Hi,
first my opinion about the Fluke 332/335:
Those are very nice instruments, and you really need such instruments,
if you really want to calibrate your DCV gear: Only having a 10V
reference as 732A/B, or all those amateur grade 2.5 / 5 / 10V references
won't help, as you always need the Cardinal Points (1000V, 100V, 10V,
1V, 100mV) for calibration.
Therefore, those old 332/335 calibrators are basic instruments, to
generate those voltage in first instance.
You have to make sure, that they already contain the MOSFET chopper card
(not the mechanical chopper).
The reference also comes in two variants, either the older zener diode
oven, or the Reference Amplifier version.
Both are ok, but will not give more stability than about 10ppm/ 2months.
There is an elaborate teardown/ripoff of the 332B/AF on EEEVBlog, where
you can see additional stability parameters I have measured.
http://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393609/#msg393609
From those measurements, I really doubt, that old 332/335 calibrators
will really get better over time.
The reference is too simple to be principally stable, that begins with
the simple oven, and ends by the non buried zener RefAmp. The
cummulative drift over 40 years was about 500pm, so you cannot really
state, that its stability had improved over time.
The noise of the instrument presumably increased over time, as all the
capacitors deteriorate, and also all the transistors loose their gain hfe.
As simply a precision, stable high voltage power supply, a working
332D is very fine.
The stability level is several ppm in any aspect.
But as a standard, there's one important further problem:
The 332D itself needs calibration, first linearity, second the range
calibration.
That strictly requires additional equipment, as a Fluke 720A, or a
HP3458A, and a Fluke 752A.
I have designed my own 100:1 / 10:1 Hammon divider, see EEVBLOG also,
and for example, the 34401A is also capable to adjust the linearity.
At least, I recommend to buy a Fluke 5440/5442.
Its stabilities are ten times better in all aspects, and it features
ultra high linearity by design,(my device: ~0.1ppm), and a Autocal
feature, like the 3458A.
The additional 1V and 100mV ranges have to be calibrated externally
again, but 10V, 100V and 1kV can be calibrated from one external 10V
source, similar to the 3458A.
Frank
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