[volt-nuts] Drifting 3458As

acbern at gmx.de acbern at gmx.de
Fri Dec 1 04:58:22 EST 2017

What you describe (A9 drift) would be the explanation. However question is how much you see. The A9 should not drift much in the intervals you talk about (days).
Also, you should run the ACAL to determine the cal constant not several times a day but with several days inbetween, and then divide by number of days to determin drift
Frequent ACAL may not give you good results (random fluctuations such as noise, temp... and their impact during cal gets higher the shorter the time between ACALs)

> Gesendet: Donnerstag, 30. November 2017 um 07:40 Uhr
> Von: "Randy Evans" <randyevans2688 at gmail.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: [volt-nuts] Drifting 3458As
> I have been testing two 3458As against a known good Fluke 732A. Each 3458A
> was calibrated for DCV before the start of the measurements so they both
> started at the same point.   I have been running a series of tests
> consisting of measuring the Cal Constant as detailed in Service Note 18 for
> each meter several times a day (to calculate the drift per the procedure in
> SN 18).  I also measure the 732A voltage in each meter each time using NLPC
> 100 and NRDGS 100 and then recording the STDEV, MEAN, MAX, and MIN values.
> What I have observed is that the Cal Constant is acceptably low but the
> MEAN value per measurement  is drifting up in one meter and drifting down
> in the other.   The unit drifting down has a new A3 board installed and the
> unit drifting upward is an Agilent 3458A only a few years old so would not
> be expected to have a drifting A3 board.  It was only calibrated 1 time per
> the internal REV number, so would have likely been re-calibrated if the A3
> board was replaced.
>  My question is what is the likely cause of the drift in the MEAN voltage
> reading if the Cal Constant value is relatively constant?  SN 18 says the
> drift rate of the Cal Constant is an indication of drift in the A3 AD
> board, but I believe it assumes the A9 voltage ref board has a constant
> value over time.  If this is not true (e;g., the voltage ref is drifting),
> I think this would explain the drift in the voltage reading even though the
> Cal Constant is relatively constant.  Any opinions on this?
> Thanks,
> Randy Evans
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