Weird temp issues - WarmGlass.com

Weird temp issues

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Lisa Allen
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Weird temp issues

Postby Lisa Allen » Thu Mar 18, 2004 11:33 am

I had a strange firing yesterday and wanted to get some feedback.

I was fusing a couple of 1/2" pieces and took them up to 1500 as I have done many times in the past in my evenheat oval. WHen they cooled I noticed that there were sunken areas where the bars butt up together like it wasn't taken high enough to allow the glass to flatten. 1500 has always been enough in the past to get the same type of pieces fully flattened, so I am at a loss as to why it id not working anymore....

I left them in the kiln and took it up to 1550 last night and made sure to watch the kiln at different stages to make sure that it was actually reaching temperatures and that the top and side elements were both on and it seemed to fire fine. But at 1500, I could see through the peep hole that the sunken gaps were still there. By the time it reached 1550, it was too hot to see what was going on, but it looked as if it had flattened out. I'll know more tonight when it is cool enough to open. But my question I guess is, is it unusual for a kiln to stop full fusing at a temp that it originally could? Does this indicate that my elements are going or that my thermocouple is going?

Any thought would be greatly appreciated.

Lisa
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rosanna gusler
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Postby rosanna gusler » Thu Mar 18, 2004 11:42 am

did you have more mass in there than usual? thicker molds, more posts extra shelf? elements can fail slowly. thermocouples can erode. can you test with ceramic cones or an additional pyrometer? rosanna

Phil Hoppes
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Postby Phil Hoppes » Thu Mar 18, 2004 11:47 am

If everything was equal to what you've done in the past, ie schedule, final temp, hold, dams and size of load then I would suspect one of two things either your thermocouple may be degrading (reading incorrect temps in this case it would read higher that it actually is) or your elements are degrading and they are not heating evenly. You can check both if you have access to a hand held pyrometer that you could probe at various places inside your kiln during a firing. You can check the elements if you know what there resistance is suppose to be and then measure what they actually are. This can only be done at room temp with power disconnected. I wouldn't recommend it if you are sure what you are doing.

Phil

Bert Weiss
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Postby Bert Weiss » Thu Mar 18, 2004 12:50 pm

Lisa

My guess is that 1" of glass is a considerable mass and it was taking longer for the glass to reach temp. Perhaps a longer soak would have done the trick.

Unless of course you meant 2 different pieces of 1/2" in which case it could still be a question of soak time...

What do you think caused the depressions, bubbles settling out or different colors behaving differently so one sagged out or ...?

It could have been a temp thing where a particular color needed more temp to get the viscosity in to the flow zone.
Bert

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Lisa Allen
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Postby Lisa Allen » Thu Mar 18, 2004 1:07 pm

Bert Weiss wrote:Lisa

My guess is that 1" of glass is a considerable mass and it was taking longer for the glass to reach temp. Perhaps a longer soak would have done the trick.

Unless of course you meant 2 different pieces of 1/2" in which case it could still be a question of soak time...

What do you think caused the depressions, bubbles settling out or different colors behaving differently so one sagged out or ...?

It could have been a temp thing where a particular color needed more temp to get the viscosity in to the flow zone.


2 different pieces of 1/2", set up in the exact same manner as many others. I had a similar issue with the sinkhole areas a few months back that had been fired up to 1450 and remedied it with a hotter 1500 firing. It looks like the bars are not flowing together and leaving depressions at the seams in some places. But these pieces are using the same set of colors as the previous ones that flowed and flattened at 1500? I think that the 1550 firing last night worked from what I could see, but I won't know for sure until I can see them cold.

I called Evenheat to see if they had any idea and they suggested that the thermocouple was going. They mentioned that even in a new thermocouple there is a certain amount of drift and that as it got older the drift can fluctuate even more, giving false temp reading either up or down. So I ordered a new thermocouple from them and I probably should get a hand held pyrometer too. I'll know more tonight when I can open the kiln, but if 1550 solved the problem, then it will make more sense that the drift has grown to the point of needing everincreasing temps to accomplish the same full fuse. The new thermocouple should get me back to predictable schedules at least for a while!

Lisa
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Postby Tony Serviente » Thu Mar 18, 2004 2:42 pm

I was surprised to hear what Evenheat said. I have been using Stainless clad thermocouples from Omega for many years and have never seen this effect. I have had rare failures, and they almost always are in the form of the thermocouple opening, in other words total failure. I once had one read correct at room temp, but above 1000 or so read a few hundred degrees low. If there was a drifting of the reading, I would know as the kind of production work I do shows those things right away. Wonder if this is a real effect, or if it only applies to the lower end thermocouples. The Omegas are not terribly pricey, and they last a long time, and are tough. Other comment is on element degradation. I use Kanthal on Mullite tubes and have not seen this effect either. Mine either open, or work fine. there has never been an in between. If you can see all of your elements glowing, my bet is that they are fine. I'd focus on the thermocouple. Good luck.

