[volt-nuts] What's All This Low Thermal EMF Test Lead Stuff?
acbern at gmx.de
acbern at gmx.de
Wed Aug 20 04:57:09 EDT 2014
if you buy a voltage source that is cal'ed to 6ppm you do not end uop with a factor of 10 (60ppm). the fatcor of 10 is often used to be on the safe side, but in high percision cals 10 is not achievable anyways. some mil standards call for 4, but what you should do is to analyze the error propagation and then determine the likely final uncertainty (you would do this with a certain confidence level, say 95% which is usual). so you would look at the different contributors (temo variation, aging since call'ed, error due to emf voltage and so on). you would add these up by the rss (root sum square) method.
there is a lot of literature out there for this, also free on the net, and it would not be possible to describe the details here, but I would suggest to do this, as the real error will be much better I am sure.
> Gesendet: Mittwoch, 20. August 2014 um 03:03 Uhr
> Von: "Stan Katz" <stan.katz.hk at gmail.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] What's All This Low Thermal EMF Test Lead Stuff?
> If you reserve those pure copper bananas strictly for infrequent cal. of
> something like an HP3458, or other transfer standards in your lab, they
> may be good for some years.
> I envision the "beer nuts" to be a rather relaxed group of individuals,
> who are perfectly satisfied to know the alcohol content of their favorite
> brew to no better than +/- 60ppm ;-)
> Why +/- 60ppm? A selfish reason. I plan on bringing home a beer-nut-NIST
> volt for my Fluke 731B using one of these standards
> They're only good to 6ppm according to the seller. Rule of thumb is primary
> must be ten times the accuracy of secondary, that leaves me with an
> uncertainty of +/-60ppm....does seem a bit much....oh well, if necessary,
> I'm willing to be the only beer-nuts member.
> On Tue, Aug 19, 2014 at 4:12 PM, Todd Micallef <tmicallef at gmail.com> wrote:
> > Stan,
> > I recently picked up some of these ...
> > http://www.douglasconnection.com/Furez-TSTWP30NP-Bare-Copper-Banana-Plug-Connectors-Pair-FZTSTWP30NP.htm
> > They are a little pricey and are made for 12ga wire. I think they are OK
> > for semi-permanent use. A lot of use will probably scratch the soft metal.
> > I plan on trying them with my Keithley 181 plugged into a low thermal
> > scanner.
> > The original Keithley cable will be difficult to terminate. I may have to
> > go with crimped spade lugs.
> > Also, I am all for joining a beer nuts group.
> > Todd
> > On Tue, Aug 19, 2014 at 2:07 AM, Stan Katz <stan.katz.hk at gmail.com> wrote:
> > > 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
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