James Freeman on wed 29 jul 09
Hi, all...
A technical question for the electric kiln geniuses. I believe I have
a bad thermocouple, but wanted to run the problem by you folks.
My small kiln, an 8-sided Evenheat with a Bartlett controller, has
just acted up. For years it has fired dead on, the cones I stashed
inside corresponding perfectly to what the controller said it did. A
few days ago I did a cone 04 glaze firing. My textured glazes were
all smooth and glossy, and the cone was a shiny flaccid mass. I had
no cone higher than 04 in there (I don't watch the cones, just use
them as an after the fact gauge), but judging from the conditions, I
estimate that the kiln overfired by about two cones. The next firing
was a cone 4 glaze firing. I set a 45 degree thermocouple offset,
then let the controller have sway. This time cone 4 was a puddle and
cone 5 was a shiny flaccid mass. The glaze, or rather engobe, showed
a few blisters on the outside of the forms, and evidence of boiling on
the inside. I estimate the firing at something greater than cone 6.
Today I did another cone 4 firing. This time I set the thermocouple
offset back to zero, and additionally inserted the probe of my Fluke
pyrometer. I also included cones 3, 4, and 5 in a place where I could
actually see them.
At the beginning of the firing the pyrometer closely agreed with the
controller's readout. The following are readings as the firing
progressed:
Kiln Fluke Difference
1015 1176 161
1721 1856 135
1776 1906 130
1869 1993 124
2000 2117 117
2016 2130 114
2050 2162 112
2062 2176 114
At this point cone 4 was down, so I shut off the kiln manually.
On the way down, the two thermocouples came more into line:
Kiln Fluke Difference
1502 1533 31
1439 1471 32
1365 1397 32
1320 1353 33
983 1013 30
888 915 27
So, the question is, is this type of behavior indicative of a failing
thermocouple, or is it something more serious? I would have assumed a
failing thermocouple would gradually drift, but in my case the shift
was sudden and dramatic. I would appreciate your thoughts.
Thanks.
...James
James Freeman
"All I say is by way of discourse, and nothing by way of advice. I
should not speak so boldly if it were my due to be believed."
-Michel de Montaigne
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/clayart/
Arnold Howard on thu 30 jul 09
From: "James Freeman"
> My small kiln, an 8-sided Evenheat with a Bartlett
> controller, has
> just acted up. For years it has fired dead on, the cones
> I stashed
> inside corresponding perfectly to what the controller said
> it did. A
> few days ago I did a cone 04 glaze firing. My textured
> glazes were
> all smooth and glossy, and the cone was a shiny flaccid
> mass.
James, I would check the distance the thermocouple extends
into the firing chamber. If a shelf has bumped the
thermocouple into the kiln wall, the kiln will overfire as
you describe. If the thermocouple extends into the firing
chamber by the correct distance, then I would replace the
thermocouple.
As a general rule of thumb, the thermocouple should extend
into the firing chamber by four times the diameter of the
thermocouple.
Sincerely,
Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com
Dave Finkelnburg on thu 30 jul 09
James,=3D0A=3DA0=3DA0=3DA0 Do you know what type of thermocouple (TC)=3DA0y=
ou have?=3D
=3DA0 Is the TC bare or=3DA0protected by a thermowell?=3DA0 A type S,=3DA0p=
latinum-=3D
rhodium TC should be bullet proof for life in a kiln.=3DA0 The following is=
o=3D
nly in the case that what=3DA0you have is a=3DA0type K TC,=3DA0which does d=
rift o=3D
ver time due to corrosion (oxidation) of the metal.=3DA0 IF the thermocoupl=
e =3D
is bare, and the wire diameter has changed due to oxidation, that accounts =
=3D
for the "drift."=3DA0 Type K TCs are not particularly expensive, around $25=
o=3D
r so for 1/8-inch diameter wire, plus S&H. See http://www.wardburner.com/ot=
=3D
herproducts/pyrometerscontrollers.html=3D0A=3DA0=3DA0=3DA0 BTW, you do see =
why it's=3D
good to look at the cones near the end of the firing, right?=3DA0 :-)=3D0A=
=3DA0=3D
=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0 Dave Finkelnburg=3D0A=3D=
A0=3D0A=3D0A________________=3D
________________=3D0A=3D0AFrom:James Freeman =
=3D0AS=3D
ent: Wednesday, July 29, 20098:56:02 PM=3D0ASubject: Help with thermocouple=
p=3D
roblem?=3D0A=3D0A=3DA0I believe I have=3D0Aa bad thermocouple, but wanted t=
o run th=3D
e problem by you folks.=3D0A=3D0A=3D0A=3D0A
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