Author Topic: So what exactly do you need a 6-1/2 digit multimeter for again?  (Read 23071 times)

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FenderBender

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So what exactly do you need a 6-1/2 digit multimeter for again?
« on: July 02, 2012, 07:48:44 pm »
After watching the SignalPath video on the Rigol 6.5digit meter and reading another discussion...I was wondering, what field of electronics requires such a high resolution test instrument? When will it matter whether you have 5.12345V vs 5.12344V?

I understand for test equipment calibration or perhaps frequency standards (in satellites or cell towers) or maybe NASA stuff, but why would would we need 6-1/2 digit meters on our benches?

Just wondering..

HLA-27b

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #1 on: July 02, 2012, 08:03:44 pm »
For marketing purposes of course...

if you want to outsell competitors 5.1 model you need a 6.1 model.

If the competitor comes up with a 3D  multimeter you need a 4D multimeter to outsell it...

FenderBender

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #2 on: July 02, 2012, 08:20:41 pm »
Just as I expected.

rr100

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #3 on: July 02, 2012, 08:29:44 pm »
When will it matter whether you have 5.12345V vs 5.12344V?
Do not confuse resolution with accuracy! If we take a look at the specs for DM3068 in three months to one year from calibration you should get, for 5V (that needs 20V scale):
+/- (0.0040% reading + 0.0005% range)
That is
0.004/100*5 + 0.0005/100*20 = 0.0003
So your:
5.00000V reading (for example, to keep things simple) can mean anywhere between 4.99970 and 5.00030.
The range can be 60 times more than what you implied. You are barely sure about the "milivolt" digit, if that.

Now run the numbers for your $50 multimeter (with 3 1/2 digits) and then see if you can properly test even a LiIon charging circuit. robrenz • Super Contributor • Posts: 3035 • Country: • Real Machinist, Wannabe EE Re: So what exactly do you need a 6-1/2 digit multimeter for again? « Reply #4 on: July 02, 2012, 08:32:06 pm » Personally I find I gain a deeper understanding of a topic when I attempt to learn all the nuances involved in high levels of precision. Being a machinist my real appreciation for high precision started when I bought my first electronic indicator with 1 micro inch resolution (38 years ago and I still use it frequently). My eyes were opened when I could breath on a piece of metal and watch it grow or press on my lathe (which I always assumed to be infinitely rigid) with one finger and watch it deform. Did I actually need that indicator when I bought it? No, but the educational value and gut level understanding of how materials behave was invaluable. The same applies for me with electronics. I bought a 6.5 digit 24ppm accuracy bench meter. Do I actually need it? No, but I am learning a lot by being able to see things happening that are not apparent with a regular meter. As with all metrology, knowing what is involved in making accurate measurements at the bleeding edge of precision helps you improve your understanding and the accuracy of your every day measurements. olsenn • Frequent Contributor • Posts: 993 Re: So what exactly do you need a 6-1/2 digit multimeter for again? « Reply #5 on: July 02, 2012, 08:34:11 pm » For simple repair work and debugging (checking if you're getting between 4.5 and 5.5 volts for example), you would not require a 6.5-digit multimeter. However, if you are testing/designing a precision voltage reference for an ADC or DAC, or testing the stability of a system for example, having a high accuracy, high precision DMM is crucial. Furthermore, the accuracy rating for 6.5-digit meters (good ones) are high higher than for say a Fluke 87 and so instead of the instrument guaranteeing that it will be within +-0.3V of the displayed value, it will guarantee between +-0.01V for instance... this can save your ass in numerous applications. WorldPowerLabs • Regular Contributor • Posts: 62 Re: So what exactly do you need a 6-1/2 digit multimeter for again? « Reply #6 on: July 02, 2012, 08:51:55 pm » I've also seen the argument that this sort of resolution is good for keeping an eye on subtle battery behavior (self-discharge, etc). FenderBender • Super Contributor • Posts: 1118 • Country: Re: So what exactly do you need a 6-1/2 digit multimeter for again? « Reply #7 on: July 02, 2012, 08:59:55 pm » I've also seen the argument that this sort of resolution is good for keeping an eye on subtle battery behavior (self-discharge, etc). Not arguing with you, but hell, I think you could get away with 3-1/2 or 4-1/2 meter for things even like that. When you get out into 6+ digits, subtle changes could practically be caused by anything in the environment. I don't know. FenderBender • Super Contributor • Posts: 1118 • Country: Re: So what exactly do you need a 6-1/2 digit multimeter for again? « Reply #8 on: July 02, 2012, 09:07:16 pm » When will it matter whether you have 5.12345V vs 5.12344V? Do not confuse resolution with accuracy! If we take a look at the specs for DM3068 in three months to one year from calibration you should get, for 5V (that needs 20V scale): +/- (0.0040% reading + 0.0005% range) That is 0.004/100*5 + 0.0005/100*20 = 0.0003 So your: 5.00000V reading (for example, to keep things simple) can mean anywhere between 4.99970 and 5.00030. The range can be 60 times more than what you implied. You are barely sure about the "milivolt" digit, if that. Now run the numbers for your$50 multimeter (with 3 1/2 digits) and then see if you can properly test even a LiIon charging circuit.

