[volt-nuts] Resistance standard

WB6BNQ wb6bnq at cox.net
Wed Dec 16 06:02:18 UTC 2009


I want to apologize for making a snap judgement of your background and capabilities.  In all honesty
that was the way it came across to me.

In my opinion, I think you may have wasted your money on the lead compensator.  You would be better
served obtaining a Fluke 750A Reference Divider.  It is an extremely good, stable fixed division
divider.  There is a non-destructive modification that was done at the Navy’s Primary Standards Lab
to make the comparison section use a 10 volt reference instead of the standard cell as was
originally envisioned.

The way the metrology world maintained the Volt for decades was through tons of measurements,
cross-measurements and statistical evaluation of Standard Cells.  The standard cells used would not
permit any current to be drawn.  It was always done in a null configuration, i.e., a source compared
against many standards cells through a null meter; no current drawn.  These standard cells were used
in large groups and constantly inter-compared to 1.) determine the bank mean and 2.) To guard
against errant cells that misbehave and, obviously, inter-compared between other labs both
nationally and internationally.

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 about the oil and it's
affects.  The same problem with a potting material of some sort, particularly with regard to leakage

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 binding 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 ?) through a 10 meg Ohm
resistor between the post and the panel monitoring the current flow.  There shouldn’t be any but
that is obviously unrealistic.  At least you can measure it.

Again, I apologize for my snap judgement.


Rob Klein wrote:

> Bill,
> Thanks for your comments, they are appreciated.
> WB6BNQ schreef:
> > Rob,
> >
> > Fluke has over 60 years of real experience correctly make highly accurate  stable
> > resistors.  Actually they figured it out in the very beginning of their
> > business.  Without a serious metallurgic background, plus a few more disciplines,
> > I doubt seriously you would even come close.  However, there is nothing wrong
> > with trying.
> >
> But I am not going to try to actually make a resistor, I 'merely' want
> to build several resistance standards, out of
> commercially available ones. And I certainly do not have any intention
> of beating Fluke at their own game.
> > First off, what is your purpose for such accuracy ?  What do you intend to do
> > with these ?
> >
> Purpose? This is the volt-nuts group, isn't it? :-)
> I'm a self-employed electronics engineer with 'a thing' for precision
> measurements. I have, over the years, built
> up a nice collection of multimeters, calibrators, etc. and a resistance
> standard is going to make a welcome
> addition to that.
> > I see a number of problems that need to be addressed.  First, it does no good to
> > have the highest possible quality resistor if you do not have the proper means to
> > compare it to other items.  To maintain that level of quality, for that given
> > resistor, you would need to duplicate its’ measurement environment precisely,
> > i.e., temperature, humidity, etc.
> >
> While my workshop isn't a cal-lab, it is nonetheless pretty stable,
> environmentally. It has to be, in order to keep
> my pick-and-place machine happy.
> As I'm not aiming for sub-ppm precision, it will more than likely suffice.
> > Having a resistor, in of itself, serves no purpose without the proper equipment
> > to compare it against other items or to use it in a measurement process.  Items
> > like a highly accurate voltage/current source and a very good null meter (Fluke
> > 845A/B) are but a few things needed.
> >
> Let's see:
> - Fluke 720 KVD, check
> - Fluke 845, check
> - Various high stability voltage sources, check
> Add a recently aquired lead compensator (bought "as is" and not yet
> tested, so maybe not) and I think I'm
> reasonably well set-up.
> > Nonetheless, some of your intended construction ideas need to be reviewed.  You
> > talk about wanting to equal or beat Fluke, yet you only intend to buy 0.01%
> > instead of 0.001% resistors.  What’s up with that ?
> The VHP202's I'm buying *are* 0.001%, as I stated.
> >  Those are not considered standards by any means.
> Really? Then kindly explain how the metrological world was able to
> succesfully maintain the Volt for decades,
> using standards that deviated almost 2%.
> As long as the value is known and stable to within the required
> uncertainty limits, I'd almost say that anything
> could be considered a "standard".
> > Just series/paralleling a bunch of resistors is not
> > going to help you unless you know, precisely, the temperature coefficients are
> > for each resistor.  In order to play that game you would need to buy a whole lot
> > (like thousands) of them, not just a couple.  Then you spend an inordinate amount
> > of time testing the temperature coefficient for each and then mixing and matching
> > in trying to achieve a zero temperature coefficient.  Or at the least the
> > smallest variation of resistance verse temperature change.
> >
> The RTC of Vishay's Z-foil resistors is as low as it gets, not by
> selection, but by design! The series/paralleling
> is mainly to increase the long term stability
> > On the one hand you talk about buying expensive resistors then decide to get the
> > non-hermetically sealed models because they are cheaper.
> I intend to use *both*, so that I can build several different types, in
> order to compare their long term behaviour.
> It will be interesting to see just how good these non-hermetic types
> behave over time.
> > Using an oil filled container is not going to help if the resistor is not hermetically sealed.
> But the point is to use an oil filled *hermetically sealed* container to
> hold the non-sealed resistors. Fluke did this for the
> main resistors of the first decade of their 720 KVD, Tegam, formerly
> ESI, do it for their SR104, widely considered to be one
> of the best, if not the best, standard resistor there is. So it appears
> this idea might not be as daft as you make it seem.
> > The
> > same goes for using any other kind of fill material.  It is going to impact the
> > non-hermetically sealed resistor and its going to impact the leakage.
> >
> How, exactly, is it going to impact the resistor?
> > The biggest
> > point for leakage is the connection post insulation material; most are junk !
> >
> Hmm, this got me curious enough to dig out my insulation resistance
> tester. Unfortunately it seems the batteries
> are flat :-( To be continued ...
> > The second biggest point will be dirt between the connection posts.
> >
> I am well aware of the neccesity of keeping a clean shop.
> > However, it is not so simple as just pouring in some oil.  Oil has a multitude of
> > properties and the selection is not going to be an easy one.
> Eh, yeah, that's one of the main reasons I came here for advice :-)
> > You talk of baking the resistor/can combination.  What are the limits of the
> > resistors relative to heat ?  Heating it to 85c is going to change its
> > characteristics and would require you to re-test the resistor all over again.
> >
> I have already 'pre-cooked' the first batch of 9 Z201's for three weeks
> at around 60 °C and found a
> downward shift of maybe a few ppm, as compared to a 1kOhm reference; a
> 20 year old VHA412, from Vishay.
> The TCR of this older resistor is, at around 2 ppm/°C, a lot higher than
> that of the resistors under test and
> may have had a considerable influence on the measurement.
> I have not been able to find any references to changes in TCR, caused by
> temperature cycling, in these devices.
> The baking may also help relieving some of the stresses that might built
> up during soldering.
> > What about that low emf connector ?   You need to consider what that material
> > (insulation) will do when you “bake” it at 85c.  Most likely will ruin it.
> >
> The binding posts are rated at 115 °C max. operating temperature and are
> not going to be baked anyway.
> Best regards,
> Rob.
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