The best spec's that I could find said that the teflon insulated probes
were fine for continued use at 450F, and for intermittant use up to
500F. I have used teflon insulated TC's with teflon as a probe in the
bean mass of my poppers without any trouble.
The fiberglass insulated ones are good to a much higher temperature. I
uses this type for monitoring the post-heater, pre-chamber air
temperature, which gets much hotter than 500F. The spots that get close
to the heater wirres lose the color in the glass insulation, but the
glass stays put. Touches between the TC and the heater destroys the
As far as making the TC joints go, any solid electrical joint the can
survive the temperature that it is exposed to will work fine. The thing
that I worried about in setting mine up was the fact that by using a
disimilar metal (like solder) to make the joint that you would be
introducing additional thermocouples that might make your measurement
wrong. This turns out to actually be relatively unimportant, as long as
the TC wire 1 - solder - TC wire 2 junction is isothermal.
The reason this is so, I believe, is that the dominant voltage that you
measure is actually developed along the wire, and not specifically at the
junction. The is a very small voltage that exists between disimiliar
metals, but the workhorse of the TC is the fact that any material with a
hot and a cold end will develop a voltage difference. ( Think of the heat
as influincing the velocity of the electrons at each end -- at the hot end
they run away faster, so at equilibrium, there must be fewer around, --
the voltage you get then comes from charge accumulation). TC work because
the hot end - cold end voltage difference is different for the two metal
types that make up the thermocouple.
My setup confirms the unimportance of the specific junction material. I
use both spot-welded junctions, and tin-lead soldered junctions. Since
I measure the TC voltages with a straight voltmeter (about 22 microvolts
per degree F), I use a reference junction setup: two back to back TC's.
The voltage measured in that case is the temperature difference between
the two junctions. (If you want to use NIST calibration tables, for
example, this is the voltage they list, with one of the junctions cooled
with an ice bath.) But anyway, the voltage that I measure with the two
sensors at the same temperare (one is welded, the other soldered with
ordinary tin/lead) is really small less than the
equivalent of a couple of tenths of a degree. So the most important thing
is that the joint does not melt!! not so much what its made of.
On Fri, 24 Jan 2003, Jason Molinari wrote:
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