Barbara Dunshee on thu 13 oct 05
Dear ClayArt,
I'm mostly a lurker, via the index, but have had nice correspondence with some ClayArters here
and there, mostly regarding small updraft kilns. I posted a message a few months back about how
uncooperative my updraft was and soliciting advice about turning it into a downdraft. That's all
changed. Now I'm wondering what I was making such a fuss about.
Much thanks to Roger Graham, who wrote about making one's own oxygen probe in an article for
Ceramics Monthly (March 2003). I initially wrote to him for a little advice based on his article. He
was HUGELY generous with his time and knowledge regarding my particular set up and endured
numerous back & forth emails. Obviously I can't keep all his help to myself so wanted to share
with those who might have a similar set up who could glean anything from this.
I have a kiln SIMILAR to the Olympic torchbearer, run on propane. I initially started by putting the
auto oxygen sensor at the exit flue, trying various holding brackets to channel the fumes for
proper reading. But those wildly whipping flames coming out the top of the kiln made for wildly
whipping meter readings. I finally put the oxygen sensor into the 2nd pyrometer hole at the back
of my kiln. As luck would have it, I just inserted it and it was a perfect fit. The sensor part sticks in
there horizontally, and the back end, which needs fresh air, sticks out of the kiln, no bracket
needed.
Here's Roger's explanation: "The idea is to support the sniffer so the hot end gets an undiluted
flow of kiln exhaust, while the cold end remains sheltered from the nasty hot gas, but exposed to
the clean atmosphere."
I tried to find a used oxygen sensor through a wrecking yard but that was too time consuming.
Then a local auto parts store took the time to look in lots of boxes to find me a two-wire oxygen
sensor. He found a Bosch Premium Oxygen Sensor #12028. I think it cost me around $35. My local
wrecking yards were charging $25 plus for used ones.
The meter can plunge up and down quite rapidly, but as Roger pointed out:
"Further to the rapidly fluctuating readings from the sniffer, that's quite
normal. In an auto engine the onboard computer expects to read the composition of the exhaust
gases several times per second, and adjusts the
fuel injectors almost instantly to keep the fuel/air ratio correct. So the
sensor is made to respond very rapidly indeed to changes in the atmosphere.
If the sniffer reading flickers rapidly while you look at it, say
650/677/645/665/672 etc etc, that's just a sign that it's doing what it
should. Just estimate a likely midvalue (say 670) for the log book."
After seeing my set up via pictures, Roger suggested that I make sure I can accurately measure my
dampering. I WAS adjusting the 6 inch round exit flue with two soft bricks on either side. I could
measure the open gap in between them, but that made for difficult rapid adjustments. Now I slide
a 3rd brick into that gap and make 1/4 inch markings on the top of my kiln so one little push
makes and measures the adjustments. Sometimes a sixteenth of an inch makes the difference
between reduction and not reduction in my kiln. I'm getting much better success with copper reds
now. I pack lots of skirting around the bottom after I shut off the kiln to slow the cooling.
Another thing. The kiln now fires so evenly top to bottom, both in bisque and in glaze firing. So
much so that when I look in and see that the cones are drooping at different rates I do nothing
because I know they'll even up by themselves by the end of the firing.
I've run the kiln under this new system about 5 bisques and 5 glaze firings. After two miserable
years I'm sure this happy firing is all a fluke, and I will wake from my dream to the mini-
nightmares I'm used to, but I'm in lala land now.
Hope this info is relevant to somebody on clayart. Thanks again Roger. Would still be pulling my
hair out without your help.
-Barbara
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Barbara Dunshee
http://www.bdpottery.com
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