[volt-nuts] Guidance requested.

[Charles Steinmetz]

David wrote:

> So please how best to proceed from here?

The Fluke 732A is by far the most accurate and most stable voltage 
reference you have.  There are only a few cal labs worldwide that can 
calibrate it to an uncertainty that does it justice (1/10 ppm).  Most 
good to excellent cal labs have 10v DC uncertainties from 1-10 ppm.

SO:  First, have the 732A calibrated by one of those few labs.  I 
strongly favor the Fluke US cal facility (Fluke Park Laboratory, 
Everett, WA, USA, NVLAP Lab Code 105016-0), with an accredited DC 
calibration uncertainty of 100nV at 10v.  However, it may be an issue 
getting it back to you still under power from there (IIRC, you are in 
the UK).  I maintain three 732As and calibrate each one annually on a 
rotating schedule, one every 4 months.  I built a shipping container 
with internal batteries that will keep the 732A powered for three days, 
if necessary, and an internal charger so all Fluke has to do is plug the 
shipping container in to recharge them.

If the choice comes down to a lab that can calibrate the 732A with full 
accuracy (1/10 ppm), but it will arrive back to you "cold," and a lab 
that cannot calibrate it to that standard in the first place, I'd choose 
the former.  In my experience, 732As generally retrace within 0.25 ppm 
if re-powered within a few days.  IOW, I recommend the Fluke US cal lab 
unless you have a trustworthy cal lab with 1/10 ppm uncertainty local to 
you.  (But although experience suggests that you can rely on the 
re-powered standard to be within 0.25 ppm of the calibrated value, you 
will have lost NIST traceability.)

So whatever you do, have the 732A calibrated *properly* before you do 
anything else.

Also, put the 732A on its own dedicated 2-3 kW "online"-type UPS.  If 
the UPS runs nothing but the 732A, it should carry you through all but 
the longest power outages.  Adding external batteries is a good plan 
(industrial "online" UPSs support this).  I have a backup generator, but 
even so my UPS system will run the 732As, my primary frequency 
references, and transfer DMM for more than a week if necessary.

Note that "calibrating" the 10v output of a 732A generally does not 
involve adjusting it.  Rather, the cal lab will tell you its actual 
voltage to 1/10 ppm, and you will calculate from that value when you use 
it.  I usually have a new-to-me standard adjusted the first time I have 
it calibrated, unless I obtained it with its full calibration history 
from an unimpeachable source.  I obtained two of my 732As this way, one 
I repaired.  I had that one adjusted at its first two calibrations.

Finally, note that proper calibration of a 732A is not cheap.  Mine are 
on a calibration contract, and I still pay over $1000 (plus overnight 
shipping two ways on 40+ pounds).  That said, the real value of a 732A 
is its calibration history.  Without a proper calibration and the 
calibration history, it is just another voltage reference that needs 
characterization before it can be relied on.

Best regards,

Charles