[volt-nuts] Resistance standard

Charles Black cblack at centurytel.net
Sat Jul 23 17:14:47 UTC 2011


The traditional oil for standard resistors and oil baths is Mineral Oil. 
The cheapest source of Mineral Oil USP  for small quantities is out 
local grocery store at about $5.50 per pint. It is a highly purified 
paraffin oil. If you need a large quantity then check out Exxon Marcol 
72 which is available for about $12 per liter from Measurements 
International. I am trying to find a much better price since I could 
potentially use 157 liters if I am able to get a used bath that I know of.


On 7/23/2011 3:03 AM, m k wrote:
> Hi,
> 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.
> Mike
> 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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