[volt-nuts] Resistance standard

[Electronics and Books]

Look at :
www.metrology.pg.gda.pl/full/2009/M&MS_2009_183.pdf

These are temperature regulated resistor compared to a standard.

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--- On Mon, 12/14/09, Rob Klein <rob.klein at smalldesign.nl> wrote:

> From: Rob Klein <rob.klein at smalldesign.nl>
> Subject: [volt-nuts] Resistance standard
> To: volt-nuts at febo.com
> Date: Monday, December 14, 2009, 5:01 PM
>    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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