masta@UMICH.EDU on sat 3 jan 04
Antoinette Badenhorst writes:
>Hi Guys.
> Over the past few years I've been working with
> thermocouples in my
> electric kiln that is made out of high fire wire ( guess it
>is the same material that the elements are made out of, just
>much thicker)
Thermocouple wires are really 2 different metals or alloys, and the
key is the junction between them. So no, they are not the same as
the elements, exactly. But the two alloys in K-type thermocouples are
Chromel (Nickel-Chromium) and Alumel (Nickel-Aluminum), so the
Nickel-Chrome is going to be quite similar to Nichrome wire. However,
the alloys for thermocouples are specially formulated so that they
conform to the published specs for the type (K in this case).
If your kiln or pyrometer uses a K-type thermocouple, then you
can indeed substitute other K-type wire as long as you know
what you are doing. The main thing to be concerned about is the
wire insulation near the hot kiln; not all K-types use high-temperature
insulation. Inside the probe part iself the wires are usually bare
but threaded through ceramic beads or tubes to hold them apart
except for the very end where they are welded together. So if the
insulation on the wire you obtain is able to withstand the temeratures
of the outside end of the probe, after the ceramic insulation, then you will
be OK.
You may also find that thicker wires last longer, but I haven't tried
experimenting with thin wires.
My own are 24 ga, purchased on Ebay from a guy named Frank Joyner
(djbjoy) several months ago. This was a lot of 5 thermocouples, each
7 feet long, insulated with high-temperature plastic that seems to be
holding up just fine. He may have more to sell... I think I paid about
$20 for the whole lot. You may also see them for sale with various
metal sheaths, etc, but be careful of those. The metal probably
can't withstand kiln temperatures, and the thermocouple wire may
not be easy to remove from the sealed metal probe. (Sealed
stainless steel probes are standard industrial workhorses
for use in the several hundred degree F range.)
Note that K-type wire insulation is color-coded red and yellow, with
brown for the overall protective sheath. Other thermocouple types
use different colors. Watch out fot type J (red and white) which is
the most common for lower temperatures but won't work in a kiln.
The thermocouples I bought are really "kits" because they aren't
welded at the tip. This is easy to do yourself if you are a little
adventuresome. You need to get an old non-alkaline battery,
preferably a D-cell, peel open the metal case, and remove the
carbon rod that forms the core. This is easier on some batteries
than others, so if it seems too stuck in the first one you try,
try another brand. Or you may have some other source for
carbon rods.
Assemble the thermocouple with the ceramic beads or tubes, leaving
the two bare ends protruding. Twist those together with needle-nose
pliers and trim off the excess so you have a nice twisted section,
maybe 1/8 to 1/4 inch long. It's OK to have a short bare section of the
two individual wires leading up to that, as long as they are stiff
enough that they don't short out.
Now comes the fun part! Get a pair of jumper cables for your car.
BEFORE connecting to the car battery, grip the twisted thermocouple
wires with the positive (red) jumper. This may take a little fiddling to
get a good grip on the wires, and still leave the twisted tip exposed.
Gip the carbon rod with the negative jumper. Then connect to the car battery.
(Use an assistant for this, or prop the rod/wire ends apart so they don't short
while you are fiddling under the hood.) Now touch the twisted wires to the
carbon rod for a moment. You'll get a red glow and a little ball will form.
If you don't like the looks of it, try again until you get the hang of it.
If you consume the entire twisted section, you'll have to start over.
Insert the finished masterpiece in its ceramic sheath and you are
ready for business!
A word about "leads": These are lower-quality thermocouple wire
used to run from the thermocouple in the kiln to the pyrometer, etc.
For Platinum-based thermocouples (R and S types), it's a little
cheaper than running the "good stuff" over any distance. For K
types, you can just run the same K wire the whole way since it's not
that expensive to begin with. (You also gain a small amount of
accuracy, in theory.) You want to keep the pyrometer end of
the wires a good distance from the heat of the kiln, since the
connections there make additional thermocouples that reduce
the measured value by their ambient temperatures, relative to
whatever the pyrometer is expecting them to be at. This is not
trivial, since the error may be 10s of degees F, more than the
difference between cones.
Hope this helps!
Robert Masta
dqatech@daqarta.com
D A Q A R T A
Data AcQuisition And Real-Time Analysis
Shareware from Interstellar Research
www.daqarta.com
| |
|
