[volt-nuts] Resistance standard

Roy Phillips phill.r1 at btinternet.com
Wed Dec 16 16:01:03 UTC 2009

It would be very interesting to have the details of his "improvements" to 
the Fluke 731's, if this is possible.

From: "WB6BNQ" <wb6bnq at cox.net>
Sent: Wednesday, December 16, 2009 12:03 PM
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Subject: Re: [volt-nuts] Resistance standard

> 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.
> Bill....WB6BNQ
> 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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