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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