Temperature Rising During a Hold on an AFAP Cool Down

[glassy]

I am questioning whether other kiln users have this same thing happen when doing a cool down. When I do an AFAP to cool down the temperature it always rises back up (about 30 degrees) at the beginning of the holding time. After being in the hold for about 5 minutes, the temperature then quickly goes back down to the target temperature and stays at that temperature for the remainder of the hold. Here is an example of it happening:

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*AFAP-960-20….When it gets down to 960 it immediately starts climbing back up until it reaches 990 or so. During the hold it stays steady at 990 or so for a length of about 3 to 4 minutes (12 to 11 minutes are remaining in the hold). Then it quickly drops back down to 960 and stays at a fairly steady temperature (955-960) until the hold is finally done.

700-700-Off

Does anybody have any suggestions? Why is it doing this? The kiln company says this is normal. They say it takes the digital computer a while to recognize it should be staying at 960. They had me add another step in my schedule before the AFAP-960, and it is now happening during that step. Here is their schedule:

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*AFAP-980-5… Goes quickly up to 1010 degrees and stays there for almost the Entire hold of 5 minutes!

AFAP-960-15

700-700-Off

This problem has been happening for 3 months now. I have been discussing this issue with the kiln company for about 2 months. My warranty will expire in less than 3 weeks!! Am I getting the run-around from their company? I think my kiln is a faulty kiln. I have told them I deserve a new kiln if they can’t rectify this! They said that if my kiln does die within a few months (who knows what they mean by “few”) they will honor the warranty even if it has expired in the meantime. Help me!

[Stephen Richard]

Go back to your original schedule.  You have the answer from the kiln company – they are not giving you BS.  The controller has to “learn” how much heat needs to be put into the kiln to keep the temperature stable.  So it “thinks” more heat is required to keep the the temp at 960 than is the fact.  The few minutes the kiln is staying a the higher temperature is the time the controller needs to correct the heat input.  (you will see an overshoot on the top temperature on the way up if you go fast, and for the same reason.)

Second, the anneal soak is not the most important part of the soak.  It helps to equalise the temperature within the glass.  The annealing is taking place in that 55C below the AP.  This is what needs to be slow and steady.  700F/hr is neither.  I don’t know the size of pieces you are firing, although I suppose they must be small, given the fast rises in temperature.  So maybe 100C per hour to abour 370C would give you a more stable result.

 

Stephen Richard

blogs at: http://www.verrier-glass.blogspot.com/   and  http://www.glasstips.blogspot.com/

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