[volt-nuts] Buying HP-3458A

[acbern at gmx.de]

well, some time ago, when I bought my first 3458A, I looked into the data sheet of the 3458a and I figured that the basic accuracy of their standards is not terribly good. 2ppm each, you need to add this to all their specs. dont get me wrong, for most it is really sufficient, but as we are all a little nuts, I felt I would rather spend the money needed to calibrate the 3458a on a yearly basis for something else.
so I use a 732a and a 4910 as voltage references, and a very stable resistor. the 732a can be calibrated to appr. 1ppm at relatively little cost here, the 10k resistor is in the 2ppm range, but simply because I did not want to spend the money for sub 1 ppm. calibrating both on a yearly basis is much cheaper than sending the 3458a for cal. I still need to establish the stability of the references, but the voltage references should be below 1ppm drift pa, the 10k below 0.5ppm (fluke also has a good appnote on establishing sub ppm stability with te 732A). so eventually I can move to a cal once every 2 years or so. and of course now, I can do the cal every 90 days to maintain 90 day accuracy. do that through an eternal lab, costs you a fortune.

now with these, you have only adjusted the meter, you still need to do performance verification. to do this, you need to have more precision gear, which i already had to some extent. the good thing is, some of it does not need cal., or is self-cailbrating.
all dcv is valiated with a 752a divider (self-calibrating) and a nanovolt-nullmeter. current validation is with a set of precision resistors, these are derived from the calibrated 10k with proper error propagation calc. it turns out that using the voltage divider method with the high linearity of the 3458a meter (in conjunction with a very stable voltage source) achieves precise enough resistor accuracies to do current verification. for ac you need a set of thermal converters or an ac transfer standard (ballantine 1605 e.g.) and a ratio transformer. question of course is, do you really need the ac function verified, most applications really focus on the dcv and ohms measurement. so here some money can be saved on gear.
again, this allows to keep the 3458a at a much higher acc. level than using an external cal lab, especially if you do it on a 90 day basis. or you could opt to just verify selected functions, even on a more regular basis. all depends on your needs.
performance verification is time consuming though, if you do it well. 



> Gesendet: Sonntag, 06. Juli 2014 um 19:18 Uhr
> Von: "Attila Kinali" <attila at kinali.ch>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] Buying HP-3458A
>
> Moin moin,
> 
> On Sun, 6 Jul 2014 10:45:51 +0200
> acbern at gmx.de wrote:
> 
> >  so for me, since I am also calibrating the unit myself,
> 
> I would be very much interested in how you calibrate the 3458 and
> what you use as calibration standard.
> 
> 			Attila Kinali
> 
> -- 
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