mzzj, your results are a bit dissapointing to say the least. The 25R results especially look terrible - Edwin Pettis claims his wirewound resistors typically drift less than 3ppm/year whereas yours appeared to take 5 years or so to settle down to < 3ppm/year.
Your results are well adrift (pun intended) from Vishay's claims. According to figure 11:
http://www.vishaypg.com/docs/49789/vfrguide.pdfBoth a 103R and 157R VHP101 moved less than 1 ppm over 8 years, in a random walk rather than the monotonic curves you measured. Perhaps more importantly, Vishay specify drift at typically less than 2ppm in 6 years in their datasheets.
So either:
a) VHP201Z drift performance is not as good as the VHP101 (I'd be surprised if there is much difference)
b) Your devices suffered some environmental conditions (shock, temperature extremes etc) either before, or after you received them, which caused large shifts in values requiring a few years to recover. What temperature ranges have they experienced during your tests?
c) You were unlucky (8 times or twice from a batch POV),
d) Vishay got lucky with their test samples.
e) Vishay published the results for the best performers out of the batch of many on test (surely not!)
f) Vishay's results are for parts that had been around for a year or three before they started the test
g) Vishay's test had been run for rather longer than 8 years and they only showed the last 8 years measurments (surely not!)
h) Some combination of the above or something else.
To be fair to Vishay, Dr Frank and zlymex had very good results with VPR hermetics WRT drift. zlymex had drift << 1ppm over several years, but they may have been well aged parts - see reply #30:
https://www.eevblog.com/forum/metrology/teardown-standard-resistors/25/The above resistors are also much higher resistance and it may well be that the extremes of values, high and low, may drift rather more than those in between. It may well *not* be the case of course, in the absence of more data.
Since long term drift performance of Vishay hermetic foil resistors is a key characteristic and selling point I assume they will have hundreds, if not many hundreds of parts on long term tests including each of the different models and parts from different batches. Given the importance of the drift characteristics I'm surprised that they seem to have been *very* economical on the amount of data they have made public. Perhaps I have been looking in the wrong place but I can't recall seeing results for more than a handful of individual specimens.
It would be marvellous if they would publish much more drift data but it clearly ain't gonna happen - commercially sensitive or embarassing perhaps.
Big customers may get sight of the data but surely small to medium customers will account for a significant percentage of their production given that these parts are expensive and relatively specialised. I wonder how many of the latter customers take Vishay's claims on trust and how many run their own long term and expensive qualification testing?
[EDIT] added comments on 25R performance.