Corinne Null on sat 29 jun 96
Have just received and set up my Bailey 2327 kiln with a Bennett controller,
and done the 2 empty firing break ins, and my first bisque fire.
2nd empty firing to cone 6-9. Put a small cone 10 in the sitter, programmed
the controller to cone 9 (2301 F) fast fire. Firing ended when the cone
bent and the controller gave an err msg (temp not rising). Last time I saw
the controller reading it was 2000 F. (hope going to cone 10 didn't hurt my
kiln!)
bisque fire to cone 07. Put a small 07 cone in the sitter, programmed to
cone 07 (1786 F) slow fire. Firing ended when the cone bent and the
controller gave an err msg. Last time I saw the controller reading it was
1565 F.
What is going on? I presume the small cones are accurate. Do I therefore
assume my controller readings to be inaccurate? Do I have to buy large
witness cones to observe another bisque fire in more detail before risking a
glaze fire? Or do I trust the small cones and just go ahead and fire a
glaze load?
Help! The problem solving just never ends!
Thanks,
Corinne Null
Bedford, NH
cnull@mv.mv.com
Bob Kavanagh on sun 30 jun 96
I don't understand. Is the small cone in your sitter shutting down
you kiln and that is creating the error message? I use an orton
sitter on a cone art kiln and occasionally check against regular (large)
cones placed here and there in the kiln. The controller is very
precise and accurate as far asI can see. In addition, apart from the
use of cones, the controller is consistent to the point now that I
use cones only occasionally, ususally after I suspect that my elements
are getting tired.
b.
Don Kopyscinski on sun 30 jun 96
Hi everyone,
On Sat, 29 Jun 1996 Corinne Null wrote:
> Put a small cone 10 in the sitter, programmed the controller to cone 9 (2301
F) fast fire. Firing ended when the cone
bent and the controller gave an err msg (temp not rising). Last time I saw the
controller reading it was 2000 F. (hope going to cone 10 didn't hurt my kiln!)<
The sitter cone isn't meant as a shutoff only a safety back-up. Their published
bending point is when they are upright (at an 8 degree angle or so leaning
towards the "bending face") position as with large cones. First make sure your
sitter is in alignment, using the plate that came with the sitter with the plate
resting on the cone supports and the sensing rod in the center hole the weight
should just clear the hook on the rod. The weight of the rod adds extra force on
the cone, making it bend early. I use a sm. cone 12 in my sitter as a safety
back-up for a cone 10 (at 2 o'clock firing). A sitter cone cannot be expected to
be an accurate indication of the firing and over time the sensing rod will bend
slightly and deteriorate, so it needs regular alignment and replacement. What I
expect happened was the sitter kicked before the endpoint of your programmed
firing. Your controller is probably programmed to give an error message in the
event the firing profile isn't meeting the reqested temperature rise (often
factory set as less than an 18 degree per hour temperature rise = an error, shut
the kiln down for safety). So the sitter kicked off, the kiln temp. began to
fall, and the controller shut itself off because it was asking the kiln to heat
but it wasn't able to because the power had been cut off by the sitter. (I doubt
there was any damage to the kiln).
> Do I have to buy large witness cones to observe another bisque fire in more
detail before risking a glaze fire?<
You do need to get large witness cones and put them in every firing where you
can see them to guide you, this is the most accurate indication of heat work and
firing termination point and will help you to troubleshoot when things don't go
as planned. You should also use them to fine tune the setting on your
controller, as there are so many variables that to think you can set it and
forget it is just not quite right. Use the cones as your guide, your kiln
controller as a tool to get where you want to go, and your sitter as a safety
back-up, and all should go well.
>The problem solving just never ends!<
NEVER!
Regards,
Don Kopyscinski
Bear Hills Pottery
(Who has kept away from his computer to make pots and not write about them. I
know I owe a few of you responses to e-mail and I should get to that next week.)
Ron Roy on sun 30 jun 96
For Corinne,
Never fire a kiln without at least a large witness cone in there some where.
I recommend several when firing a new kiln.
This is assuming you want to know what happened in there.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Paula Coleman on mon 1 jul 96
>I presume the small cones are accurate. Do I therefore
>assume my controller readings to be inaccurate? Do I have to buy large
>witness cones to observe another bisque fire in more detail before risking a
>glaze fire? Or do I trust the small cones and just go ahead and fire a
>glaze load?
