Hello,
I guess you mean if you could do a ZERO CAL w/o affecting the GAIN CAL?
In the calibration manual, p.23, that's called 'Front Terminal Offset Adjustment' and 'Rear Terminal Offset Adjustment', either performed by command CAL 0.
I think, that's indeed possible, and will not affect the gain calibration of DCV and OHM (CAL 10 and CAL 10000).
I have written several articles about optimizing OHM readings, i.e. by using 4W, APER 1 instead of NPLC X command, OCOMP with proper delay, and how to improve noise by using shielded PTFE cable, a 'silent' and stable environment, statistics and holding your breath during this measurement.
You can either use internal statistics function on e.g. 16 samples, or read out via GPIB, to achieve (stable) 8 digits resolution.
This way, I am able to achieve 0.2 ppm transfer accuracy and about 0.15 .. 0.2 ppm rms noise (StD), better than specified in the FLUKE 3458A/HFL data sheet (but not at all in the HP3458A manual).
The DUT resistor has to be designed for that purpose, i.e. it should have a proper shield around the resistive element, no leakage and small polarization at it jacks. Otherwise, you would inherit additional errors or noise from there.
Frank