[volt-nuts] Guidance requested

John Phillips john.phillips0 at gmail.com
Mon Jul 10 04:07:40 EDT 2017

I believe that 3458A will be more stable if it is never turned off.
The references are cooked in. There was a problem when production was moved
offshore, the references were stored cold after they had been cooked into
spec. When these units were shipped they did not meat spec. There was a
memo stating that if left on for 6 months the references should return to
We stored all our 3458As powered up even if we did not anticipate using
them for months.

On Sun, Jul 9, 2017 at 11:24 PM, Frank Stellmach <frank.stellmach at freenet.de
> wrote:

> So I have:
> 1) A Datron 4808 (not calibrated recently AFAIK, and no way to check),
> 2) A 3458A which is about 6 months out of calibration (sadly not by
> Keysight),
> 3) Three 7081s two of which are also 6 months out of calibration against
> the 3458A,
> 4) A 720A.
> The readings of the output of the Datron for the 1V, and 10V, 100V and
> 100V ranges on the 3458A (NPLC100) are:
> 1.00000177V (high limit 1.00000480V),
> 9.9999964V (low limit 9.9999720V, 99.999380V and climbing slowly (low
> limit 99.999550V)
> In spec some five or so minutes later.
> 999.98xxxV (and falling) (low limit 99.99450V) So definitely out of spec
> To be fair both the 4808 and 3458A have only been on for about 8 hours -
> things may improve :).
> So please how best to proceed from here?
> Thanks Dave
> Dave,
> at first, all instruments need a stable temperature environment, otherwise
> ppm measurements are meaningless,
> I recommend setting up the lab in the basement, if available.
> The 3458A is stable after about 2h, or 4h at most.
> Check its inernal temperature, and use ACAL DCV frequently.
> As the 3458A and the 4808 agree to <1ppm on the 10V range, there's good
> reason that the basic DCV calibration of the 3458A is still fine. Both
> these instruments do not drift that much if they are old vintage, and if
> they are not powered on 24/7/365.
> If the 3458A was unpowered most of the time, its reference very probabaly
> did not drift at all.
> If your 732A agrees also on 10V within a few ppm, you would have another
> fix point.
> Maybe you can send in the 732A for comparison to another volt-nut, or
> build a transportable 7,15xxxV reference with an LTZ1000, as described in
> the eevblog thread.
> Because the 3458A is an AUTOCAL instrument, the 1V, 100V readings are
> precise to about the same level, as the 10V range.
> The 3458A makes better ratio transfers than the 720A, butter latter is
> good for linearity check, anyhow.
> 1kV depends on the 100:1 divider of your special 3458A instrument, it can
> be precise as a few ppm only, but unprecise as 12ppm, as described in the
> specification. For 1kV measurements, all instruments need at least 1min
> stabilization before making measurements, due to power induced temperature
> drift.
> To use the 720A on 1kV, may not give better accuracy, either, again due to
> the high power drift.
> A 752A would be required, or another ACAL instrument, like the 5440A, or
> the 5720A.
> The references inside the 7081A may drift the most, so I would trust them
> less.
> For Ohm, you may want to follow TiNs proposals.
> Frank
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*John Phillips*

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