[volt-nuts] 732A and Prologix received

[Don@True-Cal]

Tellurium Copper is usually not used for a device's terminal posts but used as the lead wire due, as you say, for the malleability to crimp well and flexibility. The point I was making is to use the same interconnect test lead material throughout as the DUT terminal posts. The 3458A and the 732A both use Beryllium Copper alloy making that type interconnect lug or plug the best choice to minimize the dissimilar metal EMF or Seebeck voltage. The 34420A uses pure copper rather than an alloy terminal and for the same reason, minimal Seebeck voltage is realized with a pure copper interconnect. 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.

The NI website had this chart that quantifies the Seebeck voltage best:
"When two, dissimilar metals are joined a voltage is created. This voltage is known as the thermal electromotive force (EMF) or the Seebeck voltage. The Seebeck voltage is dependent on the temperature of the junction and the composition of the metals joined. The specific metal-to-metal junctions result in specific temperature coefficients (µV/°C), also known as Seebeck coefficients. The following table lists the most common metals and their respective Seebeck coefficients."

Junction		µV/°C
Copper-Copper		<0.3
Copper-Gold		0.5
Copper-Silver		0.5
Copper-Brass		3
Copper-Nickel		10
Copper-Lead-Tin Solder 1-3
Copper-Aluminum	5
Copper-Kovar		40
Copper-Copper Oxide	>500

Granted, Gold and Silver are the next best choice, and is certainly why they are satisfactory, but using either warrants a more critical temperature gradient issue. If your measurements were satisfactorily convincing, than you probably had no appreciable junction temperature differences. 

Don Johnson

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of acbern at gmx.de
Sent: Monday, August 25, 2014 5:37 AM
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] 732A and Prologix received

I have used the pomona spades, mainly to interface the low emf pomona banana cables to binding posts. I have stopped this, reasons being, they are large and worse, that the pomona spring loaded insulation tube that covers the banana plug conductor uses such a strong spring that slowly the plug works its way out of the spade. this btw also happend to me when I used the pomona low emf binding posts together with the pomona low emf banana cables. overall I m not happy with these. 
so, due to lack of options, I changed to self-made twisted shielded pair of high grade teflon/kapton silver plated copper cable with gold plated copper spades (crimped). I use them not only with the 3458a but also with nanovolt meters. these have higher resolution and accuracy in low level measurements than the 3458a. emf voltages were never an issue with these cables if properly used. I have posted some results doing 34420a stabilty measurements on the pmel forum, and the results are convincing (purpose was actually not to test the cables but the stability of the 34420a, but the emf issue is a part of this of course. we use the 34420a to do low voltage precision measurements on thermal converters where the full scale signal sometimes is 1mV). 
that btw also relates to don's statements below, I do not concurr with his comments about copper telurium as cable and spade material and so on. this material, as stated here many times, is used because it is machinable, for copper spades one would not use it. the 34420a factory cable uses copper cable and copper spades, not telurium-copper. if there was a problem, it would be worse with the 34420a than with the 3458a because of its low level ranges. and again, I have not seen any problems in a chain of (output to input):
1.copper-tellurium post from e.g. 8 digit calibrator 2.crimped copper spade, gold plated 3.silver plated tsp copper cable 4a.crimped copper spade to copper-tellurium post or 4b.soldered copper connector(34420) my consistent results over more than a year using them.



> Gesendet: Montag, 25. August 2014 um 06:33 Uhr
> Von: "Orin Eman" <orin.eman at gmail.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] 732A and Prologix received
>
> On Sun, Aug 24, 2014 at 1:46 PM, Don at True-Cal <truecalservices at gmail.com>
> wrote:
> 
> > Randy & all,
> >
> > You have correctly concluded that some (maybe not all) of your measurement
> > problem is thermal EMF being added or subtracted in series within your
> > measurement interconnect. This thermal EMF is generated at the junction of
> > dissimilar metals when accompanied with thermal gradients between the test
> > lead and device terminals. You have to eliminate both the dissimilarity of
> > the metal junctions as well as minimize the thermal differences. The
> > terminals of the 3458A as well as the 732A are Beryllium Copper so you want
> > to use the same test lead terminals. Forget the typical Tin plated lugs or
> > even Gold plated as both are not Beryllium Copper and constitute dissimilar
> > metals. The best solution (as usually the most expensive) is to use a set
> > of
> > Fluke 5440A-7005 (48") cables. I also have just as good results using the
> > much more flexible Pomona 11174A (lugs end always stay connected to the
> > 732A) or 11058A with more convenient shielded banana plugs. The Fluke cable
> > has the added Guard built in but be sure to also use a Guard lead with the
> > Pomona cabled. The Guard lead does not need to be low thermal EMF. DIY
> > cables is usually not a good idea because the lead wire to terminal also
> > constitutes just as critical of junction. The above cables use Tellurium
> > Copper wire which is usually hard to find and hard to crimp properly and
> > NEVER solder.
> >
> 
> 
> 11058A and 11174A are discontinued at Keysight.  However, Pomona 5295 spade
> to banana cables are available (5295-36 at Mouser et al) and claim that
> they are designed to minimize thermal EMFs.  Datasheet is here:
> http://www.mouser.com/ds/2/159/d5295_1_01-51722.pdf  Any comments on these
> as an alternative?
> 
> Orin.
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