none of these results are "astounding".....
If someone is inclined to tie up their equipment for LONG term testing.....then ok....+/- 2 ppm would be "somewhat impressive"....
Keep in mind the REF102C has an "enhanced" stability rating, after "168hrs stabilization period".....
A two day test shows very little....as I can get a piss poor MAX6350, on a piss poor perfboard layout....in NO enclosure, to keep +/- 1.5 ppm on my home bench DM3068....across 2 days....and that ain't no 3458A (nor even close)....
If you guys want transfer standards....
www.voltagestandard.com makes some fairly decent units.....with some realistic quotations on "specs"....
At some point, when doing this kind of design, you have to take a step back and set a "realistic expectation".....
I have played with parallel REF102C, as shown in Ti's applications schematics.....it does help noise.....but again I am not sure at what point you try to make something "better" than stated in the datasheet, and then at what point you start going off into volt-nut land and wind up with a wholly impractical design....which you can't profit from.....
At some point you will spend as much as one would just buying a JJA primary standard.....which you will need to verify your own little standard against, at some point anyhow.....
When you get down to these kinds of numbers....then every last little detail matters.....you might even consider "shock mounting" the components, to avoid harmonic resonances from cosmic rays impacting the poor little ref....and other such nonsense....which obviously Awesome14 has done....with his "heatpipes".....it just doesn't make sense, from an practical standpoint....that one should spend such vast resources and effort to try and build such a thing.....I mean what is it's real practical use?
At some point, like I said above, you will have to have it sent out for comparison against a primary standard....and that will cost more than you will ever net back from such a thing.....
I've been down this road, and gotten to the point of diminishing returns......no sweat....we all go a bit nuts sometimes......it's a lot of fun, but I don't think you would ever be able to provide such devices, on the open market and turn a profit.....
I would be fun to have a community effort to design an easy DiY EEVblog voltage standard.....NOW that sounds like fun, because we could all measure our "identical" standards against eachother....and log the results....which would actually be a HUGELY useful set of data to take academic indicators from.....imagine a loose comparison of a few dozen identical standards, against all the varying conditions each individual "lab" has.....that sounds like a nice statistical effort and might actually lead to some discovery, for those that don't have any experience in this field and want to learn....and even those who think they know alrady