John,
I agree with you regarding the seebeck effect and its lack of importance
with welded junctions, with some reservations; we must remember that if the
thermocouple leads are shorted behind the junction, that point effectiely
becomes the junction.  If one shorts the leads of a type K (chromel/alumel)
thermocouple with a copper wire, for instance, there are two junctions
formed - chromel/copper and copper/alumel.  The voltages of the two juctions
add, and I believe the resultant voltage will be the same as if the leads
were directly shorted (comments anyone?), assuming the equal temperature at
both junctions.  Just as with a "twisted" junction, however, the result is
only as reliable as the connection, and since type K wire is rather
unsolderable...

In addition, I'd like to point out that the exact temperature at which the
melting point "plateau" (due to heat of fusion) is observed can be
significantly impacted by local variations in solder alloy, as well as
superheating.  And thermocouple error is certainly not to be disregarded.

Bottom line is I prefer to mount thermocouples with a SMALL bead of
high-lead solder (such as Pb97.5, Ag 1.5, Sn 1; mp 309C)and failing that
(such as mounting to non-metallic substrate) with a non-conductive epoxy
material (again, small amounts).  I've also had great success mounting
thermocouples in small holes in epoxy materials using high-temp thermally
cured epoxies (such as Omegabond 100 or Omegabond 200.  Although these are
also electrically conductive to a degree, there is no detectable effect on
accuracy of the thermocouple.

Fritz Byle

Rockwell Automation/Allen-Bradley
At 08:29 AM 5/29/97 -0500, you wrote:
>	You are right to a point.  The Seebeck effect come in to play with
dissimilar metals in the conductive circuit.  If you are using a
thermocouple with good metal to metal connection, the solder can't penetrate
this junction.  It is true that the solder give an alternate path for the
current, and does impact the measurement, but the effects are minimal.  I
have clearly seen the reflow of solder, which shows up on the profile as a
short plateau, at 181 to 184.  Is the error do the that fact that the
thermocouple is mounted in the solder or error in the thermocouple itself?
Personally, I don't know, but since that error is within the error
associated with thermocouples, I don't care.
>	On the other hand, if you are in the habit of twisting your thermocouples
together, than this could be a really big problem.
>
>John Guy
>American Competitiveness Institute (EMPF)
>317-655-3673 x130
>[log in to unmask]

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