[time-nuts] V standards

Neon John jgd at johngsbbq.com
Thu Dec 11 14:03:11 UTC 2008


On Sat, 29 Nov 2008 19:27:34 -0500, "Mike Monett" <XDE-L2G3 at myamail.com>
wrote:

>This discussion of voltage standards is very informative and useful, and my
>thanks to all who are contributing. 
>
>It is clear why precise frequency standards are needed - there are
>innumerable applications such as GPS, VLBI, secure spread spectrum radio,
>deep space navigation using doppler, and so on.
>
>But I wonder why extreme accuracy is needed in measuring voltage? Don't get
>me wrong - I have a HP 3456A, and I would love to have a 3458. But the
>prices on eBay can reach $6k, and I can't see spending that much money for
>two more digits. As Bill would say, six digits should be enough for
>everyone:)

Because it's still an analog world out there.  One instance that immediately
comes to mind are truck scales, especially the enforcement ones.  I recently
serviced one that had a 200,000 lb capability, a 5 lb resolution and nailed my
personal body weight to within the resolution with no last digit dithering.
That indicates to me better than 2.5 lb internal resolution.

Calibration of this particular scale is done in the digital world - roll a
50,000 lb calibrated trailer onto the scale, tell the scale processor that it
weighs 50,000.0 lbs and it figures out its own conversion factors.  HOWEVER.
Servicing the thing means working down in the kind of precision levels we're
talking about.

I have the 4 lightning-struck load cells and the matched summing box in my
shop ($10,000 from the scale company) so when I get time I can see how they do
it that precise with so few parts and with the load cells and summing box out
in the weather.  There are only 4 wires coming from each load cell so the
temperature compensation has to be built into the mechano-electrical design
and not externally compensated.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
In theory, there is no difference between theory and practice.  In practice, there is.




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