I tried to get a Rasp Pi talking to the 3458A to record the results but it was beyond my programming skills so I tried the RF Scientific freeware as I also had a Prologix GPIB USB adapter. I found it pretty easy. Below are my first results. DrFrank's comments re EMC effects were quite significant , about 10-50uV or so even with shielded cable bonded to the metal box of the reference and the guard post of the 3458A. Similarly door opening and closing in my small 'shack' ( I am an amateur radio op - total floor area ~12m>2).
Below are my results, the upper graph show my data over 8hours or so, the shack temperature slowly rose during the day from 17.5 C at the beginning to 18.2 at the end. The temperature inside the reference box was between 29.7 and 30.1C.
The two/3 dips in the raw data coincide when the 3458A was sampling just after I opened the door.
Settings 10V manual range, 4hr warm up then DC ACAL. NPLC 100, sampling every 120secs
The internal temp rose a little (31.5 to 32.1C) during the day and I recall its Internal last cal temp was in the mid 30s . I can't recall the Basic command to get it out of it again!
I am pretty happy with the results in terms of stability . I will repeat the whole setup soon to see if I get the same!
I wonder if the age of my unit (15+yrs or so) may have allowed the DMMs reference to stabilise. My unit was obviously used pretty seriously before as it has been calibrated 16 times before, the last was by Agilent in 2009.
Any comments , suggestions or questions appreciated.
Thanks to dave, TiN, DrFrank, pelule, VintageNut, mimmus78, quarks and plesa and other EEV metrology contributors.