[volt-nuts] Which meter?

WB6BNQ wb6bnq at cox.net
Sun Nov 15 02:47:21 UTC 2009


Hi Greg,

I did mention using a 732 or the more recent (number escapes me) voltage
reference.  It would be the basis for using the rest of the equipment I mentioned
to accomplish the job.

That 10ppm spec you quoted is for 60 days and that is an absolute value, i.e.,
includes all variables when referenced at standard conditions.  AND, again at
standard reference conditions, the stability of the output is +/- 0.0005% +7uv
for 30 days.  NOT bad for early 1970's technology and no digital either !  In
reality the 335D, for example on average, performed much better in the short
term.  The reference used was the same as in the 731 series voltage reference.

Yes the 332, 335, and the 845 are no longer current.  It is truly a shame that
Fluke quit making the 845 as there is nothing else on the market that can even
replace its quality and functionality.  Even though the 335D had essentially the
same null meter circuitry, it did not have the isolation of the 845A/B and did
not go as low in scaling.

BUT then again, all the "new" people are getting piled on by the digital
revolution to the point of not understanding how to properly use the "OLD" style
of equipment anyway.  Self guided calibration processes with the computers makes
having to think obsolete.

The big problem with the digital domain is the ability to display a lot of
numbers for a measurement.  It is human nature to want to accept all those
numbers as being absolute when we really know they are not.  The problem with the
old gear is the "human" had to do a little work to average out variations and
account for systemic issues and uncertainties.  In the digital domain, the smart
machine does all this math behind the scenes and presents a nice tidy number on
the screen which the "human" never seems to question.

However, if Bram really wanted to have a class act, he should consider snagging
one or preferably more of the 845's off of Ebay, as well as, a 720A, 750A and a
335D.

The 750 is a damn good divider for fixed ratios, if you stop to think about it.
Most voltage references are set to cardinal points anyway.  The few that are not
(mostly in the digital domain) can be easily handled by the 720.  Besides those
off cardinal voltages used in the digital domain are not of the same quality that
is being discussed here.

Of course one of the big problems with the older gear was understanding how to
properly adjust the calibration of each of them.  It looks straight forward, but
you could easily get into trouble if you do not have the experience.  Especially
with the 720 Kelvin divider and the 332/335.

One other point, how many people truly think about the wire and the connections
?  These add more error then any of the equipment I mentioned.  Damn few binding
posts are even worth considering for real work, but no one considers that
either.  For serious work those laboratory gold plated beryllium-copper cables
are very expensive.  How many people have those laying around ?  Not many !

Bill....WB6BNQ


Greg Burnett wrote:

> Bill - The Fluke 845A/B, 750A, 332 series and 335 series are obsolete.
> Spec'd at only 10ppm accuracy, how would the 750A reference divider fit
> Bram's project? (For comparison the 3458A's linearity is > 100x better than
> the 750A.)
>
> Also how would the 332 or 335 series be good enough sources for a LTZ1000A
> UUT project? (For comparison the 332 & 335 short-term noise might be
> typically 50x greater than the 3458A, according to my measurements.)
>
> :) Greg
>
> ----- Original Message -----
> From: "WB6BNQ" <wb6bnq at cox.net>
> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Sent: Saturday, November 14, 2009 2:13 PM
> Subject: Re: [volt-nuts] Which meter?
>
> Bram,
>
> You need to consider a different method !  You would be better off renting a
> Fluke 732 voltage transfer standard with current calibration and a Fluke
> 845A/B
> (desk version - not the rack mount version) null meter.  The desk version
> has a
> high isolation resistance then the rack mount unit.
>
> The 845 series null meter has a DC output on the rear that is a full scale
> representation of whatever scale you have selected on the front panel.  This
> way
> you can use just about any piece of crap meter with computer connections to
> log
> the results.
>
> I am assuming you are setting the LTZ1000 to a standard 10 volts.  If not
> you may
> need to use a good divider; either a Fluke 750A or the Fluke 720A Kelvin
> divider
> if you have a weird voltage level below 10 volts.  If you have a voltage
> that is
> higher then you may need a good high quality voltage source like the Fluke
> 332 or
> 335 or one of the newer ones from Fluke.
>
> Bill....WB6BNQ
>
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