[volt-nuts] Guidance requested

David C. Partridge david.partridge at perdrix.co.uk
Mon Jul 10 05:41:55 EDT 2017

Frank and others,

>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. 

The 3458A readings for all ranges except 1000V are now within the spec of the Datron 4808 (between 2.8ppm and 5.6ppm depending on range)

On the 1000V range for voltages up to and including 700V the 3458A reading are consistently low low but within the Datron spec of +/-5.6ppm
For voltages of 800V and above the 3458A reading starts off just within or close to spec of the Datron of 5.6ppm, but then falls away as the divider warms up (I think).

Here are the settling levels:

800V is 799.9946V 7.3 to 7.4ppm low
900V is 899.9901V 9.9ppm low
1000V is 999.9886V 11.2ppm low

Whether it is the Datron or the 3458A I'm not yet sure. That's why I'm doing the 720A alignment so I can cross check.


-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Frank Stellmach
Sent: 10 July 2017 07:25
To: volt-nuts at febo.com
Subject: [volt-nuts] Guidance requested

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

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.


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