[volt-nuts] What's All This Low Thermal EMF Test Lead Stuff?

[Stan Katz]

I'm a self described volt-nut-near-beer. I don't own a 732A/B, or an
HP3458. I do own HP3456 DMMs that are at the top of my instrumentation
pecking order.  I have all the necessary gear to calibrate these DMMs
according to ancient HP documentation. At the top of my cal. chain is the
731B, called out in HP3456 original documentation.  This hierarchy places
me in the near-beer, or junior member status of the group. I look forward
to being educated, and/or corrected on my understanding of the use of test
leads with precision instrumentation.

 I don't find much ancient HP documentation on test leads. The only
recommendations in the era of the 3456 back to the 1960's is to use as
thick a solid lead of pure copper wire as you can find, and insert the wire
into the drill hole on the banana terminal. If the copper is pure, and has
been properly cleaned, the thermal emf's on both identical length leads
should all be balanced, and cancel out. In any case, pure copper-copper
connections generate the lowest thermal emf. I  will agree that manhandling
16 gauge solid wire can be very inconvenient.

 The path I have taken recently is to order Nakamichi gold over copper
stereo banana plugs for my connections ( I deal in low voltage work
exclusively), as well as gold plated spade lugs to go under the banana
screw-downs. ( My budget ruled out gold over beryllium copper Pomona brand
spades. )  I will then experiment between the two connector types. As for
connections, it seems to me the best course is just to screw down the
banana plugs, or in the case of the spades, just crimp. I'll wing it on the
crimping, and see if simple tools can perform adequately. I would avoid
solder, since how can one form identical topological spots of solder on
every connection, deposited at the exact same place on each connection, and
ensure the exact same weight of solder, to the microgram, on each
connection. Since my modus operandi is to aim for balanced emfs, I think
solder is out. Of course, if one wishes to risk one's health, and that of
the family, one can track down a source of cadmium, and mix up a lot of low
thermal emf solder (cadmium solder is banned in the US, and EU)...not for
me.

Don't worry, I won't be using tin plated wire, I managed to pick up a spool
of silver plated 16 gauge wire for my investigations.  Am I on some other
planet for choosing the balanced thermal emf approach for precision test
lead applications, as opposed to going to great lengths to eliminate every
trace of emf?

Please forgive me if these emf discussions have been between engineers
working outdoors in the oil fields of North Dakota, or out on a North Sea
drilling platform. In that case, go for all the super low emf techniques
you can lay your hands on.

Good Luck,
Stan