30 years ago I watched a major safety NRTL (you may be able to guess
which one) weld thermocouples - be glad you didn't get the job doing that.
Basically, the whole apparatus consisted of a pencil stub, a high current
power supply, a bare contact, an open pool of mercury and a light bulb. The
contact. Mercury pool, and light bulb were all in series with the power
source. The "technician" first cut and stripped the ends to be soldered.
Then he wrapped them around the cone shape of the pencil stub and gave them
a couple of twists. Next he cut off all but one or two of the twists. Now
the fun part. He took one end of the thermocouple and touched it to the
first contact. The with the other hand he dipped the twisted end into the
mercury pool. A small arch at the pool and the light bulb glows brightly for
a second. An viola a nice round thermocouple weld, and if he did everything
just right - a nice breathing technician, which wasn't that hot of an idea
anyway given the mercury vapors. The made bazillions of them and I don't
remember a time when the technical got himself across the open contact and
mercury pool with out the shorting thermocouple, but I always passed on the
opportunity to try it myself.
Gary
From: Peter L. Tarver [mailto:[email protected]]
Sent: Tuesday, April 27, 2004 8:42 AM
To: PSTC
Subject: thermocouples
I weld my own thermocouples and yesterday reflected on discussions in this
list regarding acceptability of thermocouple welds to third party safety
agencies. There is an apparent conflict in the lore of thermocouple welding
and the fundamental theory of operation for thermocouples that I am hoping
someone can shed some light on.
The lore -
Some number of posts have stated that some safety
certification agencies have problems accepting data taken
with thermocouples welded in anything other than a low
oxygen environment. The idea being that the presence of
metal oxides in a thermocouple bead will cause inaccuracies
in the resulting temperatures indicated by a measuring instrument.
This idea is supported by the (several years old) Omega Temperature Handbook
in my office.
The theory -
Thermocouples produce a voltage difference at their free
ends, based on the Seebeck Effect. Seebeck showed that a
wire with ends held at different temperatures will have a
small potential difference along its length. In a thermocouple, two
dissimilar metals with joined ends held at a figurative common point produce
different potential difference along their lengths, allowing a potential
difference to exist at their free ends through which a temperature at the
common point can be inferred.
(The common point is figurative, because practical thermocouples have
"beads" formed by the welding process of finite size and the wires rarely
leave the bead at the same
location.)
The conflict -
If we assume:
* the thermocouple bead has homogeneous distribution of the two
metals (I say two for convenience, even though many thermocouple wires are
alloys) and their oxides, nitrides, etc.,
* that there is negligible migration of the oxides,
nitrides, etc., into the wires,
* the wires leave the bead sufficiently close to one
another that the distance separating them can be ignored,
* the bead is sufficiently small that radiative and
convective heat loss can be ignored, and
* the bead is sufficiently small that it can be thought of
as "saturated" at the temperature of the intended measurand heat source,
the bead should also be ignored. If the bead is small
enough, The temperature determined will be at the location
where the wires leave the bead, rather than in the bead
itself or at the surface it touches (the intended measurand source).
How, then, does the presence of oxides in the thermocouple
bead have any effect on the temperature measured?
It may turn out that the above assumptions are part of the problem I'm
having. It's also true that theory and practical physics often depart from
one another to some degree.
Whatever help anyone can offer would be appreciated.
Regards,
Peter L. Tarver, PE
[email protected]
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This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
Visit our web site at: http://www.ieee-pses.org/
To cancel your subscription, send mail to:
[email protected]
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Ron Pickard: [email protected]
Dave Heald: [email protected]
For policy questions, send mail to:
Richard Nute: [email protected]
Jim Bacher: [email protected]
All emc-pstc postings are archived and searchable on the web at:
http://www.ieeecommunities.org/emc-pstc
