Ian Currie on sun 14 jan 07
On 14/01/2007, at 8:32 AM, Steve Mills wrote:
> Pyrometers measure pure temperature as seen at the moment you view
> them.
> They cannot accurately measure accumulated heatwork, and despite
> various
> modifications to replicate that function I doubt they ever will.
This is quite correct, though accuracy of the measurement are another
problem.
I've been promoting the use of cones for years in my teaching,
whether or not you use other temperature or heat measurements. To
illustrate Steve Mills' point, it is possible to mature a cone 10
glaze at a temperature we normally associate with cone 5 or 6, if you
hold the kiln at top temperature for long enough. I can drop large
Orton Cone 9 at under 1200 deg C. (2192 deg. F.) by soaking at around
this temperature for 8 or 10 hours. There are reasons to do this for
certain glazes.
Another illustration is what happens in computer controlled electric
kilns as the elements age. Eventually you find your glazes being
overfired. This happens if your computer controller faithfully
attains the programmed temperature but taking longer time to get
there. The extra heat work at the end of the firing is overfiring
the glaze in spite of the fact that it is still (eventually)
attaining the programmed temperature.
One point I have not seen made on this thread on pyrometers is the
matter of exactly what temperature is the pyrometer measuring. It is
NOT measuring "the temperature inside the kiln". The pyrometer
measures the temperature DIFFERENCE between the hot end and the cold
end (the "reference junction") of the thermocouple. (One ambiguity
I've had trouble resolving here is whether the "cold end" is the end
of the actual thermocouple or the end of the extension leads that
connect the thermocouple to the meter. This web page seems to
indicate that it is actually the end of the thermocouple itself:
http://www.omega.com/techref/thermoref.html
If this is the case, it is obvious that as the outside of the kiln
heats up, if this is also heating up the "cold end" then the
temperature reading on the meter will be reading low. If for
hypothetical example the temperatures of both the hot end and the
cold end are heating up at the same rate, there would be no rise
recorded on the meter.
And I agree with some other commentators on this thread, the absolute
accuracy of the measurement is often less important than using it to
see if the temperature is rising or falling. Especially when firing
in reduction you want to know as soon as you make a change to the
settings what the temperature is doing. This often will give you
some indication what is going on. For example if you reduce the fuel
but the temperature starts rising faster, it's a good bet that you
were using excess fuel and had some degree of reduction. I use a
digital pyro with 0.1 degree accuracy that indicates immediately if
the temperature changes. As far as the actual accuracy of the
temperature measurement, I'm not interested as I am firing by the
cones, and sometimes with new glazes I'm using draw trials too.
These are taken out at the end of the firing to see how mature the
glazes are. Dangerous in an electric kiln unless you turn off the
power!
In summary, we need to take control. The "set and forget" philosophy
is very problematic. I realise that some have no alternative. If
you have to use this method then a computer controller that you
calibrate with cones (and re-calibrate regularly) and learn how to
use is the best way. I would not use a kiln sitter shutoff even for
a bisque!
Regards
Ian
http://ian.currie.to
My glaze books are available at my website
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