[volt-nuts] Resistance standard

Tue Dec 15 05:40:20 UTC 2009

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.

First off, what is your purpose for such accuracy ?  What do you intend to do
with these ?

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.

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.

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 ?  Those are not considered
standards by any means.  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 problem is these composition type resistors will be homogeneous in their make
up and most likely they will all exhibit the same or very similar temperature
coefficient.  Fluke did it by making their resistors out of different types of
metal that had different temperature coefficients.  Thus they were able to
compensate over a wider temperature range by mixing the different metal wires
that made up the resistor.  Your only choice with a homogeneous composition is to
provide a constant temperature chamber.

On the one hand you talk about buying expensive resistors then decide to get the
non-hermetically sealed models because they are cheaper.  Using an oil filled
container is not going to help if the resistor is not hermetically sealed.  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.

“LEAKAGE” is going to be your “other” big problem.  The best environment is going
to be dry clean AIR.  Very hard to get !  Besides inside a container, it is also
important outside of the container.  ERGO, not going to happen.  Things used to
“POT” the inside of a container is only going to make matters worse.  The biggest
point for leakage is the connection post insulation material; most are junk !
The second biggest point will be dirt between the connection posts.

You mention “oil” as a filler.  Even the oil is going to cause “LEAKAGE” issues.
The only reason you would use a carefully selected “OIL” is to provide a thermal
time constant.  Yes, even Fluke used “OIL” in some of their arrangements.
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.  The oil that I
would use is Johnson & Johnson’s BABY oil.  For a hobbyist it is more then good
enough as other environmental effects would most likely mask any ill effects from
the BABY oil.

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.
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.

Don’t mean to come on so hard, but some of your comments tell me you are way out
of your class at the moment with no experience.  So before you spend large
amounts of money I would suggest looking at sites like http://www.nist.gov and a
number of top of the line companies who make resistor standards and see what
publications, app notes, and other engineering papers might be available for
study.  It would not hurt to look around for reference books, tables, etc., that
show the properties for various metals and insulation materials to grasp their
contribution the final product.

Bill....WB6BNQ

Rob Klein wrote:

>    My first post to the group, and it's Ohm-nut, rather than Volt-nut, but
>    I hope you'll forgive me for that :-).
>    The nutty idea is this: To build a Volt-nuts grade resistance standard.
>    Or, actually, two, perhaps three.
>    The first one is quite simple, but rather pricey: I have ordered four
>    Vishay VHP202Z's, at $ 96 each (ouch!). Expected to be delivered late
>    february/early march. They will be placed in a series/parallel
>    configuration to deliver a 10kOhm resistor that should easily rival a
>    Fluke 742A,
>    probably be even better. When finished, I intend to have it calibrated
>    at regular intervals and use it as my house standard.
>    The other two are a bit more involved, but it will be interesting to
>    see the results.
>    For these two, I will use 9 each Z201's from Vishay. These use the same
>    chip as the VHP202, but are molded, rather than hermetically sealed.
>    Also, I will use 0.01% types, rather than the 0.001% VHP's. These are
>    much cheaper (the 18 I need to make two standards cost less than the
>    four VHP's!), but also less stable over time.
>    To overcome the stability problem, I am looking at two ways to *make*
>    them hermetically sealed. The first is to house the 9 (three in series,
>    three
>    sets in parallel) in an RF shielding can, fill this up with oil and
>    solder it shut.
>    The can I have in mind is a PCB mounted type, for which I shall have to
>    design a board. The PCB area inside the can will be solid copper,
>    extending
>    some way beyond the outside, so I can make a proper seal. Connections
>    to the outside world will be through glass-sealed, solder mounted
>    feedthrough capacitors of low capacitance. After mounting the resistors
>    and a thorough cleaning, the whole thing will be baked at ~85°C
>    overnight
>    to get rid of any moisture, then filled with oil and soldered shut.
>    This assembly will be placed inside a sturdy metal box (Hammond model
>    [1]1457K1201), which will hold four low EMF binding posts (Pomona
>    3770).
>    For the second solution, I want to use much the same method, but rather
>    than filling the can with oil, I want to fill it with epoxy or maybe
>    polyurethane resin.
>    This is a much simpler solution, because there will be no need for the
>    feedthrough caps and no need to solder the can shut.
>    However, of course, I am aware that neither of these resins will
>    provide an actual hermetic seal, since they *will* absorb some
>    moisture. I am curious
>    though, as to how much of a positive effect can be gained from this
>    method, as the shear volume of the resin, as compared to the quantity
>    use to mold the
>    actual resistors, should at least greatly diminish any effects of
>    moisture.
>    So, if you're still with me after this, I would very much like the
>    knowledgeables of this group to comment on these ideas. Are they
>    feasible? What potential
>    pitfalls might I have overlooked? What oil to use (as an avid cook, I
>    know my olive- from my sesame oil, but I haven't a clue what type of
>    mineral oil to
>    look for :-( ).
>    Thanks,
>    Rob Klein.
>
> References
>
>    1. http://www.hammondmfg.com/pdf/1457K1201.pdf
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