[volt-nuts] 732A and Prologix received
stan.katz.hk at gmail.com
Wed Aug 27 17:50:31 EDT 2014
As a non-scholar in metrology, I tend to want to simplify the results of an
academic debate to make the results of the debate useful to me. One thing
is clear from my web search, copper alloyed with Tellurium, or Beryllium,
still oxidizes, only at a slower rate. It appears that a big disadvantage
of Beryllium oxide is its very hard, a useful industrial characteristic.
Not a nice property in the metrology lab.
For the purists with lab grade metallurgical abrasives, polishing
equipment, and oxide removal chemistry, only, copper, whether alloyed with
Beryllium or Tellurium is all they want to see in their lead terminations,
and they will try to procure only instruments with copper/copper alloy
terminals. These purists will clean/deoxidize all connections in a
controlled atmosphere, and dry them in completely dry nitrogen, or other
inert gas. The connections will then be used immediately during the
measurement and the purists will have assured themselves that all
instrument to lead connections are tight enough to be oxygen free.
Can anyone mention any precision metrology instrumentation in current
production with pure copper, or copper alloy connectors? I have a 38 year
old Hp 740b, and yep, it's got Gold flashed connectors. This Gold flashing
seems to be a tradition.
The practical metrologist, will accept the fact that precision metrology
instruments are meant to last many years, and the terminals supplied with
these instruments must provide a stable thermal emf profile over time.
Thus, they accept instruments that come with gold plated terminals. The
Gold may need to be cleansed of debris, and degreased from time-to-time,
but the procedure is much simpler, and not as time consuming as oxide
removal. The leads to these instruments are meant to be used day in, and
day out, as well. Therefore, the leads also terminate in some form of gold
plated connector. The practical metrologist is fastidious in controlling
temperature, and air movement in the lab, to minimize thermal unbalance in
his/her lash up.
Is this a foolish simplification of the thermals debate, or can I feel
vindicated using my homemade, gold plated lead terminations with my old
740b, and 731b?
On Wed, Aug 27, 2014 at 3:45 PM, M K <m1k3k1 at hotmail.com> wrote:
> On 26/08/2014 16:05, Mike S wrote:
>> After some more research, I think I've answered some of my own questions -
>> Tellurium copper is used for binding posts, not because it has any
>> special thermal or EMF mojo, but because it machines much better than pure
>> copper. And, I suppose, because it sounds like it's extra special.
>> The Seebeck coefficients (uV/C, relative to Cu) of some relevant
>> Cu 0.0
>> Ag .2
>> Au .5
>> Yellow brass 1.5
>> Phosphor bronze 2.0
>> 63/37 solder 3.0
>> Sn 3.1
>> Stainless steel 3.1
>> Beryllium copper 5.0
>> Fe -12.3
>> Ni 22.3
>> Te -49.25
>> Based on the extreme Seebeck coefficient of pure tellurium vs. copper,
>> I'd expect that there might be some coefficient between Cu and CuTe (0.5%
>> Te), but I could find no reference. The relatively large number for CuBe is
>> interesting, since that's a common material for banana plug springs, where
>> one might expect the greatest temperature differential to occur in such a
>> connection (between the thermal masses of the binding post/jack and the
>> bulk of the banana plug). Heat has to flow a considerable distance through
>> the springs, very much more than when it flows through a surface plating.
>> The Pomona (Fluke) EM5295-48-0# uses CuBe (gold plated) for the spring
>> contacts. It seems there might be an improvement to be had by using the
>> older style pin plugs, where a solid pin was partially sliced into 4
>> sections which were then spread apart a bit to create tension. That could
>> eliminate relatively large thermocouples at a thermal gradient, and might
>> also be expected to have less thermal resistance, allowing the connection
>> to settle quicker.
>> But maybe not - I'm still not clear on how plated conductors behave in
>> this situation. For a high impedance voltage measurement where almost no
>> current flows, the gold plating may carry the signal, so there is no real
>> thermocouple (or more correctly, it's entirely contained within the
>> connector). But if that's the case, why fool around with special copper
>> connectors when common brass ones would be easier/cheaper? For current or
>> resistance, the signal would also flow through the base metal, so does this
>> have an effect (especially for tinned copper test leads, where there may be
>> a larger temperature difference between the ends???
>> Nickle is avoided as a contact material largely because it is subject to
>> fretting corrosion. Tests done by AMP (http://www.te.com/
>> documentation/whitepapers/pdf/p154-74.pdf) show that a Ni to Ni contact
>> can increase from 8 mOhm to 5 Ohms (sic!) in a short time due to this,
>> while Ag and Au plated contacts exhibit negligible changes.
>> Cu (with Be for better machining) seems to be used as the base material
>> for jacks/plugs to get thermal EMF cancellation to the wiring on both sides
>> (i.e. use copper everywhere except where there is a minimal thermal
>> gradient, like platings).
>> Beryllium copper is a springy material, Tellurium is the material added
> to aid machining without adding too much seebeck coefficient.
> I remember someone on this list a long time ago saying that NPL used van
> damme star quad cable and bought bulk quantities of spade lugs that they
> strip all the coating off before crimping and clean before each use.
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