[volt-nuts] Calibration of voltage standards

[acbern at gmx.de]

for those interested, and for simplicity, wanted to add this from David which was in voltnuts in 2012.


________________________________
 From: Charles P. Steinmetz <charles_steinmetz at lavabit.com>
To: Discussion of precise voltage measurement <volt-nuts at febo.com> 
Sent: Saturday, 15 December 2012, 10:28
Subject: Re: [volt-nuts] Fluke 335A versus HP 740B
 
David wrote:

> I have a Keithley 155 Null Detector how does that rate?

The three usual suspects are the Fluke 845AB, the HP 419A, and Keithley 155.  I have one of each, and they are all good meters.  The received wisdom is that the Fluke is the one to have.  However, in my view, the Keithley is the best of the bunch.  It is a bonus if you find one with the rare 1554 AC power module (the 1554 mounts to the rear panel and allows AC operation -- otherwise, it is battery-only).

The main problem today with the HP is that it uses impossible-to-find batteries.  Not only are the original batteries unobtainable, I have yet to find a satisfactory replacement strategy.  It also does not have a +/- 1 uV range, although I do not count that as a major fault since thermocouple noise in the measurement setup frequently prevents taking full advantage of the 1 uV range.

The Fluke's batteries (sub-C NiCd cells) are readily replaced (though not inexpensively, if you get the best cells).  Keithley used four, # 246 9 V carbon cells, which can easily be replaced with common 9 V alkalines or 9 V primary lithium cells that simply plug into the existing connectors.

The HP and Fluke both use photocell choppers.  Fluke published pre-release information indicating that they had designed a FET chopper for later production, but I have never seen an 845 with a FET chopper or a schematic of the FET chopper.  (Does anyone here have either?)  The Keithley was designed with a MOSFET chopper from the start.

Not only is the Keithley the most modern design and the most likely to remain reliable, it also performs the best in my lab.  I have had fewer ground loop and shielding problems with it than with the Fluke, and it has less noise and less drift.  It is not enough better that most people should sell their Fluke to get a Keithley just for the performance difference (reliability may be another story), but -- IME -- it does perform better.

They are all good meters, but IMO the Keithley is the best of the three.  If I had only one, that is the one I'd want.

Best regards,

Charles


> Gesendet: Mittwoch, 11. Februar 2015 um 08:55 Uhr
> Von: wb6bnq <wb6bnq at cox.net>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] Calibration of voltage standards
>
> To amplify Chuck's point,
> 
> The only meter worth considering, if you are going to get one, is the 
> FLUKE 845.  Preferably, the battery operated version which has a higher 
> isolation leakage resistance.  However, the 110
> volt rack mount model would do just as equally.
> 
> Bill....WB6BNQ
> 
> 
> Chuck Harris wrote:
> 
> > To do a comparison of the sort you are asking about, the
> > sensitivity of the null meter is much more important than
> > its ultimate accuracy.
> >
> > So, neither of your meters is really the right meter to
> > use for this task.  What you want is called a null meter,
> > and is generally sensitive to the microvolt region.
> >
> > -Chuck Harris
> >
> > Ken Peek wrote:
> >
> >> Hi Group,
> >>
> >> I have heard of a few different ways to measure one 10V voltage standard
> >> against another 10V voltage standard.
> >>
> >> Assume we have two 10V voltage standards.  One is calibrated, the other
> >> not only needs to be calibrated, but probably adjusted.  For the sake of
> >> simplicity, let's say the two standards are Fluke 732B's.
> >>
> >> I *think* the best way is to connect the two units' (-) terminals
> >> together, then connect a calibrated meter in between the (+) terminals,
> >> and measure the difference.  I have also heard that to remove thermal
> >> EMFs, you should use a low-thermal-EMF DPDT switch or a low-thermal-EMF
> >> relay to reverse the connections on the DMM, so you can take the reading
> >> forward and reversed, then split the difference.  There is the
> >> possibility to introduce thermal-EMF errors from the switch/relay as
> >> well, so I'm wondering if this is a good idea.  This sort of makes sense
> >> to me, but I'm not a metrologist, so I would like to hear what others in
> >> this group think about this.
> >>
> >> So, just what is the proper way to accomplish this task?
> >>
> >> BTW-- I have an Agilent 34420A and an HP 3458A, which would be the
> >> better instrument for this task?
> >>
> >> Best Regards,
> >> Ken Peek
> >> =============================
> >>
> >>
> >> _______________________________________________
> >> volt-nuts mailing list -- volt-nuts at febo.com
> >> To unsubscribe, go to 
> >> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> >> and follow the instructions there.
> >>
> > _______________________________________________
> > volt-nuts mailing list -- volt-nuts at febo.com
> > To unsubscribe, go to 
> > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> > and follow the instructions there.
> >
> 
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.
>