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Author Topic: How do you know a volt is really a volt?  (Read 6073 times)

Steve M Smith

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Re: How do you know a volt is really a volt?
« Reply #10 on: December 06, 2017, 03:08:34 am »

The real question is, does it really matter?

Quite often, absolute accuracy isn't too important.


Steve.
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Mike Sokol

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Re: How do you know a volt is really a volt?
« Reply #11 on: December 06, 2017, 07:53:20 am »

The real question is, does it really matter?
I would say yes. I teach an audio electronics class at University level, and have my students bring in whatever digital meter they want to class. It's amazing just how many different voltage readings you can get from the same source. Now, being 2 or 3 volts off on a 120-volt reading may not be significant in many cases. However, I do a lot of ground and power troubleshooting where a volt or two difference can send my failure theory in different directions. Also, other mundane things like measuring battery charging voltage really need tenth of a volt accuracy on a 12-volt circuit. And back when I built missile guidance systems for the military we had to manufacture components down to 2ppm accuracy. So the more accurate your reading are, the better job you can do. I have a high-end Fluke multi-meter with the factory calibration seal that I use for my class voltage reference.
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Chris Hindle

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Re: How do you know a volt is really a volt?
« Reply #12 on: December 06, 2017, 08:09:57 am »

I have a high-end Fluke multi-meter with the factory calibration seal that I use for my class voltage reference.
..... BUT, when was it last calibrated ?
Chris.
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Keith Broughton

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Re: How do you know a volt is really a volt?
« Reply #13 on: December 06, 2017, 08:19:47 am »

Chevy has a Volt :-)
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Pete Erskine

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Re: How do you know a volt is really a volt?
« Reply #14 on: December 06, 2017, 08:26:43 am »

The "international volt" was defined in 1893 as 1/1.434 of the emf of a Clark cell. ... Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called "standard cells".
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Mike Sokol

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Re: How do you know a volt is really a volt?
« Reply #15 on: December 06, 2017, 10:36:58 am »

..... BUT, when was it last calibrated ?
Chris.

It does have a calibration date on it, and I'm sure it's past due. But I'll take the word of a $1,000 Fluke meter that was factory calibrated and certified over a Harbor Freight giveaway meter any day.
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John Roberts {JR}

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Re: How do you know a volt is really a volt?
« Reply #16 on: December 06, 2017, 10:49:10 am »

Nothing I measure requires standard reference precision.

I recently bought a modest Fluke, and a cheap $10-15 wonder... so far the cheap VOM looks pretty respectable.

I have seen really cheap VOM that were extremely unreliable, but the modern cheap (chinese) VOM, maybe one step up from the cheapest, probably is usable. 

When my one VOM broke, I felt blind...  now I have back up.  8)

JR

PS: The errors in a "too cheap to be accurate" meter are probably related to scale and/or rectification, so a several volt error at 120VAC will not be the same several volts around 0V.
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Jonathan Johnson

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Re: How do you know a volt is really a volt?
« Reply #17 on: December 06, 2017, 12:07:35 pm »

I recently bought a modest Fluke, and a cheap $10-15 wonder... so far the cheap VOM looks pretty respectable.

It was probably a fluke. But not a Fluke. ;)

Quote
PS: The errors in a "too cheap to be accurate" meter are probably related to scale and/or rectification, so a several volt error at 120VAC will not be the same several volts around 0V.

Sometimes, your measuring device doesn't need to be accurate, as long as the variation is consistent across the range you are measuring. If you're looking to identify voltage variations between two different circuits, and you're more concerned with the difference rather than the actual voltage, it doesn't matter if you're off by -3.2 volts, as long as you're off by -3.2 volts in each measurement you take.
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Chris Hindle

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Re: How do you know a volt is really a volt?
« Reply #18 on: December 06, 2017, 12:22:46 pm »

It does have a calibration date on it, and I'm sure it's past due. But I'll take the word of a $1,000 Fluke meter that was factory calibrated and certified over a Harbor Freight giveaway meter any day.
No worries Mike, I was  just Pokin the Bear.
When someone asks, I have a habit of saying "Around 122 volts", or "It LOOKS dead"
I'd trust an old Fluke over pretty much anything else. Even new.
Never been bitten by a fluke.
Dad gave me a Simpson 260 when I started High School.
Not small, but a great meter.
Chris.
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Geoff Doane

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Re: How do you know a volt is really a volt?
« Reply #19 on: December 06, 2017, 12:59:35 pm »

Makes me wonder what the precise definition of watt and amp is.

An amp is one coulomb per second.  6.3 x 1018 (roughly) electrons passing a certain point in one second.  How fast can you count?  ;)

GTD
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ProSoundWeb Community

Re: How do you know a volt is really a volt?
« Reply #19 on: December 06, 2017, 12:59:35 pm »


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