[volt-nuts] Resistance standard

[Rob Klein]

   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