In wtxplc.com e.g. there is a vishay prec group glossary that under shelf life declares complete reversability by drying/baking, which would also indicate no hysteresis effect.There are also other docs. Of couse, not to be confused with distructive life tests such as pressure cookers.
Generally important is to keep the resistors under 10mw load to avoid drifts.
Sorry, on my hermetically sealed, oil filled VHP202Z, I measured a pronounced hysteresis of 5ppm after heating them to 125°C.
Therefore, that has nothing to do with humidity inside, obviously.
This technology is similar to DMS elements (resistive strain gauge), which also suffer from hysteresis effects
I was able to remove that shift only by temperature cycling.
Clearly, I always have treated the resistors for 'shelf life' mode only, P < 10mW.
Measurements of voltage drop on two resistors, relative to each other, one at constant temperature, one in an oven/peltier heater/coooler, powered by a precision voltage such as 732a or 5440/5700/5720 or 4808, using the very high linearity of the 3458a adc allows precise measurements of small drifts. I also use this methode to calibrate other resistors from the 10k, which saves cost. the error calculation shows surprisingly small errors piling up as you move away from the 10 k. It is a pitty agilent has not specified transfer accuracy of res measurements.
Yeah, that is no easy measurement, and you have very sophisticated gear, haven't you?
I 'only' use the 3458A for such measurements.
I could demonstrate a transfer stability of 0.2ppm, over 20min. and at constant temperature of +/- 0.1°C.
It must be noted that the z-foil such as the z201 is specifed to 0,2 + 0,6ppm/k max over the full military temp. Range, so if you have measured 1ppm, it was out of spec., the 0.05 ppm arround room temp is usual, getting to above 0,5 is pretty unlikely.
Well, Vishay in the past published several too optimistic specifications, with very low T.C.s in the describing text
Reading more carefully revealed that those were typical values only, and in the small table on the first page, at the bottom, they always specified +/-0.2 ppm/K + 2ppm/K max. spread for the T.C.
Therefore, my resistors of 5 years ago were well inside the actual spec, which was the best one available.
When I complained about those resistors, Vishay at that time could not deliver better guaranteed T.C.s, as they obviously could not control the process.
You are right, lately they again specified resistors ,VH102Z, I think, with 0.6ppm/K max. spread.
But if this is true now, and whether they cancel that spec again, I don't know.
The spec looks odd, and the parts are not oil filled. Strange.
I had the 10k calibrated to 2ppm, and hope the lab was correct, at least i paid well for it, unfortunatelly. I have no other way to verify at that level, other than redoing it in a year's schedule. I would certainly be interested in some cross comparisons. I have also decided to spend the money at least for the 10, 100, 1000 and 100 k hermetic resistors to get less drift. Got a good deal.
Well, that explains how you got such a low uncertainty.
Definitely, the VHP resistors are very stable over time and very predictable over temperature, even with high T.C.s , so they really make very good precision standards, and the calibration is well worth its money. Although, an ESI SR104 would be more convenient.
Perhaps you really have got better technology inside your resistors, meanwhile.
Which T.C.s did you measure on your resistors?
I'm also very keen to make a comparison with your gear!
Just tell me place and time.
Frank