille, In my latest firing,my target was 04,( bisque) since I knew it was about a cone off I set the program to 03 medium speed, no holds. I planned to keep my eye to the peep hole so to speak. about 7 1/2 hours later I tried to see the target cone thru the peep ( unfortunately I was delayed in retuning home or would have checked sooner) and the pyrometer showed the temp. as 1,080 Celsius (1,976 F I think) AS I could not see the cone I assumed it was down, and the temp was so high, I shut off the kiln. When opening, the 04 was down and the 03 cone ever so slightly bent. I am assuming the temp. might not be accurate as that temp should be for a cone 03. Not sure how much longer it would have fired if I had not shut it off.

I previously spoke with Skutt and they walked me thru a temp.adjustment for cone 06.( which is what I test fired to when I bought the kiln) Unfortunately he said the adjustment must be made for each cone . He said there was no overall adjustment .


Val,

I think some of your issues here might involve a lack of a true understranding of what cones measure???? Cones do not measure a precise temperature. Pyrometers are the tool for doing that. Cones measure the effect of the application of heat energy upon ceramic raw materials over a period of time. The temperature / time relationship is usually refered to as "heat work". Within reason, you can get the same "heat work" on your raw materials by firing to a lower temperature over a longer peiod of time, as you can firing to a higher temperature over a shorter period of time.

The rate of climb is VERY important to this discussion. The faster the rate of climb you fire at, the higher the end point temperature (on the pyurometer) will be when a given cone finally deforms so that the tip is equal to the base.

(For this following discussion I am using large Orton cones.)

At 7 1/2 hours from room temperature to your reading of 1976 F (1080 C), that represents an average rate of climb of about 263 F per hour. If you look at the Orton cone charts at the various end point temperatures based upon rate of climb, the 270 F per hour column is the closest to the rate that you fired at. So that says that the temperature the kiln will be at (on an accurate pyrometer) when a cone 03 is "tip to base" is at 2019 F. Your rate of climb is just slightly lower than that rate listed....so the end point temperature will be a TAD less than that number. Your reading of 1976 F when you shut the kiln off would be more commensurate with Orton Cone 04 when fired at a 270 F rate of climb........ which is pretty much EXACTLY what you observed.

You didn't let it get to the roughtly 2019 F temperature that the 03 should have been down at.

Sounds like maybe the kiln is accurate to me, unless I'm missing something.

best,

..............john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#10 Val

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Posted 01 January 2013 - 04:51 PM

Thanks John, That makes sense, and I do understand somewhat at least the concept of" heat work". What has, and is confusing me is that I thought a kiln where you programs in what cone to fire to took into consideration the rate of climb when it decided what temp. to shut off the kiln. At least an L & L kiln I have used in the past was like that. I just chose the cone and slow med.or fast and the program did all the "heat work" calculations. Maybe this Skutt being 12 or 15 years old is not really that smart, and I assumed it was.??
Thanks for all the help everyone
Val

#11 thrival

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Posted 01 January 2013 - 05:09 PM

Maybe try a new thermocouple, here they are,cheap

#12 weeble

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Posted 02 January 2013 - 03:27 AM

Our Skutt is the same age, Val, and if you read the manual, it actually says to expect to change the pyrometer after 50 firings. Now, I happen to know ours fired way WAY more than that before I replaced the it at 10 years, but we glaze fire at cone 5. If yours was used, it could be that its just time! Seriously, they're not that expensive and its not too hard to do. I think I had to drill the hole out a bit, as the old model had just a single wire to the probe and the new one had a ceramic sleeve, but the instructions that came with it explained it well. You don't usually need to replace anything more than the thermocouple that goes into the kiln, it gets worn but the electronics are fine.
Maryjane Carlson

Whistling Fish Pottery

#13 neilestrick

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Posted 02 January 2013 - 10:48 AM

This is most likely not a worn out thermocouple issue. When thermocouples wear out, they typically read off by way more than one cone. It is not at all uncommon for kilns to be off by one cone even with a brand new thermocouple. In kilns with only one thermocouple in the middle it is even more likely to happen. In some models of controllers (depends on what type and year your is), you can set an overall offset for the thermocouple(s) that will cause them to read higher or lower at all times, or you can set an offset for each individual cone. Since thermocouples are generally less accurate the hotter you go, the cone offset is a nice feature. Check you manual.

Skutt and L&L controllers are both made by Bartlett Instruments. They have some different features, but the main guts of how they measure het work are the same.

I would either program an offset for the cones you most often fire to, or simply add a hold time to achieve the perfect cone. For bisque, I wouldn't worry about it at all.
Neil Estrick
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Owner, Neil Estrick Gallery, LLC
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#14 Val

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Posted 03 January 2013 - 10:23 AM

Thanks everyone for your input ! As usual with ceramics there are 100 possible causes :) I think I 'll go with Niel's for the moment and see how it goes........




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