Still learning my instruments. And probably fundamentals on all of this.
After the perfect agreement (which didn't seem to go away for about six days now of continuous running, while leaving the instruments alone, and I kept tracking logs to document it past what I already posted here), I decided to experiment some more, and leaving the P6048 on (and not touching it), run a fresh INT CAL on the F5440A. The result is essentially the same 1.5ppm lift-up of the reading on the P6048. The symmetry of this is, probably, relevant - for reference, I refer here to the identical event where a nulling of the P6048, while leaving the F5440A alone, had apparently the exact same effect - and, surely, has a certain beauty in the spirit of ancient Greek esthetics. Which, for the record, I'm not a big fan of (symmetry as beauty).
I am reviewing several prior pages to this thread to see if anything in the feedback gives me a basis to work with to explain this (other than that anything around 1ppm and lower is subject to fairly random laws, which I regard as legitimate). Being symmetrical, I'm thinking:
- EMF potential over my cables (low-emf AB sourced). Though I think dietert1's input was emf shouldn't account for this much.
- Some sort of consistent tempco (can't imagine how).
From the possible hypotheses presented this far, nothing stands out to me as a compelling explanation. I know we've circled this issue quite a few times already, but after performing the parts asked to perform I think these jumps still remain unsolved. Or at least to me, but if I am missing it I'll appreciate a pointer. Possibly missing the forest because of the trees.
It looks like any of the long term logs I am recommended to do rely on explaining this persistent realignment (or else I risk random, but persistent 1.5ppm offsets). I am trying to recall how this went away the last time, but I think it may just be nothing other than time passing. By which I mean given enough time the two instruments seem to fall back into agreement regardless what operations (nulling, INT CAL, etc.) have been performed in the meantime.