OK, enough pissing contest. Let's get back to the original topic, and that is calibration of 732a at Fluke Everett Service center, and why they will give you 0.3ppm or 0.2ppm uncertainty of measure traceable to NIST.
Part of it has to do with NIST's side of the equation, and NIST being able to verify manufacturer's claims...
I had a nice long chat with a PhD metrologust directly at NIST this afternoon to clarify some questions about NIST's involvement of the certification process.
1. There is ONE legal volt in US, and that is the NIST version. If you do business internationally then you would specify something along the lines of traceable to SI thru NIST...but none of our needs are met by that. Fluke's volt is Fluke's Volt, and does not count as legal anything, at least for what we need. JJ-Array at point A does not necessarily legally equal JJ-Array at point B, and that is why there still has to be a means to inter-compare the two, either by a fleet of 732b's or transferring compact JVS systems between labs. A JJ-array is close to an intrinsic measure, but not quite...For instance in case of war and the GPS system goes down and we lose a common worldwide time/ freq reference, there will be extra uncertainty if we have to use a different means to keep excitation frequency of the JJ-As matched, but that's a different story.
2. NIST uncertainty (95% confidence) is spec'd to 5nV/V on their JVS. Fluke's uncertainty (95% confidence) their JVS is currently is 5nV/V PLUS extra 50nV if you want a traceable measure (That's where Fluke's measure spec uncertainty of 100nV or 0.01ppm on 10V scale comes from). NIST has the luxury of being the golden standard, so they don't have the extra added transfer uncertainty. They ARE the definition of the volt for US. That being said they still intercompare with all the bigger labs - everyone is trying to keep the standards in close agreement.
3. For our needs and doing business with our customers, we MUST have have the words "traceable to NIST" on both absolute value and uncertainty. "Traceable to Fluke" is worthless a lot of times. You will see in several places in Fluke documentation that yes you can get a measure at Fluke to better than 0.1uV/V U95 BUT that is relative to Fluke's Volt. The NIST traceability for uncertainty will be larger (that part appears in the fine print).
4. YOU CAN cut out Fluke and send your Fluke 732a / b to NIST for Measure and Cal, and they will measure it against the NIST JVS for 30 days, and give you a characterized report. The fee is $2311 and you pay for all shipping.
https://www.nist.gov/calibrations/voltage-measurements-calibrationsThe official reports will use 1uV/V uncertainty. For the rest of this post I will assume we are on a 10V scale so that 1uV/V is 1ppm, and 95% confidence.
NOW - As explained by NIST, for any known Zener Diode Reference, about the best anyone is going to do is about 0.2ppm uncertainty. They have seen it go as low as 0.1ppm uncertainty but that is rare. So the Calibration is quoted as the best you'll get is 0.19ppm, and the normal range of uncertainty on any Zener Diode Vref they've seen is around 0.1 to 0.5ppm
The limiting factor here is the noise of the Vref. This includes Zener Noise, Temperature, Humidity, Pressure effects on the system. So in the words of NIST: They watch your Vref
for 30 days at a measuring resolution of down to .005ppm uncertainty and then look at the overall drift and noise - and assign an absolute voltage value and 95% confident uncertainty to the Vref, which will be no better than 0.19ppm.
The other problem NIST pointed out: You can measure a 732a/b to some ridiculous fine resolution on a JVS and come up with some value. The problem is making use if that measured value: Unless you're going to use that measured value within a very short time frame, trying to get a transfer measure out of your Vref much below 0.1ppm uncertainty is like trying to measure bowl of jello. And forget transporting the 732 and expecting that measure to still be valid at a very small uncertainty.
5. Zener noise of 732a's is spec'd at 1uV RMS or less (typically way less than that when they age, ours are down around 0.35 or 0.50uV). 732b is spec'd at 0.60uV rms (.1 to 10Hz), and usually measure around about that. You are never going to get a Cal certificate better than 0.2ppm uncertainty because anything below that uncertainty is going to be a measure of mostly the noise of the Zener Diode Vref - and not a very stable or usable measure.
So the bottom line is YES you could get your 732 measured at NIST - but all that happens is you might get 0.19ppm uncertainty. Send it to Fluke and measure against their JVS and you've dropped all the way down to 0.2ppm uncertainty and saved a few bucks. Plus Fluke will perform an adjustment if you need it.
NIST is only going to work with the "Traceable to NIST" uncertainties with values they can verify. And that lower uncertainty limit - for now at least - is about 0.19ppm for a zener diode reference.
Have Fun!