[time-nuts] Aging rate of crystals

[iovane at inwind.it]

I learn from this discussion that the aging rate claimed by manufacturers would refer to the 
aging of the whole assembly, not the crystal alone. And for practical purposes that is correct. 
And even in the case of sealed assemblies, components other than the crystal itself may affect 
the overall measured drift.
So my original question on this subject seems to lose any sense, because we will never be able 
to measure the aging of the crystal alone (if any, at this point) and hence variations in the 
aging rate either.
Anyway some doubts of mine are not yet fully answered by this discussion, and I would appreciate 
your opinions.
Given a good quality sealed OCXO running in constant ambient temperature, what kind of aging 
curve  should one expect, a fluctuating one? (I understand that this might be the case, due 
to the interaction of known "intrinsic" aging factors having different timescales, as I've just 
learnt on this list. A "regular" curve would be hard to get).
May it happen that fluctuations in frequency due to "external" causes such as tides, geomagnetic 
storms, or so, and not actually affecting the "aging rate", are interpreted as fluctuations 
in the aging rate?

I'm running a simple test comparing an OCXO (option 04E on a military Racal 1992 counter) to 
rubidium (LPRO), the counter being counting the LPRO. The test is running since about two weeks, 
and I started recording three days after power up. In the first days the OCXO showed a decreasing 
drift starting with some 3x10e-10 per day until it reached a stability within +/- 1x10e-10 in the 
last 5 days (that is, since 5 days back, the counters reads always the same value +/- the occasional 
uncertainty of the rightmost (11th) digit (10 seconds gate time). The OCXO specs are <= 5x10e-10 
per day. I didn't notice whether it is sealed, and won't check right now. I don't expect that the 
counter will always stay there, and I don't know what to think when the drift (aging rate?) will 
change.

Thanks,
Antonio I8IOV