My ancient Kerr 666 kiln recently had to have the pyrometer
replaced. The former owner had melted the front of it, making it
almost unreadable at anything hotter than 1000F. Ever since the new
pyrometer was installed by a local jewelry supply shop, it has not
provided consistent readings. I tried to fire some regular PMC, which
has to be held at 1650F for 2 hours. Before the pyrometer ever reached
1500F, the PMC had melted into little balls. Yesterday we had to cast
some sterling silver, and at the same setting that took the kiln to
the point that ruined the PMC, it never got hot enough to burn off
the ash from the investment. I turned it up to complete the burnout,
then turned it back down until the flasks were not glowing. When we
cast, every flask came out with heavy firescale on the silver.
Obviously it was still too hot.
My main question is, does this sound like the kiln itself needs
repair, or is this just a mis-calibrated pyrometer?
And if it needs repair, would it be better to just replace the poor
old thing? If so, what company makes a good affordable small (4-6
flask) burnout kiln? (I also have a large 220volt kiln, but no 220
power line at the moment).
Since the pyrometer has been replaced, was it replaced with an
identical part or functionally similar?
Was the thermocouple also replaced? There might be a mismatch
between them. I don’t know the type of TC originally built into the
Kerr but when I replaced the pyrometer on mine (with a digital w/
more functions) I had a similar situation but after I exchanged the
TC to a compatible type it runs as always.
What you have is a burned out rheostat - that’s the rotating switch.
It’s supposed to do a thermostat thing: turn on when it’s cold, and
then turn off as the pyrometer sends it signals that it’s hot.
There’s a good chance that that’s what happened to the pyrometer in
the first place - the switch burned out the pyrometer, not
vice-versa. I say it’s likely, but it really doesn’t matter. Now the
rheostat is staying full on all the time, and your kiln is running up
to maximum as time goes on. I have taken them apart and fixed them,
but that takes some tinkering. It’s about a $50 part, I guess,
depending on the kiln. I also say it probably went first because it
is the part that goes out on kilns - I’ve done about 6 rheostats and
never changed a pyrometer. It’s a moving part (electrically moving),
and it’s the only moving part on the kiln.
I think you need to calibrate the Pyrometer. I use the Orton
Pyrometric cones with great success. These should be available from
any ceramic shop as that is what they use in their kilns. Here is a
link to their website where there is a lot of on what
they are and their use.