Ken Chin-Purcell on wed 30 mar 05
Earlier a Clayart regular said:
> you would never want to use silicon carbide shelves in an electric kiln
> Silicon carbide is an excellent conductor of electricity,
I did a quick check on the 'net, and it seems that SiC products come
in a wide variety of conductivity, although none of the numbers I saw
seemed "excellent" (e.g. like metal).
So I whipped out the 'ole multi-meter and tried to measure the
electrical resistance of my Advancer shelves. The meter measures up
to 20 million ohm. The elements checked out at 13.1 ohm and across my
wet finger about 300 thousand ohm, which is what I've measured before.
No amount of scratching or pressure with the probes produced a
resistance reading, i.e. the shelf resistance is above 20 million ohm.
This is no matter how close the probe tips are. Even if they a
hair's breadth away on the shelf the resistance reading is still off
the scale.
So Advancer shelves do not seem to conduct a measurable amount
electricity. This corroborates Marshall Browne's findings at Skutt.
I am very comfortable with using them in an electric kiln.
I'm not a cheer leader for Advancers, but I wouldn't want someone to
not get them out of some unfounded fear of zapping themselves. For
high fire work there are some big advantages to them. I'm very glad I
went with them.
-- Ken Chin-Purcell
www.bungalowpottery.com
Ken Chin-Purcell on wed 30 mar 05
On Wed, 30 Mar 2005 16:13:12 -0600, Louis Katz wrote:
> I would suggest a more sophisticated test. Measuring the the resistance
> by measuring the amps at the voltage you are interested and calculating
> the resistance.
As far as I know that's exactly what Marshall's testing at Skutt did.
They tested safe. I just did a quick sanity check on my own, and it
jibes with his findings. These are first hand experiments, not
heresay, and I'm at least convinced.
-- Ken C-P
Bob Masta on thu 31 mar 05
Ken Chin-Purcell wrote:
> No amount of scratching or pressure with the probes produced
> a resistance reading, i.e. the shelf resistance is above 20
> million ohm.
> This is no matter how close the probe tips are. Even if
> they a
> hair's breadth away on the shelf the resistance reading is
> still off the scale.
I don't use Advancer shelves (yet!), but I do use SiC rods
as heating elements. For those who have never seen
these, they have an active length that goes inside the
kiln and gets hot, and bonded to the ends of this are
lengths of a different grade of SiC that protrude through
the kiln walls. The very ends of these sections are
metalized where the power cables connect.
With the probes on the active section, the resistance
is above 20 Megohms no matter how close the tips,
just as Ken finds for the shelves.
But measuring the resistance of the entire bar by
putting one probe on each metallized contact end,
the resistance is about 4 ohms. (The hot resistance
is about half that, judging by voltage and current
readings while running at temperature.)
So clearly the bar is pretty conductive, just not on
the surface. On a broken bar, the interior of the
active portion appears to be slightly more silvery
than its surface, which is black, maybe slightly lighter
than a carbon block. With both probes on the
broken end, the resistance varies all over the place
from about 10 to 1000 ohms depending on exact
placement of the tips.
So, it's possible that the shelves are also very
conductive in the interior. It's also possible that
the surface becomes a much better conductor
at firing temperatures. But I still wouldn't hesitate
to use them in an electric kiln. Consider that a
shelf simply touching a conventional wire heating
element in one spot isn't going to do a thing... it would
have to bridge between two spots on the element that
are far enough apart to have a significant voltage
between them. If it bridged across the whole
element (where the wires enter/exit the kiln),
AND if the hot shelf happens to be as conductive
as my heating elements, then it would simply
pop the breakers.
I imagine things like this could happen if a post
broke and the shelf crashed into the wall of
the kiln, but then I guess there will be more
serious issues to worry about, like damaged
elements and wares.
Best regards,
Bob Masta
potsATdaqartaDOTcom
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