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Postby Bert Weiss » Thu Mar 18, 2004 3:04 pm

Tony Serviente wrote:I was surprised to hear what Evenheat said. I have been using Stainless clad thermocouples from Omega for many years and have never seen this effect. I have had rare failures, and they almost always are in the form of the thermocouple opening, in other words total failure. I once had one read correct at room temp, but above 1000 or so read a few hundred degrees low. If there was a drifting of the reading, I would know as the kind of production work I do shows those things right away. Wonder if this is a real effect, or if it only applies to the lower end thermocouples. The Omegas are not terribly pricey, and they last a long time, and are tough. Other comment is on element degradation. I use Kanthal on Mullite tubes and have not seen this effect either. Mine either open, or work fine. there has never been an in between. If you can see all of your elements glowing, my bet is that they are fine. I'd focus on the thermocouple. Good luck.


I'm in agreement with Tony here. I use inconel sheathed thermocouples and they last a long time. I believe that they only drift in one direction. I think that is down as I recall. Mine are now 13 years old and if they have drifted it hasn't effected production yet.

I will guess that the evenheat thermocouples are exposed. I don't care much for that setup.
Bert



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Lisa Allen
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Postby Lisa Allen » Thu Mar 18, 2004 3:13 pm

Tony and Bert-

The thermocouple that I have is an exposed ceramic sheathed one. It came with the kiln attached to a Rampmaster II. Is there a thermocouple like you guys are talking about that would work for this setup, wired to the controller and protruding through a hole into the kiln?

Thanks, Lisa
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Postby Tony Serviente » Thu Mar 18, 2004 3:37 pm

Lisa-Omega's number is 800-222-2665. I use part no. TJ36-ICSS-14U-12. This is a type J stainless sheathed 1/4" ungrounded. I use the fat ones cuz they can really take some hits and keep working fine. The thinner probes will respond faster, but are too delicate for the ham handed likes of me. The price is in the mid $30's. They are my gold standard. Good luck.

Lisa Allen
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Postby Lisa Allen » Thu Mar 18, 2004 3:57 pm

Tony Serviente wrote:Lisa-Omega's number is 800-222-2665. I use part no. TJ36-ICSS-14U-12. This is a type J stainless sheathed 1/4" ungrounded. I use the fat ones cuz they can really take some hits and keep working fine. The thinner probes will respond faster, but are too delicate for the ham handed likes of me. The price is in the mid $30's. They are my gold standard. Good luck.


Tony, the Rampmaster uses a type "K" thermocouple. Any idea if Omega has those available in a stainless, hardier version than what I have currently?

Thanks, Lisa
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Postby gthomson » Thu Mar 18, 2004 4:31 pm

Lisa Allen wrote:
Tony Serviente wrote:Lisa-Omega's number is 800-222-2665. I use part no. TJ36-ICSS-14U-12. This is a type J stainless sheathed 1/4" ungrounded. I use the fat ones cuz they can really take some hits and keep working fine. The thinner probes will respond faster, but are too delicate for the ham handed likes of me. The price is in the mid $30's. They are my gold standard. Good luck.


Tony, the Rampmaster uses a type "K" thermocouple. Any idea if Omega has those available in a stainless, hardier version than what I have currently?

Thanks, Lisa


Yep.

http://www.omega.com/ppt/pptsc.asp?ref=TJ36-ICIN&Nav=tema07

Prolly #TJ36-CASS-14U-12 would work.

Cheers,

Glenn

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Warning

Postby Lauri Levanto » Thu Mar 18, 2004 5:54 pm

Contrary to a suggestion abowe
DO NOT insert pyrometer during firing!
Switc off the power for that moment.

-lauri

tob
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Postby tob » Fri Mar 19, 2004 10:56 am

Tony Serviente wrote:Lisa-Omega's number is 800-222-2665. I use part no. TJ36-ICSS-14U-12. This is a type J stainless sheathed 1/4" ungrounded. I use the fat ones cuz they can really take some hits and keep working fine. The thinner probes will respond faster, but are too delicate for the ham handed likes of me. The price is in the mid $30's. They are my gold standard. Good luck.


Tony, using a J type thermocouple for slumping and fusing is operating on the ragged edge of that thermocouple's upper operating limits. Omega lists the maximum temp for a type J as 1382 F, although I have seen recommendations of a lower maximum temp than that. If you check the technical literature you will find that the longer and closer to that limit one is operated, the less accurate it will be. Since J type thermocouples are not designed to go to the firing temps that slumpers and fusers commonly use, I would suggest that the thermocouple you are using is probably not reading very accurately, whether or not you realize it. I personally wouldn't use a type J above 1100 F.

