Arnold Howard on fri 16 dec 05
Maurice, it is interesting that your kiln underfired when your thermocouple
extended only 1/8" into the firing chamber. I'm not sure why that happened.
Maybe someone on Clayart can offer a suggestion.
During firing, the temperature in the firebricks is lower than the firing
chamber temperature. If the firing chamber is 2000 degrees F, the outer
firebrick surface may be only several hundred degrees. The closer you go
toward the outside of the firebrick cross-section, the lower the
temperature.
So if the thermocouple is pushed into the firebrick wall, the thermocouple
will register a lower temperature than the firing chamber. This will cause
the controller to keep the elements turned on longer to compensate for the
low reading, resulting in an overfire.
If the thermocouple extends into the firing chamber by 1/8" instead of the
recommended 3/4" - 1", the overfire will be only a few degrees. If you are
firing slowly and the heat has soaked into the firebricks, there might not
be an overfire at all. But if the thermocouple tip has been pushed well past
the firebrick surface, the overfire could be as high as several hundred
degrees F.
Another factor is distance to the elements. If the thermocouple hole is near
an element, the kiln might not overfire at all when the thermocouple tip is
flush with the firebrick surface.
It is very important not to disturb the thermocouple, either by bumping it
with a shelf or twisting it. Twisting the thermocouple can cause the two
thermocouple wires to touch each other at some point inside the kiln wall.
This could short out the thermocouple tip and cause the thermocouple to read
the temperature from inside the firebrick wall where the thermocouple wires
are touching. This would overfire the kiln.
Sincerely,
Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com
----- Original Message -----
From: "Maurice Weitman"
> At 1:23 PM -0600 on 12/15/05, Arnold Howard wrote:
>>The thermocouple
>>reading will be cooler than the actual kiln temperature if the
>>thermocouple
>>is not pushed far enough into the firing chamber.
> Two weeks ago I was firing my small (Aim 88) test kiln with about 15
> raw-glazed test tiles to cone 6. I had the (S-type) 'couple inserted
> incorrectly; it was only about 1/8" into the kiln. > My point (you were
> wondering, huh???) is that it made sense to me at the time that since the
> tip of the thermocouple was so close to the hottest part of the kiln, the
> bricks and elements, it would be reading high, thus underfiring.
> What do you think about that theory?
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