The question, does the LM399 have a tendency to drift up or down in voltage with age and heat cycles? Or is it completely sample dependent?
The reason for the question, I have a band new 34465a that tends to read 8-10ppm low, so I'm wondering if it may have been offset calibrated to allow for aging. I know this is well within spec, but curious non-the-less.
Virgin LM399 tend to drift randomly, I've salvaged Joe Gellers measurement on 78 EA National LM299, and that does not reveal any special tendency. See picture.
If LTs 399AH behave the same, I can't tell.
The pre-aged and monitored references in Keysights DMMs should be stabilized already, to meet the specified drift rate.
I measured several 34465A, freshly from manufacturing and calibration, and in the 10Vdc range, they all were spot-on (1ppm), after delivery.
You need to use ACAL, of course.
With FW 2.14, the 100 and 1k DCV ranges should also read within a few ppm, but no 10..15 ppm deviation any more, as in earlier versions.
My latest 34465A still is spot-on in the 10VDC range, after 1 1/2 years of use.
So, please check FW version, ACAL, and calibration report and its date.
Frank