Randy, the crucial thing is definitely the JJ multi junction structuring, let it be a Nb-Al/Al2O3 Superconductor @ 4.2K of liquid He(4) or the HTC - YBCO @ 77K of liquid Nitrogen. Can't be done at home, and is for sure very expensive. A can of 50l liq. He is not that expensive compared to the cost of the JJ element and the rest of the electronics. At the Juelich institute, associated to the RWTH Aachen, Technical University, and my Physics Alma Mater, they have shown that HTC superconductors also can give a reasonable good accuracy of about 10^-8: http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf Anyhow, this is also not accessible for us as a homebrew standard, I think. Too much expensive gear for high vacuum, sputtering and structuring is necessary for the production of such a device. Frank