Overall's on sat 2 jul 05
What the hell happened???
I just bought a ten inch probe - reason being I'm firing in a regular 9" brick door kiln and will use it in the future in my 3" brick conversion kiln.
I've used two separate twelve inch probes in two separate kilns without incident. When I use mine yesterday for the first time, all was going fine. Started at 7am, began reduction at 8.9. At 3pm (9.5 on probe) I bumped up the gas to 3-1/2" wc after about an hour into the body reduction. That's when the glitch happened. The meter stopped reading atmosphere. I readjusted everything like it was before bumping up the gas and air and still nothing. After scratching heads and butts, I figured it had to be the probe.
We grabbed ceramic fiber and carefully pulled the probe out wrapping it quickly inside the thermal blanket to cool down slowly. We inserted a regular pyrometer into the same spot for some numbers to read. Yes, obviously I'm still learning about firing.
Kiln's atmosphere was cloudy so it was in reduction. Using the pyrometer for indications of temperature climb; I made adjustments to complete the firing. I watched the kiln's clarity and color of the flame from the peeps as the temperature climbed steadily. Top and bottom cone tens went down after twelve and a half hours total.
Back to the probe; it was FRIED on the inside. Flames from reduction had kissed that sucker to black blisters. I'll be calling Axner here soon. Not much they can do on July 4th holiday weekend.
Can anyone explain???
Also, why does the pyrometer read 300-400 degrees off from what is?
Kim
Houston
John Baymore on sat 2 jul 05
Kim,
As to why the pyrometer reads "off" from the actual temps.......
Your question was not quite clear to me. Doe you mean the pyrometer
function of the oxyprobe itself... or the reading on the OTHER pyrometer
you inserted?
If you mean the oxyprobe... which should be pretty accurate for
temperature...... then that was probably a good sign of the early
malfunction of the unit. Sometimes the joining of the wires inside the
tube can be faulty ...often due to mechanical shock... that can cause all
sorts of problems. They are pretty delicate units that don;t like a lot
of knocking around.
One question is how do you know it is 300-400 degrees off ? Do you
mean that the two units disagreed with each other?
IF... and that is a big IF..... the second pyrometer you inserted was an
analog type... then there are many reasions for the possible discrepancies
between the two or with any otehr method of measureing you used. Most of
the cheapie analog pyrometers that potters have are pretty inaccurate
units. Even the cheap digital are typically more accurate. (Assuming
they aqre in good condition.)
First of all... cheap analog pyrometers meter movements are accurate to a
certain percentalge of the full scale of the meter. Full scale on most of
these units is 2500 F....... so a 2 percent accuracy ...which sounds good
at first.... can mean a variance from "actual" of about 100 F (plus 50 to
minus 50 degrees).
Second as a thermocouple is used, it's electrical characteristics slowly
change.... called "drift". In industry they get recalibrated either
manually or automaticaly (by software). Combine the drift with the
accuracy of the meter... and things are headed "south" in a hurry .
Then there is the possibility that the cold junction of the analog unit is
not "compensated". Where the wires connect to the meter actually is
another "thermocouple". As the temperature of this junction changes... it
can affect the reading.
Then there is the question of whether the meter circuit was set up
properly. If the thermocouple is attached with extension whire... is it
the right type of wire? You can't typically just use copper wire for this
use. Then the circult resistance has to be set up correctly for the
extension wire. So this might be a culprit too.
Then we come to the type of thermocouple used. The typical type K that
most potters use (to not have to pay for the platinum thermocouples )
go non-linear in the upper reaches of the cone 9-10 range. Cheap analog
meters often do not have the scale set up well to compensate for this. SO
as the temp goes up toward the upper limit of the meter....... the
accuracy degrades.
Add a few of these factors together.... and you get the potential of
serious inaccuracy.
And remember that the pyrometer only gives you are reading of the
condition at the exact location of the PROBE used. It says nothing about
the conditions anywhere else in the chamber .
If it is a digital unit that you are seeing this with....... let us know.
BTW... see the archives from a long while back on pyrometers and type K
thermocouples and such. I (and others) posted a lot of info "way back
when".
best,
..............john
JBaymore@compuserve.com
http://www.JohnBaymore.com
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
"Please use compuserve address for any direct communications"
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