I have been told that small cones are not meant to be used in cone packs and
set on the shelf. It is my understanding that they are only accurate when
they are used in the kiln sitter.
------------------------------------
Paula Coleman
Ceramic Artist
Email: potter@comcat.com
Web: http://www.comcat.com/~potter/
CaroleER@aol.com on tue 2 jul 96
I don't have a controller - but I've been checking my kiln sitter cone
against witness cones. The kiln sitter cone is probably 1 cone lower than
the witness cones. The good news is that the temp seems to be pretty even
from top to bottom. I'm gonna try a higher cone in the kiln sitter and see
what happens.
Carole Rishel
CaroleER@aol.com
Bastrop, TX
Corinne Null on thu 11 jul 96
Thanks to all who have answered my query's. I have learned SO MUCH!
So, I did my first glaze firing, programming the controller to go as follows
175* to 1000*
125* to 1500* (burn out combustables, prevent pinholes)
175* to 1900*
88* to 2290* (cone 9 is supposed to be 2301*)
1 hour hold
-200* to 1850*
The small cone 10 in the kiln sitter popped at 2019* (You bet, I watched
that baby!) The witness cones (8,9,10) looked like the 8 was down and 9 was
just starting. Hum. . . That awful wait . . .
Voila. Top cones were perfect, 10 just starting to move. Middle of kiln,
cone 10 at 2 o"clock. And bottom of kiln, cone 10 flat.
So, kiln sitter and witness cones validate each other. Must be that
controller has funny numbering system. Guess I just have to get used to it?
I'm starting another glazefire, and wondered - can I turn off the venting
system and close the top peephole (ie. button up the kiln) when I get to
soaking time, or should I wait till reaching 1850* cooldown? I'm basically
doing the same firing except switching at 800* and 1300* for pinholes, and
topping out at 2000*. Think I'll make it, or am I pushing the envelope too
close?
Thanks, y'all.
Corinne Null
Bedford, NH
cnull@mv.mv.com
John Baymore on sat 13 jul 96
From: Corinne Null
Subject: small cones vs. controller
----------------------------Original ----------------------------
........... So, kiln sitter and witness cones validate each other. Must b
that
controller has funny numbering system. Guess I just have to get used to it?
-------------------------------------------------------------------
Corinne,
This thermocouple, pyrometer, and controller thing is sort of like the
discussion that's been going on about barium and glaze chemistry....... there is
a lot more to it than first meets the eye. Now we're leaning into the realm of
kilns......... as you know, that' s an area in which I have some expertise .
Many of the things we "assume" to be true from "experiential logic" are simply
not. A pyrometer seems like such a simple thing on the surface. Stick a probe
in the kiln and attach a guage to read the result. Yet there are complexities
untold in these devices. The pottery suppliers haven't helped the situation
much either. They have us expecting a cheap "magic bullet" in these things.
The controller on your kiln uses type K thermocouples, most likely, like
99.9999999999% of all equipment supplied to the craft potters. The Seebeck
coefficients (temperature change related to voltage change created) for Chromel
/ Alumel type thermocouples (K) goes somewhat non-linear at the top end of the
use range. Seriously oversimplified, this means that one unit temp change no
longer results in one unit voltage change. That makes the software for the
controller a bit difficult for the programmer, and the conversion from
microvolts to degrees gets less accurate as the temp goes up.
For example, the Seebeck for the 50 degree interval from 600C to 650C changes .3
microvolts per degree Centegrade. As you get up to 850C to 900C, it changes
more, from 40.5 uV/dC to 40 uV/dC....... a change of .5 microvolts. From 1150C
to 1200C it goes from 37.2 uV/dC to 36.5 uV/dC, a change of .7 microvolts. From
1200C to 1250C, it changes .8 microvolts. The software has to take this
increasing "curve" into account.
Worse, the ASTM recommends type K for use only up to a max of 1260C. So a
firing to cone 10 (150C/ hr....... 9 hours+/-) is pushing the absolute limit of
their usefulness by a great factor...... 105 degrees C over the max. use rating
of the sensor. So they simply should not be used for cone 9-10 kilns. Yet they
are all the time.
Continuing this complex picture, that rating is for PROTECTED (more on that
later) thermocouples of AWG (wire thickness) of 8 ......... yes EIGHT!!!!
