|
Posted by Bob Masta on October 21, 2007, 9:45 am
Please log in for more thread options On Sat, 20 Oct 2007 14:29:27 -0700, seasa42@yahoo.com wrote:
>How long should your thermocouple be? Or is that to personal a
>question ;)?
>
>I am fixin' to get a new pyrometer, and I a new thermocouple will be
>needed to go with it, but I don't know if I should get a 12, or 8
>inch.
>
>I have a very short one now, it might be like 6". my kiln is gas,
>18.37 cu,ft, top loader 46"wide, 28"depth, 27" high inside.
>
>I wondering if all this time my readings have been inaccurate due to
>the length of my thermocouple.
>
>Any advice on types of pyrometers, anything would be priceless.
>thanks
>Sa
>
In addition to Moose Hunter's good advice on 1 inch clearance
and protrusion, I would add that while some folks use naked
probes, I am a fan of putting the probe in a protective ceramic
sheath. That should help the life of the thermocouple by keeping
it away from the corrosive effects of kiln gasses. Also,
thermocouples can be "poisoned" by these gasses such that
they give an erroneous reading, so the sheath should reduce that
as well.
You will see differences of opinion on what type of thermocouple
to use. The usual advice is to go for the platinum R or S types.
These tend to be pricey, as you might imagine. I have been quite
happy with type K (Chromel-Alumel), except for when I was just
starting out and used too-thin a wire gage on home-made
thermocouples (welded with a car battery and a carbon rod
from a dry cell). Those died in less than 2 dozen firings.
And note that whatever type of thermocouple you use, you probably want
to have the readout away from the kiln, in a cooler area. That's
especially critical with older-style pure-analog pyrometers, since
they typically contained no compensation for the temperature at
the meter. This is called "reference junction" or "cold junction"
compensation and is ABSOLUTELY CRITICAL because there are always at
least TWO thermocouples in every system, like it or not: The one
formed by the two dissimilar metals that you put insert into the kiln
(the :hot junction") and the one (or usually two) where those same
wires connect to the readout device (the "cold" or "reference"
junction). The measured voltage is the total of all the junction
voltages in the system, with the cold junction voltage opposing
the hot junction. So if you have an uncompensated pyrometer that
was calibrated for an ambient of (say) 30C and it is really at 40C,
your readings could be off by 10C, the equivalent of a cone or more.
Modern electronic readouts are probably compensated
(but it wouldn't hurt to check), but you still probably want
to keep the electronics away from a hot kiln.
Which brings up the issue of thermocouple type again,
since you can only use the same type of wire if you need
an extension (or you would have to contend with the
junction temperature at the connection). The wire gage
is unimportant here, so you can have a short heavy-gage
thermocouple in the kiln, connected to a long thin-gage
extension. But with platinum, even the extensions can
get pricey.
Best regards,
Bob Masta
DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!
| 
XML site map