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kiln quandary resolved (longish)

updated fri 14 aug 98


Mo Cain on thu 13 aug 98

Thanx to all for responses and suggestions sent both on/off list. Here's
my two penn'orth describing what I've learned and what I've rationalized
to give me a reasonable comfort level.

First I invested in a decent multimeter and have been able to eliminate
all of the suggested problems of low voltage, wrong power and wrong or
worn out elements. The kiln and power are within specs!!!

In spite of the wonders of computer controllers the only thing that
counts is the amount of heat work required to cause a cone 6 witness
cone to collapse. This heat work thing is both time and temp related. A
higher rate of temp increase ( say 270*f) leads to cone 6 at 2232*f
while a lower rate of increase (say108*f) leads to cone 6 at 2194*f.
Clearly the faster rate gets to cone 6 quicker!! Hamer and Hamer
describes (page73) this and indicates that a kiln fired very slowly
might reach Harrison cone 5 (2201*f) in three days at an indicated temp
of 2165*f. Conversely a very fast fire (say 700*+/hr) might get to
Harrison 5 in about three hours at an indicated temp of 2228*f. I'd be
inclined to call both of these firings an Orton cone6.

To remind you of my original quandary. My controller set to fire to cone
6 (indicated 2200*f) is shutting off when the indicated temp is 2165*f.
The witness cone is a perfect cone 6 ( to my reckoning in my kilns).That
is the tip is just touching the shelf while the cone 5 is flat. I call
this cone 6 but it takes a little over 12 hours.

The other lesson learned is that no matter what temp ramp up you program
into the controller there comes a time ( approx 1100*f) when my kiln can
no longer keep up and the clicking and clanking of the relays cycling
the kiln cease. Now I surmise that the role of the controller changes.
It becomes a temperature recorder with a very important difference. The
computer chip apparently is programmed using an algorithm which
essentially says because the rate of temp increase is declining it's
going to take longer to get to the target temp and because everything is
slowing down it is likely that enough heat work will be accomplished at
some temp lower than the programmed target temp ( in this case 2200*F)
This temp rate increase is continually changing. In my case early on I
achieve 400*F while during the final hour the rate declines to about
50*f/hour. The algorithm take all of this into consideration while it
selects the anticipated final temp and elapsed time. Thanks to the
Controller Engineers for this info. When enough is enough the controller
determines that the heat work has been accomplished and shuts everything
down... again in my case with an indicated final temp of2165*f. Open the
kiln and voila perfect cone 6 tips touching the shelf Still takes over
12 hours which I think is too long. Have done this three times now and
am consistent in the 2165*f shut off.

My next experiment will be to program the controller to cone 5 with an
hours soak at the end. I aim to get to cone 6 this way in 10 hours or
less consistently. I will however always use witness cones.

I hope that this as clear to you as it seems to be to me!!

Thanx for your indulgence...comments and differences of opinion are
welcomed. For me this is the only way to learn

regards mocain...Atlanta.