Hi enut11,
Expanded graph sections from the results above. All measurements are spot checks.
The blue line is Vz. Graph shows variation from test-to-test.
The red line is 10v output. Graph shows variation from 10v. I occasionally tweaked the 10v output to bring it back to 10.00000v.
Some of the graphs show off-chart excursions. These are simply missing data from when the Refs were traveling around Australia for the Aussie Cal Club.
I am zeroing in on Ref #5 as the best of this lot.
enut11
the variations of the 10V-output of your reference look unnecessary high. I have just one LM399-based 10V-reference which is permanently running and most of the time it is connected to a 34401A. I can't provide you with a fine documentation such as you prepared but I can tell you that my 10V output drifted 2ppm in the first year and 4,5ppm in the second year in absolute* terms.
The output drifts really slowly over time. During summertime with wild temperature swings in my appartment from 15 degrees Celsius to 30 degrees the 34401A shows variations of maybe 1,5 to 2ppm per day which provides a kind of overlay to the long-term trend.
*Absolute terms means, that the reference was 10V-2ppm at t=0 measured with a (calibrated!) 8508A from PTB's DC cal lab at the maker fair.
At t=1 year it was measured against a calibrated 3458A from PTB at the maker fair (10V flat)
At t=2 years it was measured against a calibrated 3458A from PTB at the PTB and at the maker fair (10V +4,5ppm).
Swings like 6ppm in one month only seem odd to me. As you wrote that you reset your reference after some time I assume your references contain a trimpot. Depending on the adjustement range and maker a trimpot could be a source of drift evil.
I would replace the trimpots by fixed resistors and would not modify the references anymore.
When you track your references and simply take notice of the adjusted value of 10,000 00 V at reset time you loose the drift information over the last period und you will be unable to compile a continuous drift chart.
You might consider recording the voltage BEFORE adjustment as well.
There is nothing special in my reference. The resistors used are just Yageo MF in TK25.
Best regards
try