HZ series wins in the stability department for reference purposes.
How did you deduce that? VHP101 has a lower guaranteed TC from 15 to 45C than HZ and the same shelf stability specification. Or are you saying that in your experience that HZ outperform the VHP series drift wise?
I didn't deduce it. Vishay states it in their HZ datasheet.
"When combined with the hermetic sealing and oil filling, the
HZ Series resistors become the most precise and stable
resistors available. They are used as the most precise
secondary standards for ultra precision metrology. "
Also, I was strictly referring to stability/load life of the resistor, in reference conditions, not TCR. Since most reference resistors will most likely only be used ~25C, one has to look to those specs.
If you want to compare Vishay's voodoo specs, HZ still wins. Look at the graph Figure 4 on each datasheet and across your 15C-45C zone. HZ Typical TCR curve is 0.05 ppm/C while VHP typical TCR curve is 0.3 ppm/C. Sure, you might find differing specs elsewhere.. but that's one of the spots you can get an apples to apples comparison of specs.
VH series has shelf life of 5ppm after 1yr and 10ppm after 3yr, while the VHP and HZ series both have shelf life of 2ppm after 6 years.
I guess you didn't read my post immediately prior to yours? The VH 10ppm/3 years spec is a maximium whereas the 2ppm/6 years spec for the VHZ and VHP series are typical figures. The latter may be better/worse or identical to the VH series but there is no way to tell from the datasheet.
While it doesn't state 'maximum' in the VHP or HZ series datasheets, it also doesn't state that they are typical values either. Normally to take something as a 'typical' value, it has to state that. Has anyone confirmed that the 2ppm over 6 year shelf life is a typical spec? Even so, I'd much rather have a typical 2ppm/6 years than a MAX of 5 ppm/1 year All. Day. Long.