Re: Pyrometer Creep

[Lynne Chappell]

In the last week, I've had to change two programs that I use frequently because the glass was slightly overfired. It's still such a minor thing, that the first time, I thought it was maybe just different glass firing a little differently, but the second one was a firing I've done many times before with the same glass. The last firing I did 10 degreesF cooler and that seemed more like usual. So, it would appear that the thermocouple reading has changed. The kiln is about a year old. Will this continue to change, and should I get a new thermocouple?

[Tony Smith]

Lynne,

If the thermocouple connection is coming loose or had become oxidized, you will find that the temperature will read a little low until it becomes intermittent and stops reading completely. When the temperature reads low, the kiln gets hotter to compensate.

Take the box off the side of the kiln and remove the thermocouple leads. Mark which one came from which terminal with tape because they are polarized and must be reconnected in the same way.

Pull the thermocouple out of the kiln and instpect it looking for kinks, thin sections or oxidation. Inspect the tip and make sure it doesn't have anything stuck to it that might insulate it.

Reinstall the thermocouple and make sure the terminals are tight. Try another firing to see if it makes a difference. Another possible cause for the error would be an outside air leak near the thermocouple. Thermocouples are simple devices... when they fail, they usually fail all the way.

Good luck

Tony

[wynpotter]

If your thermocouple is the thick twisted wire with ceramic spacers this is an older style and more prone to drift. There is a newer metal tude style that seems less prone to drift. Price should be 30-40 $ Type k

They can drift in temp over time. As a way of calibrating your firing, get a box of pyrometric cones either self supporting or a cone holder for the non supported type. get the cone that fires at your maturing temp. cones are more accurate than some thermocouples. This will help in evaluating the temp that the kiln reached. Find out from the kiln mfg. the type of thermocouple, most are type k , which are fine but do drift with time. if possible find out if your kiln can be fitted with a type s thermocouple. these are made with platium and less prone to corrosion and have a longer life, price $75-100. the controllers are, I believe either made be bartlett industries or orton ceramic. Both folks have very good tech support. Your kiln mfg can help there to get you the info.
Hope this helps wyndham@ac.net

[Stuart Clayman]

Lynne,
Is there something different in what you are firing. Like maybe using the new Bullseye peper as compared to the old paper? The new paper tends to fire things hotter and we are adjusting our firing schedules for that purpose.

Stuart

[Lynne Chappell]

Thanks everyone. The drift isn't so significant that I'm prepared to pull the thermocouple quite yet. It is one of those new solid tube kind (a Skutt Clamshell about a year old). It's possible that one of the firings was just a little different in terms of the glass than usual. Actually, the plate was fine, but the black stringer had started to shred, which it doesn't usually do with the program I was using. The other firing was more of a concern - it was floral formers and the glass stretched to touch the shelf on the one in the corner (not in the thermocouple corner), and the black irid (regular Spectrum) that was in the center started to fire off. I was firing to 1375 which is very near the edge of losing the irid on the black, but I've done the same firing with the same glass before and that has never happened before. I'll just turn it down a bit for now and see what happens. Maybe there's a little warp that's causing airflow, or maybe one element is firing hotter than the others?

I'm just a little concerned that the next firing or the next may be even more overfired, and eventually I'll ruin something expensive. I guess I'll probably phone Skutt and see what they think.

[Kevin Midgley]

Lynne,
Another whole different issue in obtaining consistent results is the voltage your kiln is receiving. Perhaps at night or on a weekend you are getting more or less full voltage from your power supplier than you would be during a weekday. Has a large factory/business near your studio begun summer holiday work shifts? I have consistently found faster firing times happen at night when there is less general electrical consumption. Your "problem" might not be anything to do with a thermocouple. Kevin

[davebross]

The K thermocouples do corrode over time and read incorrectly. I make my own out of small gauge thermocouple wire and at about the one year point I replace them. I just cut off the original end, strip back some more insulation, twist the ends together and melt the tip with a torch and a bit of borax for flux.As far as checking the one you have, you can check them with the ceramic cones mentioned or temperature sensitive crayons sold at welding or industrial suppliers.

The suggestion about different voltages is quite true, and reminds me of something else.

Some temp controllers run the ramp and soak by time only, with no reference to the temp.

More expensive ones reference the temp and will not go to the next part of the program until the temp is right, or will cut the power if the temp is too high too early.

It pays to know which sort you have when diagnosing things like this.