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Author Topic: How do you know a volt is really a volt?  (Read 6072 times)

Jonathan Johnson

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How do you know a volt is really a volt?
« on: December 05, 2017, 03:47:14 pm »

I was thinking about calibration of meters, and what we use for a reference. How do we define a volt? How do we know that a reference voltage source is accurate?

While the "volt" has a precise definition ("the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points"), ensuring that a reference voltage accurately represents that definition isn't so easy.

I came across this article from 2013:

https://www.nist.gov/news-events/news/2013/04/primary-voltage-standard-whole-world

It describes efforts by the Physical Measurement Laboratory at the National Institutes of Standards and Technology to develop a "user friendly*" voltage reference source based on quantum mechanics. Previous systems required extensive specialized knowledge and training; the systems currently in development (at the time the article was written) are intended to provide a more stable, accurate voltage reference that can be used by less-trained scientists.

It turns out that power and grounding issues bite them, too:

Quote
Some fixes required less sophisticated engineering or physics. "We get reports from people who have problems because they have poor local power supplies," Benz says. "We initially delivered our systems with separate components, and people were plugging them into different electrical outlets so the grounding was different. So now we've put everything into a single rack to optimize grounding and we include surge-suppressing power strips."

I really don't have much to add, I just thought I'd throw this out here for your cogitation.

P.S. -- be careful when reading the article, that you don't do as I did. Don't confuse the word metrology (the science of measurement) with meteorology (the science of weather). The article discusses metrology, not meteorology.

* As "user friendly" as "requiring access to liquid helium" can be  ::)
« Last Edit: December 05, 2017, 03:55:10 pm by Jonathan Johnson »
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Jonathan Johnson

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Erik Jerde

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Re: How do you know a volt is really a volt?
« Reply #2 on: December 05, 2017, 04:05:20 pm »

While the "volt" has a precise definition ("the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points"), ensuring that a reference voltage accurately represents that definition isn't so easy.

Makes me wonder what the precise definition of watt and amp is.
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TJ (Tom) Cornish

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Re: How do you know a volt is really a volt?
« Reply #3 on: December 05, 2017, 04:56:31 pm »

A man with one volt meter knows exactly what the voltage is.  A man with two volt meters is never sure.
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John Roberts {JR}

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Re: How do you know a volt is really a volt?
« Reply #4 on: December 05, 2017, 04:59:55 pm »

I just bought 2 new VOM so I will never know how much a volt is, or which VOM is correct.

JR
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Mike Sokol

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Re: How do you know a volt is really a volt?
« Reply #5 on: December 05, 2017, 06:49:59 pm »

I just bought 2 new VOM so I will never know how much a volt is, or which VOM is correct.

JR
Back in the 80's when I was building missile guidance systems, we had voltage references on the bench that were calibrated yearly to some voltage standard at the National Bureau of Standards. In fact there was a schedule we had to follow to test calibration of every resistance, temperature, weight, pressure, voltage and anything else with a meter on it. I'm not sure what they actually did at the NBS , but after a few weeks they would send our test gear back with documentation of the "actual" values.   
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John Roberts {JR}

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Re: How do you know a volt is really a volt?
« Reply #6 on: December 05, 2017, 09:12:27 pm »

I bought two VOM because my cheap RatShack VOM finally gave up the ghost, and I realized how difficult it is to troubleshoot a broken VOM without a working VOM....

So I bought two, a decent Fluke, and a $10 wonder... but in the meantime I fixed the broken rat shack (the fuse was open) so now I have 3... still no idea what 1.000V is...  ::)  With three I can vote with a tie breaker.

JR
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Mac Kerr

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Re: How do you know a volt is really a volt?
« Reply #7 on: December 05, 2017, 09:57:22 pm »

I bought two VOM because my cheap RatShack VOM finally gave up the ghost, and I realized how difficult it is to troubleshoot a broken VOM without a working VOM....

So I bought two, a decent Fluke, and a $10 wonder... but in the meantime I fixed the broken rat shack (the fuse was open) so now I have 3... still no idea what 1.000V is...  ::)  With three I can vote with a tie breaker.

JR

It used to be that you could get the Fluke calibrated by the factory for $75. Let them worry about what a volt is.

Mac
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Tim McCulloch

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Re: How do you know a volt is really a volt?
« Reply #8 on: December 06, 2017, 12:17:49 am »

Back when I worked in the communications radio shop (my first job out of college) they had a VTVM that was traceable to NIST via a metrology lab in some distant state.  I recall a bit of anxiety among the bench techs until it came back...

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Jean-Pierre Coetzee

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Re: How do you know a volt is really a volt?
« Reply #9 on: December 06, 2017, 01:25:15 am »

The real question is, does it really matter?

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Re: How do you know a volt is really a volt?
« Reply #9 on: December 06, 2017, 01:25:15 am »


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