Here is more info about the gain error of the 3458a in the INL report. Sorry for the long story ...
Lets starts with the drift of my LTZ1000. Between 19.06.2019 and 11.06.2020 I made two calibrations against Fluke 7009N in Bulgarian Institute of Metrology (BIM). The absolute difference was 2uV (0.2ppm), but the measurement uncertainty was 21uV in 2019 and 34uV in 2020. The Fluke 7009N itself was calibrated in BIPM France in 20.12.2017. Based on these numbers, I think that my LTZ1000 is stable enough.
When I bought the 3458a at the end of 2020, there was -5.3ppm difference against my LTZ1000 based voltage reference.
Not bad for a DMM calibrated before 8 years with 8ppm year spec :-)
The 3458a and 34401a were adjusted with same LTZ1000 based 10V reference in period of around 6 months.
The 34401a was adjusted in June 2020 by me with the LTZ1000 ref. and the 3458a was adjusted by me again in 29 Dec 2020 after adding 100K resistor parallel to the 15K in A9 voltage reference board. Because of this modification, during the whole Jan 2021, I saw 25uV down drift with 3458a measurements against my 10V reference, because of the new temp set point which is normal.
In middle of the February I got an unexpected call from Bulgarian Metrology Institute. They ask me if I would give them my 34401 and 3458 for INL test, because they need equipment to test the Josephson Voltage Standard INL test procedures with as many DMM they can found. The price which I had to paid was very low so I agreed without any doubts :-) If I know that before, I had to skip the 100K resistor modification before the INL measurements in this short 1.5 month period.
So lets return back to the gain error:
I do not have statistics about the drift of the 3458a during Feb 2021, but I expect to be around 10uV. So if I sum the Jan+Feb drift + LTZ1000 calibration uncertainty this will make around 70uV possible gain error. Not near the 115uV, but big enough to explain this big gain error.
When the 3458a returned back from BIM in February, I adjust the 3458a once more time at the end of February and during March I measure about 7uV drift against my LTZ1000. The first 10 days of April, the drift is down to 2uV, so seems the 3458's voltage reference is starting stabilizing around 3 months after 100K resistor modification. But the full stabilization can take few more months.
My LTZ1000 will be calibrated in May 2021 again against the Fluke 7009N. But this time the Fluke 7009N itself will be calibrated against Josephson Voltage Standard during April 2021. So the uncertainty will be very low.
Regards,
Krasi