No I'm not confusing accuracy with resolution. I guess the proper word should be "When will it matter whether you have measured 5.12345V vs 5.12344V?"

I'm most likely just ignorant, but in real life, are all of those digits actually all that helpful? I think 4-1/2 digits is a reasonable resolution. 6-1/2. Gosh, I just wouldn't know what to do.

I guess if I don't know why a product exists, I shouldn't be questioning it.

Bloch

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #9 on: July 02, 2012, 09:08:47 pm »
If some one gave me a 6-1/2 digit multimeter would I use it ?

Not sure.

I never had the use for one.

Would i like a more precise 3-1/2 digit multimeter

YES
« Last Edit: July 02, 2012, 09:10:24 pm by Bloch »

HLA-27b

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #10 on: July 02, 2012, 09:27:40 pm »
All of what you guys say is true of course but the real issue is that we do not have confidence in the last digit (or two) of any multimeter. Perhaps we would be better off if we simply tape over the last digit and forget that it is there at all.
So the best use for a 6.1 digit multimeter is as a 5.1 digit multimeter

T4P

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #11 on: July 02, 2012, 09:39:53 pm »
When will it matter whether you have 5.12345V vs 5.12344V?
Do not confuse resolution with accuracy! If we take a look at the specs for DM3068 in three months to one year from calibration you should get, for 5V (that needs 20V scale):
+/- (0.0040% reading + 0.0005% range)
That is
0.004/100*5 + 0.0005/100*20 = 0.0003
So your:
5.00000V reading (for example, to keep things simple) can mean anywhere between 4.99970 and 5.00030.
The range can be 60 times more than what you implied. You are barely sure about the "milivolt" digit, if that.

analex

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #33 on: July 07, 2012, 06:33:51 pm »
2 words : DYNAMIC RANGE.
That's why you need so many digits.

As 6.5 DMM has 0.1m ohms resolution, it is easier to locate the short circuit point by using 2-Wire or 4-Wire(better way) OHM.

Some of 6.5 DMM have data plot function, and DMM have variable BW(change NPLC), and uV/nA/mohm level resolution,  is very easy to find out low freq characters of a "noisy" signal. BTW, DSO can't do that, as DR/resolution reason; SA can't do that, as it can't go down to low freq range.

6.5 DMM is faster than 5.5 etc. Most of time, I set my 6.5 DMM to DCV 1000V range, so I can measure form 1mV~1000V without waiting for auto range. As this I can find out over load immediately at the power up moment, and cut off power supply before smokes.

saturation

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #34 on: July 09, 2012, 08:43:52 pm »
That would be nice.  One could be lucky today and get a used one in $1000-3000 in perfect order. Its the calibration costs that are high, if you want the best it can deliver. The 1989 designed 3458a 8.5DMM sells for about$10,000, about its new price in 1989, in could be called cheaper given the depreciation of the US $. However, with 1nV resolution and 4ppm stability, few labs can calibrate the 3458a properly. It runs about$600 for the 4ppm calibration, but the 0.2ppm version is about \$1500.

http://lvldstdslabagilent.blogspot.com/2005/01/faq-what-3458a-calibrations-are.html

http://www.febo.com/pipermail/volt-nuts/2011-August/000840.html

Only a Josephson Junction has the resolution to calibrate an 8.5 DMM these days, and even if it were made into a turn-key standard, the labor to maintain it is far more than buying a 732a and keeping it in an airconditioned room, so calibration costs won't drop soon.

Eventually we will all have to use 8-1/2d meters
Best Wishes,

Saturation

nukie

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #35 on: July 11, 2012, 02:42:28 pm »
I used mine as a coffee warmer today, it works quite well.

ejeffrey

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Re: So what exactly do you need a 6-1/2 digit multimeter for again?
« Reply #36 on: July 11, 2012, 04:43:52 pm »

The wall function was discretized in points,and matlab was reporting them as discretized values.
But the variable editor just showed the first 4 or 5 digits,not the exact value.
So 99.99999999 was displayed as 100.0000,which is nice for a cake recipe,but not for the boundary conditions checks who red "fluid" instead of "wall" on those points because 99.99999999<100.0000
So there you go,a true case when a 1E-8 can waste you an afternoon.I still have the hand drawings somewhere.
Which is why real precision mathematics needs to be done using packed bcd arithmetic. There precision is absolute.

No, the only reason to use BCD is if you need to represent exact decimal numbers, typically with currency. This works OK because there is a finite required precision, and you only have to be correct on addition and subtraction -- multiplications are allowed to be rounded before adding to the accumulator.  Physical quantities have no special preference for base 10, so BCD is rarely if every used for scientific or engineering numerical computing (it is used in CAD for geometry definition, but converted to floating point for things like FEA).  Infinite precision arithmetic, either binary or decimal, is only used in specialized circumstances, for any sort of general algorithm the required size grows exponentially with the number of steps in the computation, and in any case operations other than +,-,* are not even possible.

The real lesson here is 'don't compare floating point values for equality'.

Smf