Craig Martell on sun 6 jan 02
Jose asked:
>Craig, if I understand this one correctly you were going to a heftier wire
>and measure the length as to keep the current you could draw (same
>resistance) within the contrains of electrical gear (leads, breaker, etc.).
>Am I reading it right?
Hello Jose:
I think you have the idea. The voltage and amps(measure of current) will
be the same no matter what guage the elements are. The resistance values,
measured in Ohms, are kept the same in order to input the correct amount of
energy or BTUs. If I'm remembering this correctly, the lower the
resistance value, the hotter the element. The top and bottom elements of
most electric kilns will have a lower resistance value so they fire hotter
making up for the greater heat loss in these parts of the kiln. That's the
way my two Skutts are put together. I just bisque fire with them though.
If you can find a copy of Electric Kiln Construction for Potters by Robert
Fournier, all of this stuff is explained very well. I think this book is
out of print now but sometimes you can find it in your local library. It
may also be available through an interlibrary loan.
regards, Craig Martell in Oregon
Craig Martell on sun 6 jan 02
Jose responded:
I can not believe their design would be so accurate that a 5% increase in
final temperature would cause metallurgical changes in the element. I
believe the gauge differences obey more to the rate of energy input (less
electrical resistance) than to metallurgical or mechanical considerations,
since adding the insulation will not change the rate of energy input, only
removal, there is not a whole lot of additional stress on these elements.
Hello again Jose:
From my experience, the increase in temp from cone 6 to cone 10 will
definitely shorten element life. As the temp goes up toward cone 10 you
are causing the elements to soften and become more active on the molecular
level. This will cause changes and loss in the element mass. The heavier
the element guage, the more time you have until failure.
Even though guages vary, the resistance value should be kept the same as
specified by the manufacturer. I had very long coils wound by a kiln tech
nearby. We would cut the element to length after measuring with a volt
ohmeter to assure that the resistance was correct. In other words we
attached one lead from the meter to one end of the coil and attached the
other end at the final length where the resistance was correct. The
element was cut at this point. The I had to stretch and fit the elements
to the inner wall recess in the kiln.
regards, Craig Martell in Oregon
Jose A. Velez on sun 6 jan 02
Craig:
> From my experience, the increase in temp from cone 6 to cone 10 will
> definitely shorten element life. As the temp goes up toward cone 10 you
> are causing the elements to soften and become more active on the molecular
> level. This will cause changes and loss in the element mass. The
heavier
> the element guage, the more time you have until failure.
I do not have any experience doing this whatsoever and am only infering this
from a theoretical point of view. So, I accept and appreciate your point
that if I add insulation and fire to a higher cone I will in fact shorten
the life of the elements.
> Even though guages vary, the resistance value should be kept the same as
> specified by the manufacturer. I had very long coils wound by a kiln tech
> nearby. We would cut the element to length after measuring with a volt
> ohmeter to assure that the resistance was correct. In other words we
> attached one lead from the meter to one end of the coil and attached the
> other end at the final length where the resistance was correct. The
> element was cut at this point. The I had to stretch and fit the elements
> to the inner wall recess in the kiln.
Craig, if I understand this one correctly you were going to a heftier wire
and measure the length as to keep the current you could draw (same
resistance) within the contrains of electrical gear (leads, breaker, etc.).
Am I reading it right?
Thanks, Jose A. Velez
Roger Korn on wed 9 jan 02
The added stress when firing to ^10 vs ^6 is due to the rapid increase in molecular
re-alignment of metals as their fusion point is approached. This makes the
molecules
tend to align in the direction of the residual drawing stress and thus promotes a
condition
similar to work-hardening, where the material becomes more brittle and thus more
fragile
and subject to surface micro-cracking with the motion of the element due to
expansion
and contraction with heating/cooling cycles. Add to this the rapid increase in
surface
oxidation as the fusion temp is approached and you will find that element life
versus
temperature is a very non-linear function, with a small increase in firing temp
causing a
large decrease in element life at high temps compared to what the same relative
increase
would cause at lower temps.
By the way, the basis for comparing relative temperature differences when referring
to physical
and chemical processes should be absolute zero (zero degrees Kelvin), not zero
degrees Fahrenheit
or Celcius (Centigrade).
On making up elements to length from continuous coils, an easy way to determine
required
element length and the position of bends is to insert a 1/4" cotton rope in the
empty grooves and
make tick marks at each bend. Then remove the rope from the grooves and stretch it
straight,
along side the coil. Stretch the coil to the required length and pre-bend it at the
tick marks, using a
propane torch to heat the coil to dull red at each bend point. The resulting
element will be much
easier to place in the grooves.
