What are 6.5 Digit Multimeters Good For?

Want to explain it for me?

The one millivolt, or the lack of a clue?

Jon Kirwan a écrit :

If only that...

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Thanks,
Fred.

Depends what you are doing. In the 80's the kit I worked on used a bank of up to 8 Solartron 7060 DVMs to measure signals to ppm accuracy. Most people settled for 4 or 6 and there was an option to have just one DVM and multiplex the signals. We changed from Solartrons to the first high precision ADCs that were able to achieve the required precision in the early 90's, but a few 7060s (and their improved replacements) were needed for test and calibration. The new kit had to be provably better than the 7060 on the bench before the punters would even consider it.

These days high precision DVMs tend to be used mostly to calibrate, test and correct the linearity, drift of secondary measurement devices and sensors. I can't imagine anyone building high precision DVMs into commercial kit today but I could be wrong. We stopped doing it in the early 90's.

I can't remember the chip number but I think it was an AD part with dual slope integration and almost ppm accuracy out of the box. I did the calibration software to correct systematic errors and linearise it for our application. Obtaining true linearity is harder than it sounds.

Integration periods had to be local mains synchronous and the linearity had to be spot on for the intended application to work at all. Users were looking for reproducible 4-5 figure accuracy on the ratio of feeble ion beam signals from Faraday detectors. Dating moon rocks and similar.

Things where you need excellent long term stability and linearity with genuine 6 digit accuracy after signals are averaged for long enough.

Regards, Martin Brown

If you're developing precise instruments, you would prefer to have test equipment that has higher (preferably MUCH higher) performance than the devices you are developing. Say you have designed a 0.1% or

0.01% current source for a sensor and want to confirm how much it drifts with temperature within 1%.

It's not as easy when you approach or pass the limits of what has been done (or what you can afford to buy/rent/borrow) and you have figure out how the heck to test it.

I don't see how a meter like that would be all that useful if you were only developing digital, audio or RF though.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
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On a sunny day (Thu, 11 Mar 2010 17:56:35 -0800) it happened D from BC wrote in :

They are not needed, all you need is a 5 Euro multimeter, and in extreme cases a precise reference. That means if you use one of those reference chips, you borrow the very accurate multimeter for a day, measure your reference chip, write it down, and use that to calibrate your cheap multimeter, or o compute it's real value, Saved: 1000$

Of course there are exceptions, but in places where that counts they usually have a lot of ++++expensive stuff anyways. Usually places where nothing really useful is done, like in CERN, or ITER, or LIGO, etc.

A knat is the KDE version?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Ahh...clever. that brought a smile. Thanks!

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Rich Webb     Norfolk, VA

D from BC schrieb:

Hello,

if you carefully read the specifications of some 8.5 Digit multimeter, you will find out that you get the full performance only up to one day after calibration, at 20 +- 1 °C and only when measuring DC voltages between 1 and 10 V. ;-)

Bye

Yeah, that's kind of a "maximum accuracy" spec (repeatability + short-term drift)

You really have to read the specs on all multimeters. They can save a lot of money by "relaxing" the specs on other than the one or two primary DC voltage ranges. A meter with 0.1% accuracy on low DC voltages might be as bad as 1% for high currents, AC and high ohms, and sometimes even for high DC voltages.

Uwe Hercksen wrote in news: snipped-for-privacy@mid.dfncis.de:

one must always RTFM.

(read the F-ing manual)

--
Jim Yanik
jyanik
at
localnet
dot com

The Keithley 2100 is a cheap Chinese rebrand. It's awful. I gave them so much public grief over it that John Keithley himself arranged to take all of ours back and replace them with 2000s.

I think Array makes it...

Keithley is helpless to support the 2100. They acknowledge that the firmware is bad, but can't get it changed. The Keithley 2000 is pretty good.

The Fluke 8845A is a much better instrument, and made in the USA.

John

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If you don't _need_ the accurate multimeter, then how do you get around
the fact that unless you use _it_ to measure the reference, your cheap
multimeter is pretty much a boat anchor?

   
JF

Jan Panteltje a écrit :

Depends on what you're doing (just got a 8.5 digits 3458A)

Calibrate my 6.5 and 7.5 digits DMM? And few other things too...

anyways.

LIGO,

Trying to imply that since you have nothing more than a $5 DMM you're doing useful things? Then trash all your tools and you'll get really important...

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Thanks,
Fred.

Does your e5 multimeter resolve microvolts? If not, it's useless for designing thermocouple stuff. Can it measure resistance to 0.02%? That is useful too.

If all you do is muck around with uPs that have 8 bit ADCs, a cheap meter is almost good enough.

One cool thing to do with a good DVM is measure voltage drops in PCB traces and planes, often microvolts. That answers the eternal question. "where is the damned current going?"

John

Neato :)

Thanks :)

anyways.

LIGO,

How about mohm measurements? Maybe that's handy. My DMM only goes to 0.1 ohm. I thought of measuring DCR of coils or pcb trace resistance for sim accuracy.

e stuff anyways.

ER, or LIGO,

One use is to find shorts on circuit boards. The really accurate resistance measurement lets you home in on the offending part :)