Hello,
I have many recorded data. But it will not help you for your individual devices.
Humidity plays only a role if you have certain packages e.g. plastic DIP package
or SMD-ceramic package (like the LT LS8-package) directly soldered to a epoxy board.
Other packages like the TO-46 or TO-99 are nearly insensitive to humidity.
Ok I have one very drifty LM399 device which is also sensitive to humidity.
But I fear that this old and desoldered device is no longer hermetically tight.
If you look at the measurements: (dayly measurements with timescale (x-axis) in days)
Every reference behaves very individual.
There are some common effects: spikes that are related to contact problems during measurement.
Shifts that are related to movement of the ageing box, since the devices are not soldered into the PCB but used with sockets.
On the drift: how do you distingquish the drift of the LM399s from the drift of the measurement system.
Ok: I have other measurements where I measure 2 LTZ1000s and can see how the measurement system drifts against those.
But in the end those LTZs also drift by -1 .. -2 ppm/year normally. (and if I have a accident then they drift several ppms in 1 second).
My strategy would be:
make own measurements with the best instrument available. At least one measurement per day e.g. averaging over 1-5 minutes.
calculate a sliding standard deviation over the last 42 days (1000 hours).
Determine the device with lowest standard deviation and use that as your master reference.
Of course you also can try to measure differences between references.
In this case you have to determine the pair of references with the lowest standard deviation.
But there are more parameters which can fluctuate so this really helps only if your instruments (absolute) stray in 10V range is larger than in 100mV range.
with best regards
Andreas