Bruce Girrell on fri 12 jan 07
Ivor Lewis wrote:
> If the EMF of dissimilar joined wires can
> be given in millivolts to three decimal places can they be both
> accurate and unreliable ?
Ivor, you address the difference between accuracy and precision. A highly
precise measurement, one that can resolve small incremental changes, is not
necessarily accurate and, in fact, could be wildly off the mark. An accurate
measurement is not necessarily precise. It is providing an output that can
be verified by other means to be representative of the "true" value of
something.
Bruce Girrell
Bob Johnson on fri 12 jan 07
I'm wondering how accurate/inaccurate other people's digital
pyrometers are. I fire to cone 6 in reduction, which should be in the
neighborhood of 2232 F, but my digital pyrometer (LHT20, made by Love
Controls) always reads about 100 degrees low. It's near the top rear
of the kiln and shielded by a long ceramic tube---which, I suppose,
may cause it to read low. I hadn't expected it to be that far off,
however, so the first time I used it, I over fired by 2 cones.
Bob
Roseburg, OR
Ivor and Olive Lewis on fri 12 jan 07
Dear Erik Harmon ,
Thanks for your note. Perhaps we could ask other people to expand on the =
reasons for inaccuracy or unreliability.
My experience in gathering data for the finding the temperatures of =
phase changes in Steel was that to know precisely the temperature at =
which events were triggered and completed my specimens, which were =
conductors of electricity, were part of the thermocouple circuit. This =
would be difficult to do with ceramics.=20
The location of my cones and the position of my thermocouple are about =
300 mm apart so there can never be any correspondence that would relate =
my experience to the published temp values of individual cones. Nor =
would I expect to be able to judge energy input which is what cones =
gauge from the moment they commence deforming with temperature, which is =
an indicator of the flow of heat.
One perverse thought. If the EMF of dissimilar joined wires can be given =
in millivolts to three decimal places can they be both accurate and =
unreliable ?
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Eleanora Eden on sat 13 jan 07
Hi Bob,
My pyrometers wander a bit. I didn't know, it snuck up like scalding frogs.
I finally had a kiln in which wares were badly over-fired. Since that I have
been using cones religiously, at peeps and on shelves. Keeping a careful
record shows they wander up and down.
If I were you I would NEVER rely exclusively on the pyrometer.
Sorry about your overfired work, hope it isn't ruined. Cones are surely
much cheaper than ruined work, re-firing, battered nerves, etc.
BTW, my pyrometer probes stick out of the ceramic tubing about an inch.
Have you tried to pull back the tubing a bit to expose end of probe?
Eleanora
>I'm wondering how accurate/inaccurate other people's digital
>pyrometers are. I fire to cone 6 in reduction, which should be in the
>neighborhood of 2232 F, but my digital pyrometer (LHT20, made by Love
>Controls) always reads about 100 degrees low. It's near the top rear
>of the kiln and shielded by a long ceramic tube---which, I suppose,
>may cause it to read low. I hadn't expected it to be that far off,
>however, so the first time I used it, I over fired by 2 cones.
>
>Bob
>Roseburg, OR
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
--
Bellows Falls Vermont
www.eleanoraeden.com
WJ Seidl on sat 13 jan 07
Bob:
Re-read your pyrometer's instructions. Digital pyrometers are usually
calibrated to be "off" at one end of their scale, but accurate at the other
(can't remember where I read that). It's up to you to determine which end
of the scale is more important, and calibrate yours to suit.
And yes, the ceramic tube will cause it to be slightly lower as well.
Everyone wonders why Mel has preached for years about "learning to fire
without all the fancy gizmos" (paraphrased, quotes mine). This is a good
example. Not to brag, but it only took me a few ruined firings to learn to
trust my instinct and fire by sight; not an easy thing for someone trained
to trust what the dial is telling them. The pyro has proven it is always
"wrong" but the kiln and the work inside will tell you the truth. Once I
recalibrated mine, sticking it in the freezer showed me that ice actually
has a temperature of 86F , but it was right on at 2400F!
Operator error counts for a lot, too.
Best,
Wayne Seidl
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Bob Johnson
Sent: Friday, January 12, 2007 12:50 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Using a Pyrometer
I'm wondering how accurate/inaccurate other people's digital
pyrometers are. I fire to cone 6 in reduction, which should be in the
neighborhood of 2232 F, but my digital pyrometer (LHT20, made by Love
Controls) always reads about 100 degrees low. It's near the top rear
of the kiln and shielded by a long ceramic tube---which, I suppose,
may cause it to read low. I hadn't expected it to be that far off,
however, so the first time I used it, I over fired by 2 cones.
Bob
Roseburg, OR
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Lisa E on sat 13 jan 07
My digital pyrometer is off by close to 100 F. But I have been keeping a
log for every firing and just use my pyrometer as a guide to know when to
start watching my cones. According to my pyrometer, cone 6 is around 2000 F
which is clearly incorrect but I know that so I start watching my cones at
1980 F and onwards. Lisa
On 1/12/07, Bob Johnson wrote:
>
> I'm wondering how accurate/inaccurate other people's digital
> pyrometers are. I fire to cone 6 in reduction, which should be in the
> neighborhood of 2232 F, but my digital pyrometer (LHT20, made by Love
> Controls) always reads about 100 degrees low. It's near the top rear
> of the kiln and shielded by a long ceramic tube---which, I suppose,
> may cause it to read low. I hadn't expected it to be that far off,
> however, so the first time I used it, I over fired by 2 cones.