As far as thermocouple sheath materials go, as long as one is operating in a normal air atmosphere, a thermocouple sheath doesn't make much of a difference. But thermocouples do age and their performance degrades, regardless of whether they are sheathed or not. The higher the temperatures at which they are used, the faster that happens. In an uncontaminated oven atmosphere, using a larger diameter thermocouple wire will make more of a difference in lifespan that the type of sheath (if any). If the air is contaminated, a sheath should also help, although it will slow down thermocouple response somewhat.

I agree that a new thermocouple is definitely the answer to Lisa's problem.

Brad S (The other Brad)

Tony Serviente
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Postby Tony Serviente » Fri Mar 19, 2004 11:26 am

Brad-You're right about thermocouple type. I am using all K's, but am a bit dyslexic when it comes to keeping the J and K straight. As to degradation, I won't argue that it does not occur, but I will as to the severity. I have some thermocouples over 10 years old, with thousands of firings on them, and my firing profiles have not changed. The kind of work I do will tell me if there is as little as a 5 degree change, and even if it is a slow drift, I have the data to know it. So in my case anyway, the drift is insignifigent.

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Postby Brock » Fri Mar 19, 2004 11:34 am

I have 3 kilns with exposed type K thermocouples, all 20 years old, and all original equipment. Are they degrading? I don't know. I do all my firing in these kilns by eye. The pyrometer gets me in the neighbourhood, and I finish the firing by observation. It works very well. Brock
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Postby tob » Fri Mar 19, 2004 2:31 pm

Like Brock, most of my K type thermocouples aren't sheathed either, and they are all originals, varying in age from between 6 months and 20 years. That's a large part of why I said I don't think sheathing matters much in an uncontaminated atmosphere. But although I have made a few trips to 1800 degrees or so on a couple of them, I seldom take my ovens above 1100F. Thermocouple aging (with a K type) increases exponentially in the 2000 F neighborhood. That would put most people on the safe side except for those who try to get away with using a type K on a glass furnace. Still, chances are that with an old thermocouple the readings have drifted from what they were originally. They just might not have drifted enough to matter yet. If Tony had been using the type J he said he was there should have been a very noticeable drift.

The primary point of my last post, aside from agreeing that Lisa needs a new thermocouple, was to point out that a type J is not the right thermocouple to be using in a fusing/slumping oven. Type K is the "normal" one. Type N is even better.

Brad S

Lisa Allen
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Postby Lisa Allen » Fri Mar 19, 2004 3:33 pm

All this talk of 20 year old thermocouples has me wondering about the liklihood of mine failing......this kiln is only 2 years old??? Not that anomolies don't happen, but doesn't that seem awefully young for a serious drift?

Lisa

P.S. 1550 got rid of the sinkholes but overcooked the piece, it was dammed but the colors flowed too much for me. I have a new thermocouple on the way, so I suppose I will see what the difference is when I get it changed out.
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Postby Bert Weiss » Fri Mar 19, 2004 4:14 pm

Lisa Allen wrote:All this talk of 20 year old thermocouples has me wondering about the liklihood of mine failing......this kiln is only 2 years old??? Not that anomolies don't happen, but doesn't that seem awefully young for a serious drift?

Lisa

P.S. 1550 got rid of the sinkholes but overcooked the piece, it was dammed but the colors flowed too much for me. I have a new thermocouple on the way, so I suppose I will see what the difference is when I get it changed out.


A thermocouple is made by welding 2 wires together. There are enough variables to that process to account for short and long life to sheathed and unsheathed units.
Bert



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tob
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Postby tob » Fri Mar 19, 2004 8:07 pm

Lisa Allen wrote:All this talk of 20 year old thermocouples has me wondering about the liklihood of mine failing......this kiln is only 2 years old??? Not that anomolies don't happen, but doesn't that seem awefully young for a serious drift?

Lisa

P.S. 1550 got rid of the sinkholes but overcooked the piece, it was dammed but the colors flowed too much for me. I have a new thermocouple on the way, so I suppose I will see what the difference is when I get it changed out.


I'm not advocating using thermocouples for 20 years without replacement. The ovens my oldest ones are on don't see a lot of use, nor do they see temperatures above 1000 F. As Bert said, there are many variables that come into play in the life of a thermocouple. Just because Brock and I have some that have lasted that long doesn't mean everyone's will. Two years is a short life, but it isn't unheard of.

Are there any other variables you may not have mentioned in your problem firing?

Brad


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