User`s guide

Remarks
If you use the individual (null per function) null commands found in the
[SENSe:] subsystem, it is recommended that you not use this function,
and that you not turn on the null state with the CALC:FUNC command. If
you do, the measurement function null, set with the [SENSe:] command,
and the null set with the CALC:NULL:OFFS command, are additive.
You must select (CALCulate:FUNCtion) and turn on (CALCulate:STATe)
math operations before you set a null value.
If a null offset value is not specified with the CALC:NULL:OFFS command,
the first measurement taken is used as the null value.
The instrument clears the null value after a Factory Reset (*RST
command), after an Instrument Preset (SYSTem:PRESet command), when
the math function or measurement function is changed, or when
CALCulate:STATe ON is sent.