On 8/25/2014 9:54 PM, Don@True-Cal wrote:
Why?
Let me count the ways.You can never count on any Seebeck voltage to
be immediately offset, there are far too many variables.
...
> A set of
5440A-7002 (banana plug) cables comes with this calibrator
(5440A-7003 spade lugs for 5720A)
For those leads, Fluke says they have the advantage of "minimizing the
emf caused by temperature difference between the ends of the leads." I
don't have much argument with that - if there's a temperature difference
between the ends, you won't have an equal thermocouple offset, so
minimizing such effects makes sense.
But, since those leads use pure copper wiring ("Belden 8262 or 8719 or
equivalent"), how does having only the contact itself made of tellurium
copper (TC) make a difference? Shouldn't they be TC end-to-end, with gas
tight crimped connections? Does the tinning on that copper cable have an
effect? What's the Seebeck coefficient for a copper to tellurium copper
connection?
You said that "Tellurium Copper is usually not used for a device's
terminal posts but used as the lead wire..." But, Fluke does exactly the
opposite - those Fluke leads use TC connectors and are characterized
"while engaged in a five-way binding post of Tellurium Copper Alloy 145,
half hard." Since they use pure copper cabling, I'd think they'd work
even better with a pure copper binding post, since the offsetting
thermocouples would be in close proximity, and not at different ends of
the cable.
You earlier said "Any type of Silver or Gold plating on the terminal or
wire will introduce the undesirable dissimilar metal properties, both
at the plating junction and at the plating metal to DUT terminal." Now
you emphasize that by saying "If someone used a set of gold-plated
interconnects for this procedure, they would be laughed out of the lab
and the calibrator would be useless until recalibrated properly..."
But, the specs for the Fluke 5440A-7003 which you say are appropriate
for the 5720A state: "Connector materials: Copper space lug with gold
plating over silver plating." (Both platings!!) Is someone to be laughed
out of the lab for using the cables they're supposed to use? The
5440A-7002 specs aren't clear, but if they're not plated, are you
required to clean them of copper oxide for every use? It seems that
would be necessary, since a gold-copper thermocouple is ~0.5 uV/C, while
a copper-copper oxide one is ~1000 uV/C.
You seem to be stating one thing, but then giving evidence which
contradicts your claim. It seems that Fluke uses TC where there's
thermal mass, and pure copper where there's little, and uses plating in
addition.
Please clarify, this isn't making sense to me.
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