(pretty darn thick) The maximum use rating goes DOWN to 1090C (1970ish F) for
AWG 14 (look at household 14-2 wire for a size comparison), 980C for AWG 20, and
870C for AWG 24.
So...... what guage (AWG) are your thermocouples? I think your kiln's
manufacturer uses 8 AWG...... they do on the single pyros they sell. Not sure
on the controllers though. Look at the household wire....look at your
thermocouples. If it is AWG 14, it is only recommended to be accurate by ASTM
(within all the other constraints) for use up to about cone 4!!!!!!
Enlightening, huh?
Next, you have to look at the design accuracy of the manufacturing process to
start with. ASTM specs that the initial calibration of type K is +/- 2.2C or
..75% (whichever is GREATER) between 0 and 1200C. That is the BEST it will ever
be....when it is brand new. At 1200C, that allowable .75% is +/- 9 degrees. At
cone nine, plus or minus 9 degrees C is about 1/3 a cone (ignoring the heat-work
concept for simplicity)!
Plus, you have to deal with the accuracy of the meter (or controller). Some
analog meters are only accurate within 2% of full scale to start with. If full
scale is 1300C then it is only accurate +/- 26 degrees C, and THAT is of the
thermocouple output which is also only +/- 9 degrees C!
Additionally, type K is subject to oxidation when used IN AIR above 750C. This
oxidation results in the electrical output changing, and the need for constant
recalibration. This recalibration requires some electronic equipment that costs
a lot of $, and some skill to use. (Industry recalibrates on a regular
schedule to maintain precise production control.) Type K should not be used in
reduction atmospheres or sulpherous ones, so ASTM also says they are best used
enclosed in a sealed tube with an inert gas........ (a thing you'll never see in
a potters kiln!) BTW..... A vacume on the protection tube is also a no-no.
In general, for cone 9-10 use, type K is useless for REALLY accurate work at the
top of the temperature range. They are used on "potters kilns" simply because
they are CHEAP, and most potters won't pay for the platnium/ 10%
platnium-rhodium (type S) thermocouples and control systems that really should
be used (and the more expensive plated switches and contacts that entails).
Type K simply are more resistant to oxidation than the really cheap types E,
J,and T at temps over 500C, so they got pressed into use.
So you DO just get used to the fact that the temperature shown is not really the
temperature in the kiln (pretty bad since that is what you are lead to beleive
you are paying for! :( You also get used to the fact that the accuracy
will change with time. You "work around" the inherent inaccuracy. So all the
fancy "scientific" sounding precision of these controllers supposedly offer is
subject to a lot of variables.
What these controllers SEEM to promise (incredible accuracy), CAN be done, but
not at the price point that they have to occupy to meet the budget of a studio
craftsperson. Industry has much better stuff.
YET.................................., it is still far better than the old "seat
of the pants", look in the kiln, sniff the wind, discuss it for a half-hour,
throw salt over your shoulder, stuff we used to do (I fire a wood kiln....I
still do all that stuff!).
Luckily, most potter's applications are not all that critical at the top end of
the firing for cone 9-10 work. So the manufacturers get away with type K in
most cases. If you are getting into trouble and need more accuracy, you will
have to change controllers to one that uses the platnium thermocouples. And
it'll cost you some $$$. But from the sound of it in the thread, you just need
to understand the limitations of the tool, and work with it.
........................................john
PS: If your readings are that far off and you DO have type S (or type R or B)
thermocouples, call the kiln supplier. Something is really off there with the
controller.
PPS: Nice to see you at the Mary Rohem lecture at the Phoenix Workshops
yesterday.
PPPS: Don K. gave you a GREAT answer on the cone / controller question!!!
John Baymore
River Bend Pottery
Wilton, NH
76506.3102@Compuserve.com
http://www/CraftWEB.com/org/jbaymore/rivrbend/shtml
Shelley S. on sat 13 jul 96
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"This thermocouple, pyrometer, and controller thing is sort of like the
discussion that's been going on about barium and glaze chemistry......."
John,
Speaking of thermocouples, pyrometers, etc., on a simple analog pyrometer for a
gas kiln, if I want to change the length of the wire, I was told I would need to
change the resisters in order to recalibrate it for the new wire length. Is
this true? Is there something else I would need to do as well?
thanks for the info.
Shelley
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