I just purchased and installed a three zone controller for my kiln. I
would like to warn anyone who is considering doing this.

From my experience with three zone controllers, the problem could possibly
occur under certain conditions.

The kiln would need to be firing very unevenly in heat distribution, and I
assume this was the case to require fitting of a three zone controller in
the first place. Many of the tall top loading kilns suffer from this
problem - too hot at the top and too cold at the bottom. Also the elements
would need to be slightly low in power dissipation for the kiln size, either
through poor element design (a common problem) or age. In other words the
kiln was just managing the required firing cycle before the fitting of the
new controller.

After fitting the three zone controller, the top set of elements will be
cycled off some of the time to try to keep the top in line with the
temperature in the rest of the kiln, while the middle and lower elements are
running at full blast . The net result is that the overall power dissipated
during the firing is down enough to slow the firing cycle, or even prevent
the completion of the cycle.

Many kilns are so poorly designed that they have very little reserve power
available when new, and after a lot of firings they are struggling to reach
the end of the cycle.

One answer might be to fit a kiln vent which draws a little hot air down
from the top through the bottom of the kiln and can often make the kiln fire
evenly. The best solution may be to fit new, properly designed, elements to
compensate for the uneven firing.

A couple of questions for the original poster. What sort of kiln are you
using? Did you have trouble with uneven firings? I worked as a kiln
technician for many years, and I often had to redesign elements for tall
toploaders to compensate for very uneven firing.

Cheers

Dave

But there should never be a problem such as you describe.
You don't need any surplus power at all if you have set your
schedule to be similar to your original uncontrolled rate.
In that case, the controller will not be doing much of anything
until you hit the setpoint. Only then does it start cutting back
on the power in order to hold that temperature.


I am not an expert, so I can't give you any kind of formula to use for
determining this. However, it would seem that if you can reach your
cone reasonably by running at, say, only two-thirds of your full power
(such as running with only 4 elemnts out of 6 total), then it might
work out okay.

If not, you'll still be able to run your kiln unattended, but you will
not be able to get uniformly heated zones at the end of your fire.


Again, perhaps I have misunderstood your problem, but my
suspicion is that you have simply set an unreasonable schedule.
One thing you might consider is running the kiln the old way
and recording the temperatures every hour, then use that data
to set a controller schedule that comes close to the "natural"
kiln performance. The difference is that once you reach the
cone setting, you will now be able to hold that for as long as
you want, or to cut back to a somewhat lower setting for
crystal growth, etc.

Once you have this base ramp -up information, you can
feel free to set slower ramps over any temperature region.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

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