Dear Mr. Pettis,
I would like make some interesting clarifications about the Ohm uncertainty:
The BIPM lists on its KCDB page all National Metrology Institutes (NMI), i.e. their capabilities and uncertainties:
http://kcdb.bipm.org/When you search for the German PTB, or the US NIST, you will find the hint, that the Ohm is relative to R(k-90), that is the von Klitzing / Quantum Hall Effect (QHE) standard, not relative to S.I. Ohm!!
These NMIs either use a DCC, Direct Current Comparator, which gives 0.2ppm or less uncertainty, or the CCC, Cryogenic Current Calibrator, which allows comparisons as low as 0.004ppm uncertainty, directly to the QHE! See listing for the NIST in the attached document!
Explanation of the DCC and CCC bridges used, can be found here:
http://www.nist.gov/calibrations/upload/tn1458.pdfAccording to that, it's very well possible to make measurements /comparisons with much lower uncertainty than 0.1ppm, even outside an NMI, and without direct access to a QHE.
Therefore, this Wekomm resistor, which Dave received, and which is a prototype only, really had been calibrated to 0.1ppm uncertainty, by this Bavarian calibration lab! Dave published the Wekomm calibration document, where you can find further details:
The capability of the lab relies on their (decades old) GenRad 1444A-10kOhm standard, which is the predecessor of the IET / ESI SR-104.
This is an ultra stable artefact, much more stable than the usual SR-104, and which has been calibrated many times directly at the PTB, so its stability of about 0.01ppm/yr. has been characterized since a long time.
They also use an ultra precise bridge, an MI 6010B, which obviously allows direct 1:1 comparisons below 0.1ppm level.
(Neither do I know that bridge, nor how it compares to the 242D bridge of Mr. Pettis.)
There is obviously a demand by the NMIs, especially our PTB, for more stable working-standard resistors, than these Thomas type 1 Ohm and the SR-104, which are mostly too unstable to reach the 1e-9.. 1e-10 comparison region of the QHE and CCC bridge.
The Wekomm resistor, when a precise thermometer and T.C. characterisation will be implemented in the future, is a candidate to improve these standard resistors by orders of magnitude regarding stability. Dave already mentioned, that our German PTB is involved somehow.
I had an interesting discussion with 'engiadina', who is the design engineer of this Wekomm resistor, who confirmed that in detail.
It was a big surprise for me, what are the weak points of the current Ohm metrology, and what seems to be possible with these upcoming Wekomm resistors, implementing Vishay BMF technology.
Anyhow, in 2018 the difference between the QHE and the S.I. will probably vanish, so that the S.I. Ohm really will be precise to exactly zero ppm. requiring also better working standards.
Frank