Mo Cain on mon 27 jul 98
Be patient folks. This is not a test and will not be graded on the
curve. It is not rocket science but May be the dreaded thermodynamics.
I have this new Kiln complete with Bartlet Controller and usually fire
to cone 6.
I assume the controller switches the elements on/off to increase
temperature at a predetermined rate ( x degrees/hour) until a target
temperature is reached. Further the controller can be programmed to vary
this rate of increase on the way to the target.
For the sake of simplicity let's assume the following program...
Start temp ...65F
From 65f at 200f/hr to 400f 1.675 hours elapsed time
From 400f at 500f/hr to 1900f 3.00 hours elapsed
time
From 1900 at 100f/hr to 2200 3.00 hours elapsed time
This totals 7.675 hours to cone 6 ..a bit quick but stick with me.
In the real world this program cranked into the controller resulted in
the following...
After 3 hours the controller indicated 891F (Calculated to be 1100F}
After 5 hours the controller indicated 1503f ( calculateed 2070f)
After 7 hours the controller indicated 1870f
It took a further 5.25 hours for a total of 12.25 hours for the
controller to turn the kiln off and the indicated temperature on the
faceplate was 2174f.
During the last 2 hours of the firing the average rate of temperature
increase was in the order of 40f/ hour or less even with all peepholes
closed to move it along a tad.
Next day on opening up the kiln the glazes were OK. There was a slight
difference in achieved temperature between the levels. The top and
bottom shelves were slightly hotter than the middle two. Generally the
Cone 5 witnesses were as flat as pancakes, the cone6 tip just touching
shelf and the cone 7 on the top and bottom shelves showing a very slight
bend.
After all that background if you are still with me here is the enigma.
Is it normal for the rate of temperature increase to fall so drastically
as the kiln gets hotter? From observation it seems that after a certain
temperature (approx 1000F) the controller stops controlling by switching
on/off and becomes merely a fancy and expensive thermometer while the
kiln is on full power and doing the best it can. Can this be so and are
we doomed to have cone6 firings take over 12 hours?
The Factory said "Ah it's summertime in Georgia and you're getting
voltage drops and brown outs. Tell me this ain't so folks.
TIA Mocain ATL.
Alan Ambrose on fri 14 aug 98
Mo Cain on mon 27 jul 98 (mocain@BELLSOUTH.NET) wrote:
>>>
Be patient folks. This is not a test and will not be graded on the
curve. It is not rocket science but May be the dreaded thermodynamics...
....
I assume the controller switches the elements on/off to increase
temperature at a predetermined rate ( x degrees/hour) until a target
temperature is reached.
....
Is it normal for the rate of temperature increase to fall so drastically
as the kiln gets hotter? From observation it seems that after a certain
temperature (approx 1000F) the controller stops controlling by switching
on/off and becomes merely a fancy and expensive thermometer while the
kiln is on full power and doing the best it can. Can this be so and are
we doomed to have cone6 firings take over 12 hours?
The Factory said "Ah it's summertime in Georgia and you're getting
voltage drops and brown outs.
<<<
Hi Mo,
Well you pretty much figured the whole thing out. Rather than switching
on an off at a predetermined rate, the controller actually monitors the
rate of temperature increase against the program, and changes the rate
of switching. If the temperature is rising faster than it 'wants', it
slows down the switching, i.e. it is a dynamic process.
Near the top temperature of the kiln, the heat loss is the highest, and
the controllers are on between 90% and 100% of the time. (Heat loss
is roughly proportional to temperature.) So if you switched your kiln
elements on full throughout the firing you would get a nice curve which
is steep at the beginning and very shallow for the last third. Sometimes
the kiln manufacturers publish these curves and they are quite
interesting.
In fact, when designing the kiln, the elements and insulation are chosen
so that the maximum power from the elements will be a little more than
the heat loss through the insulation at the top temperature i.e. enough
to keep the temperature rising but not much more. A little thought says
that's the way kilns _must_ be designed for economics reasons (otherwise
they would use stronger elements and thicker insulation and would have
to charge a higher price than their competitors).
Voltage _is_ quite important since power is proportional to voltage
squared. You could always check the voltage yourself at the kiln by
having a meter wired across the supply. Apart from checking that
the kiln supply is using heavy duty enough cable (the thicker the cable
the lower the voltage drop - important for a very big kiln) I don't
think there is anything much you can do. You could think about extra
insulation, but I don't think there are many safe and convenient ways
to do this. Some potters believe that a long firing gives better
glazes (Lucie Rie's kiln used to take 22 hours to reach stoneware
temperature). I do too, and fire on the 'low speed' setting on my
controller anyway.
Alan
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