Hope this helps,
Roger
Craig Martell wrote:
> Jose responded:
> I can not believe their design would be so accurate that a 5% increase in
> final temperature would cause metallurgical changes in the element. I
> believe the gauge differences obey more to the rate of energy input (less
> electrical resistance) than to metallurgical or mechanical considerations,
> since adding the insulation will not change the rate of energy input, only
> removal, there is not a whole lot of additional stress on these elements.
>
> Hello again Jose:
>
> From my experience, the increase in temp from cone 6 to cone 10 will
> definitely shorten element life. As the temp goes up toward cone 10 you
> are causing the elements to soften and become more active on the molecular
> level. This will cause changes and loss in the element mass. The heavier
> the element guage, the more time you have until failure.
>
> Even though guages vary, the resistance value should be kept the same as
> specified by the manufacturer. I had very long coils wound by a kiln tech
> nearby. We would cut the element to length after measuring with a volt
> ohmeter to assure that the resistance was correct. In other words we
> attached one lead from the meter to one end of the coil and attached the
> other end at the final length where the resistance was correct. The
> element was cut at this point. The I had to stretch and fit the elements
> to the inner wall recess in the kiln.
>
> regards, Craig Martell in Oregon
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
--
Roger Korn
McKay Creek Ceramics
In AZ: PO Box 463
4215 Culpepper Ranch Rd
Rimrock, AZ 86335
928-567-5699 <-
In OR: PO Box 436
31330 NW Pacific Ave.
North Plains, OR 97133
503-647-5464
Joyce Lee on wed 9 jan 02
ok
----- Original Message -----
From: "Roger Korn"
To:
Sent: Wednesday, January 09, 2002 4:47 AM
Subject: Re: Electric kiln elements for hi temp
> The added stress when firing to ^10 vs ^6 is due to the rapid increase in
molecular
>
> re-alignment of metals as their fusion point is approached. This makes the
> molecules
> tend to align in the direction of the residual drawing stress and thus
promotes a
> condition
> similar to work-hardening, where the material becomes more brittle and
thus more
> fragile
> and subject to surface micro-cracking with the motion of the element due
to
> expansion
> and contraction with heating/cooling cycles. Add to this the rapid
increase in
> surface
> oxidation as the fusion temp is approached and you will find that element
life
> versus
> temperature is a very non-linear function, with a small increase in firing
temp
> causing a
> large decrease in element life at high temps compared to what the same
relative
> increase
> would cause at lower temps.
>
> By the way, the basis for comparing relative temperature differences when
referring
> to physical
> and chemical processes should be absolute zero (zero degrees Kelvin), not
zero
> degrees Fahrenheit
> or Celcius (Centigrade).
>
> On making up elements to length from continuous coils, an easy way to
determine
> required
> element length and the position of bends is to insert a 1/4" cotton rope
in the
> empty grooves and
> make tick marks at each bend. Then remove the rope from the grooves and
stretch it
> straight,
> along side the coil. Stretch the coil to the required length and pre-bend
it at the
> tick marks, using a
> propane torch to heat the coil to dull red at each bend point. The
resulting
> element will be much
> easier to place in the grooves.
>
> Hope this helps,
>
> Roger
>
> Craig Martell wrote:
>
> > Jose responded:
> > I can not believe their design would be so accurate that a 5% increase
in
> > final temperature would cause metallurgical changes in the element. I
> > believe the gauge differences obey more to the rate of energy input
(less
> > electrical resistance) than to metallurgical or mechanical
considerations,
> > since adding the insulation will not change the rate of energy input,
only
> > removal, there is not a whole lot of additional stress on these
elements.
> >
> > Hello again Jose:
> >
> > From my experience, the increase in temp from cone 6 to cone 10 will
> > definitely shorten element life. As the temp goes up toward cone 10 you
> > are causing the elements to soften and become more active on the
molecular
> > level. This will cause changes and loss in the element mass. The
heavier
> > the element guage, the more time you have until failure.
> >
> > Even though guages vary, the resistance value should be kept the same
as
> > specified by the manufacturer. I had very long coils wound by a kiln
tech
> > nearby. We would cut the element to length after measuring with a volt
> > ohmeter to assure that the resistance was correct. In other words we
> > attached one lead from the meter to one end of the coil and attached the
> > other end at the final length where the resistance was correct. The
> > element was cut at this point. The I had to stretch and fit the
elements
> > to the inner wall recess in the kiln.
> >
> > regards, Craig Martell in Oregon
> >
> >
____________________________________________________________________________
__
> > Send postings to clayart@lsv.ceramics.org
> >
> > You may look at the archives for the list or change your subscription
> > settings from http://www.ceramics.org/clayart/
> >
> > Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
> --
> Roger Korn
> McKay Creek Ceramics
> In AZ: PO Box 463
> 4215 Culpepper Ranch Rd
> Rimrock, AZ 86335
> 928-567-5699 <-
> In OR: PO Box 436
> 31330 NW Pacific Ave.
> North Plains, OR 97133
> 503-647-5464
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
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