[volt-nuts] Resistance standard

WB6BNQ wb6bnq at cox.net
Wed Dec 16 12:03:24 UTC 2009

Hi Rod,

The modification in the 750A is not terribly involved.  My friend, who was
responsible for the Navy’s primary voltage standard made the mod, but I do not
remember the exact details.  It involved disconnecting the standard cell R string
and jumping some other point I think.  I will pin him down on the particulars.

He also was responsible for a number of improvements to the Fluke 731 series.  He
modified the 750 after he characterized a large number of 731 units and kept the
best four for his use.  One of his ideas was to parallel the four 731's so the
output was a voted value.  So the four he kept had equal but opposite temperature
coefficients such that the error was basically cancelled.  I believe that Fluke used
that, among his other ideas, in one of their later units.

Yes, the Fluke resistors were of the card variety.  The oil comment regarding the
Vishays was aimed at the non-hermetically seal units that you were considering.  I
would be interested in your findings on the Pomona posts.


Rob Klein wrote:

>    Bill,
> I want to apologize for making a snap judgement of your background and capabilit
> ies.  In all honesty
> that was the way it came across to me.
>    No worries, cause no offence taken. Perhaps I should have given more
>    information about my background in that first post.
> In my opinion, I think you may have wasted your money on the lead compensator.
>    Nah, I picked it up for $25. Combined shipping with a HP3457A, so it's
>    no great loss in any case.
> You would be better
> served obtaining a Fluke 750A Reference Divider.  It is an extremely good, stabl
> e fixed division
> divider.  There is a non-destructive modification that was done at the Navy's Pr
> imary Standards Lab
> to make the comparison section use a 10 volt reference instead of the standard c
> ell as was
> originally envisioned.
>    Interesting! Do you have any more information about this mod? A link to
>    a paper or something?
> The way the metrology world maintained the Volt [...]
>    Well, yes, I know how it was done. It was a rhetorical question in
>    reply to your comment about 0.01% not "being considered a standard".
> My comment about the oil is depending upon the oil used, it could affect the non
> -hermetic resistor's
> composition.  Fluke's resistors were sealed with a shellac compound of some sort
> , so no, the Fluke
> 720A resistors are not just some resistive material open to the elements as such
> .  The same goes for
> the Fluke 750A.  They could be, but then an awful lot would have to be known abo
> ut the oil and it's
> affects.  The same problem with a potting material of some sort, particularly wi
> th regard to leakage
> paths.
>    I highly suspect that the resistors Fluke used are the card wound,
>    shellac sealed types we all know and love. Not going to open the
>    tank to take a look, though :-)
>    The resistors I'm using are not "just some resistive material open to
>    the elements" either, they are molded in epoxy. Using a good,
>    pure oil, I see little reason why they would be affected.
>    I'm thinking about dropping the resin potting idea completely and just
>    have one ensemble as is. That way I can compare the behaviour
>    of the canned ensemble agains a 'naked' version, which is more
>    meaningful.
> Regarding binding posts and such, many manufacturers specify a temperature that
> a component can
> withstand before destruction.  Does that really say the stability of the plastic
>  (or whatever
> material) binding post is going to retain its insulation abilities ?  Many bindi
> ng posts, with
> regard to leakage, are literally junk even without heating.  Before you select a
>  binding post get a
> sample, mount it to a panel and apply a kilovolt (you do have a Fluke 335D ?) th
> rough a 10 meg Ohm
> resistor between the post and the panel monitoring the current flow.  There shou
> ldn't be any but
> that is obviously unrealistic.  At least you can measure it.
>    The ones I got are [1]Pomona 3770's. Their insulation is polycarbonate.
>    I'm going to dig out my GenRad megohmmeter this afternoon
>    and make some measurements. I'll post the results later.
>    Regards,
>    Rob.
> References
>    1. http://www.pomonaelectronics.com/pdf/d3750-3760-3770_101.pdf
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