Hi Philipp,
you definitely have to clean the fan filter of your unit.. standing free
on a table, and with a clean filter, the internal temperature rise is
12..13°C at most.
Defining a metrology grade 3458A would explicitly require to define and
control the environmental conditions in the specification, like reduced
temperature range, specifying the ventilation requirements, like no use
in a rack, by using a different filter, and also by regularly checking
the internal temperature by firmware.
The 3458A is designed and specified as a system DMM, inside a rack, in a
hot manufacturing environment, like in a electronics manufacturing line,
but never was intended and built as a metrology DMM, as hp and KS still
try to advertise.
Your argument, that a sample distribution is root cause of these wide
specification limits, does not convince me:
The annual and temperature drift (with ACAL) is mainly determined by the
drift of the two internal references, due to the ACAL technique, the
rest of the circuit does contribute only marginally, but on the w/o ACAL
specifications .
Therefore, the 8508/88A simply beat the 3458A, because they use an LTFLU
running at 45..55°C only, instead of 95°C, and due to the heavy usage of
stable Vishay BMF resistors, which also contribute to the superior
short- and midterm stability of these instruments.
So only the sample distribution and yield of the LTZ and 40k resistor
affect the annual drift specification limits for the 3458A.
Concerning the LTZ reference, that is already covered by their
monitoring / selection process, where they have for sure have yield
problems already.
If they would simply reduce the temperature for metrology purposes, like
in practice done on the FLUKE DMMs, they would directly get near 100%
yield for 2..3ppm/year (@65°C), due to the ACAL DCV feature..
The 8508/88 on the other hand have to account for the drift of other
components, to achieve e.g. these 4ppm/year, although the LTFLU very
probably performs more like 1..2ppm/year (inside the 732B, @ 47°C, for
example).
I think the 3458A would at least be on par with the FLUKE DMMs if KS
would simply chose the same metrology grade resistors and oven
temperature as DATRON/FLUKE had done...
Frank
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