[volt-nuts] Resistance standard

m k m1k3k1 at hotmail.com
Sat Jul 23 10:03:49 UTC 2011


I only recently joined this group, but in answer to rob, the best oil would be a long chain parrafin, can be purchased for vacuum pumps, also a solid encapsulation will shift as it ages, and that would put strain on the resistors, so they would need an initial wrap in a silicone perhaps? definately a very compliant cover of some sort to isolate the strain.


PS I am shortly going to set up some LTZ's for a small family of references to age and compare. Doing the sums a difference measure between each one and graphed will tell me which ones are the most stable, then after a year or so splash out and get one calibrated against a known source.

> Date: Fri, 22 Jul 2011 21:24:07 +0100
> From: robert8rpi at yahoo.co.uk
> To: volt-nuts at febo.com
> Subject: Re: [volt-nuts] Resistance standard
> Hi Rob,
> How is the standard going?
> I've had renewed interest as I just "found" a pair of 200 ohm VHA516-6 Vishay Z foils in a scrap biotech unit that came in an auction lot. Reviewing the thread I revisited the Fluke 742A comparison. While the VHAs are not as "accurate" as the 742A (50ppm for the 200R) They are MUCH more stable at the 720A power levels, 2 ppm for 6 years compared to 6ppm at 1 year for the Fluke.  So they should make a grat transfer standard. 
> Robert G8RPI.
> --- On Mon, 14/12/09, 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, 14 December, 2009, 22:01
>    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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