[volt-nuts] Drifting 3458As

[acbern at gmx.de]

If you are confident that the 732A is stable to 0.1ppm then just run an acal afer a wek, and determnine what has driifted. Your A/D board, as can be seen with a drifty cal constant, or, els the A9 ref.
Dont do a checl everyday. this hides information. 



> Gesendet: Freitag, 01. Dezember 2017 um 19:49 Uhr
> Von: "Randy Evans" <randyevans2688 at gmail.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] Drifting 3458As
>
> I am running the ACAL to check cal constant drift over several times a day
> and over a 7 day period (per SN18).  The varying times of the day are to
> see how temperature sensitivity affects the cal constant drift since the
> room temperature is not constant through the day but is relatively constant
> at the same time of day each day; i.e., the furnace is off during the night
> so mornings are about 5 degrees C cooler than the afternoon.  Even after
> ACAL, I am seeing up to an absolute difference of 3.5 uV on the 732A
> voltage reading between the HP and Agilent 3458As, depending on the time of
> day.  I suspect the A9 board in the Agilent is the reason since the HP
> 3458A reads much closer to the 732A value than the Agilent 3458A over
> temperature.  Based on long term comparisons with other 732As and a 732B, I
> am confident the 732A is stable within 0.1uV.
> 
> I am using an ebay A9 board in the Agilent 3458A for the current testing
> since I suspect the original A9 board in the Agilent 3458A.  I am
> separately testing the original A9 board since I am not completely
> confident in the ebay A9 board.  I do not have enough data to draw a
> conclusion yet about the original A9 board.  My current concern is to see
> if there is agreement that the A9 board is likely the culprit.
> 
> Thanks,
> 
> Randy Evans
> 
> On Fri, Dec 1, 2017 at 1:58 AM, <acbern at gmx.de> wrote:
> 
> > 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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