nuckolls.bob(at)cox.net
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 Posted: Fri Nov 07, 2008 10:11 am Post subject: Thermocouple switching... |
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At 11:02 AM 11/7/2008 -0700, you wrote:
| Quote: |
Bob and the group,
I've read and understood the book and list info on switching thermocouples
(i.e. importance of iso-thermally balanced couples etc.) and haven't found
the answer as to which is better between the following choices related to
type "K" to copper transition connections going into/out-of the "switch
box" (which is via a single "DB" style connector).
A) terminate "K" wires with "K" type pins (Expensive, but I already have
them. They are stamped, not machined, pins. ), terminate copper wires
with standard machined gold pins -> resulting balanced couples between "K"
wire types and gold at pin/socket interface. (how important is contact
pressure to a repeatable "couple" connection?)
B) terminate "K" wires with standard gold pins, terminate copper with
mating pins -> resulting balanced couples between "K" wire types and gold
at a machined pin crimp connection (my favorite at my current level of
ignorance... but I don't know the mechanical stability of "K" type wire in
this type of crimp - at least the "K" type wire IS stranded here, and I
also don't know the voltage/temp relationship between "K" wire types and gold)
C) terminate "K" wires with copper pig-tails via single crimp over both
wires (i.e. put both wires into one side of a butt splice and crimp)
terminate copper pigtails with machined gold pins, terminate switch copper
with mating gold pins. -> resulting matched couples are gas-tight crimps
between "K" wires and copper. (seemingly best for bi-metal interface but
worse for ensuring iso-thermal connections and logistics)
D) some other option I haven't thought of?
Anyone know the voltage/temp relationship between the two "K" wire types
and gold?
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Doesn't matter if you faithfully observe the doctrine
of parasitic couple error mitigation by thermally local
pairing. For example, you can carry a thermocouple
through a d-sub connector with minimal error because the
parasitic thermocouples of wire-to-pin crimps in each
path cancel each other's effects. The pin-to-pin junctions
cancel because they are equal voltages (close thermal
proximity) and opposite polarity with respect to each
other. You can take your wires though an all copper switch
box as long as there are NO DIFFERENCES in the number,
style and temperatures amongst the parasitic joints
inserted in both sides of a thermocouple pair.
The finish and base material of the various couples
need to be minimized but for the way we use thermocouples
in airplanes, errors introduced by an array of
paired-opposing parasitic couples are negligible.
| Quote: | I except that each of the above is likely to work "good enough" but hey,
if I have to choose anyway, I might as well pick a method I can argue is
the best of the options.
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Do what you need to do to meet design goals for
function . . . disconnects, switches, etc and keep
the errors equal in magnitude and opposite in polarity.
Bob . . .
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