>
> Bob
> Roseburg, OR
>
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
--
Lisa E
Sunny Daze Design Pottery Studio
SunnyDazeDesign@gmail.com
Squamish, BC Canada
www.lisaelbertsen.com http://picasaweb.google.com/SunnyDazeDesign
Maurice Weitman on sat 13 jan 07
Bob Johnson wrote:
>I'm wondering how accurate/inaccurate other people's digital
>pyrometers are. I fire to cone 6 in reduction, which should be in
>the neighborhood of 2232 F, but my digital pyrometer (LHT20, made by
>Love Controls) always reads about 100 degrees low. It's near the top
>rear of the kiln and shielded by a long ceramic tube---which, I
>suppose, may cause it to read low. I hadn't expected it to be that
>far off, however, so the first time I used it, I over fired by 2
>cones.
I'm no expert here, but this is my take...
First of all, in Bob's case, his inaccurate pyrometer readings might
be due to a cold spot in his kiln. If the ceramic tube is properly
sized and placed, it shouldn't effect the temperature. And there's
no mention of the ramp used to reach cone 6... if it took a long time
to reach temperature, it would be possible that enough heat work was
done to drop the cone without having reached temp. But I'd first
suspect thermocouple placement and calibration.
Calibration is very important. Even if the thermocouple is dead-on
accurate, it's placement or other factors may influence its accuracy.
In general, a pyrometer is made up of two elements- a thermocouple and a meter.
The thermocouple is the device that translates the heat into
electricity. The most common (such as type "K"), and least expensive
(heh!), thermocouples are not very accurate, and worse, tend to drift
over time.
Better 'couples (such as type "S") are made with (very) expensive
wire (mainly platinum) and are much more accurate "out of the box"
and will barely degrade over time. They are a better investment if
you'll be using them for many years.
Now... here are two ways that pyrometers are used: the simplest is to
get a visual display of temperature, and the second is for that
temperature reading to be used by a controller (a computer of sorts)
to control the firing in a kiln.
In the first case, the best use for the pyrometer is to get a ROUGH
idea of current temperature. More useful in an atmospheric kiln,
though, is it's ability to convey trend (rate of climb or drop).
In that case, one should make note of the reported temperature and
the more-accurately observed reading and make a chart so that future
firings can be monitored more accurately. Still... as the 'couple
continues to drift, this will be a moving target, and other remedies
might be appropriate.
In the case of a kiln with a (digital) controller, the accuracy of
the thermocouples is very important, because the controller will use
their readings to determine how much heat to supply without human
intervention. This means that a badly failed 'couple can cause a
badly fired kiln, with annoying to disastrous results.
BUT... most "modern" kiln controllers have features to compensate for
a 'couple's errors: one can enter an offset for each thermocouple it
monitors based on empirical measurements (witness cones) and when the
offset gets too high, replace them.
When one fires a controlled kiln, it is important to "check in" from
time to time (especially at critical points -- soaks and the end) to
be certain that all is going as expected. In case of failure, it may
become necessary to turn the "auto-pilot" off and take control of the
rest of the firing like your grandfather did.
Not all controllers can use both ("K" and "S") type thermocouples, so
it might not be an easy option to switch, but I would suggest that,
especially in a digital controller, one only use type "S".
I hope there's something in here that's helpful.
Regards,
Maurice
Bob Johnson on sat 13 jan 07
Thanks for the message, Eleanora . . . and others who have responded.
I wasn't sure whether I just had a bad pyrometer or other factors
might be influencing the readings.
I do want to assure you that I never have tried to use a pyrometer in
place of cones. Rather, I use it as a means of (a) making sure I get
a gradual, consistent temp rise, (b) judging when to start body
reduction, and (c) alerting me, at the end of the firing cycle, when
to start checking the cones. I was just surprised that it would be
100 degrees or more off.
I am reluctant to try sticking the probe directly into the chamber.
Are they meant for direct exposure to the atmosphere of the kiln?
Especially in reduction? Now that I know the pattern, I can just do
the math of adding 100 degrees.
Oh, and yes---there were some pots that didn't fare so well in my
over-firing, but the load wasn't one of those being torched at the
last minute before a show. I have a fairly new ~18 cu ft kiln, and
I'm shifting my emphasis (with ash glazes) from ^10 back to ^6 to
save wear, tear, & energy. As a result, everything I have been doing
for the last few loads is in the nature of an experiment---which, by
the way, is beginning to pay off with some beautiful results.
Best regards,
Bob
At 1/13/2007 09:36 AM, you wrote:
>Hi Bob,
>
>My pyrometers wander a bit. I didn't know, it snuck up like scalding frogs.
>I finally had a kiln in which wares were badly over-fired. Since that I have
>been using cones religiously, at peeps and on shelves. Keeping a careful
>record shows they wander up and down.
>
>If I were you I would NEVER rely exclusively on the pyrometer.
>Sorry about your overfired work, hope it isn't ruined. Cones are surely
>much cheaper than ruined work, re-firing, battered nerves, etc.
>
>BTW, my pyrometer probes stick out of the ceramic tubing about an inch.
>Have you tried to pull back the tubing a bit to expose end of probe?
>
>Eleanora
Ivor and Olive Lewis on wed 17 jan 07
Dear Bruce Girrell,
I suppose in the end, by convention, all temperature and other physical =
determinations we make have to relate back to the standards we agree to =
use for comparison. In common life they used to relate back to actual =
things you could lay your hands on, like lumps of gold or the distances =
between two scratches on a slab of rock, or a small seed, the time it =
takes for a pendulum to complete a swing cycle. Temperature, to begin =
with was, based on the melting of Ice and boiling of Water. The way a =
three sided pyramid made from mixtures of minerals behaves is just the =
same. Bends so far to an agreed point.
So, how long is a piece of string and under what conditions? See what =
the International Convention says.
Best regards,
Ivor
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