Author Topic: when do you really need 6 1/2 DMM ? , I mean the res and/or the accuracy  (Read 11526 times)

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Offline eevblogfan

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hey

nince I've got 2X HP3478A and 1X HP34401 , I thought , when do you guys really use that kind of res ?  or that kind of accuracy  ?

I had to work with 2uV Chopper amp , so low voltage and good accuracy is an issue , hence 5.5Digit is needed indeed  ' but apart from that , what els ?

please feel free to shear any experiance or experiment you've had with that such need , good night guys  :=\
 

Offline Fsck

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should let you guarantee that the first 3.5-4.5 digits are 100% accurate.
"This is a one line proof...if we start sufficiently far to the left."
 

Offline c4757p

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should let you guarantee that the first 3.5-4.5 digits are 100% accurate.

That's not how it works. For example, the 3478A specifies 0.035% + 40c on the 30mV range, which means if you are measuring 20mV, the meter can read up to 20.011 mV. 300mV range (more similar to the rest of the ranges) measuring 200mV can read up to 200.019 mV, which is only 3.5 "100% accurate" digits. (You did say "3.5-4.5", but you're pretty unlikely to get 4.5 perfect without a really good meter.)

And to contrast, the Fluke 8050A (only a 4.5 digit meter) still gives you 3.5 "perfectly accurate" digits, with a 1-year accuracy on all ranges of 0.03% + 2c.
« Last Edit: May 16, 2013, 10:45:16 pm by c4757p »
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Offline Fsck

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should let you guarantee that the first 3.5-4.5 digits are 100% accurate.

That's not how it works.
maybe not 100%, since the confidence interval is probably 95%, but even if you jump to 7-8 sigma interval, and assuming the calibration is correct, the first couple digits will be guaranteed
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Offline vk6zgo

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It's so you can show it off to the other Geeks! ;D
 

Online don.r

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It's so you can show it off to the other Geeks! ;D
Digit envy? Lucky the man who owns a 3458A.
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Offline ftransform

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6.5 digit multimeter is the bare minimum for digital work. I recommend a 8.5 for analog.
 

Offline ben_r_

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6.5 digit multimeter is the bare minimum for digital work. I recommend a 8.5 for analog.
Never heard anyone make that statement before. Okay, so WHY?
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Offline eevblogfan

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6.5 digit multimeter is the bare minimum for digital work. I recommend a 8.5 for analog.
Never heard anyone make that statement before. Okay, so WHY?

yea , tell us WHY ?  :wtf: :wtf: :rant:

8 1/2 is the world's bets res ( as far as I know ) , most things on the analogue need no more then 0.05%  ( fluke 87V will do ) , that is not justifying the cost of 6.5 DMM , not tallking about 8.5 !!!?

I wonder if someone needs 10.00000V reference , I might make one for myself ( I might put my hands on kiethely 2001 , so the adjustment of that 10V reference will be done just after adjusting the 2001 ) , i presume fluke 5700 has good enough accuracy , so anything will be referenced to him :P

please PM me if you are interest in such reference :P
 

Offline mrflibble

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6.5 digit multimeter is the bare minimum for digital work.

Agreed!  :-+


(6.5 binary digits that is)
 

Offline AlfBaz

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6.5 digit multimeter is the bare minimum for digital work. I recommend a 8.5 for analog.
Ahh... That's where I've been going wrong!
Logic probe for digital and test lamps for analogue  :)
 

Offline eevblogfan

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hey

can you share with me ? 

ca you describe scenario where you was in need for 6.5 DMM within digital project ?

5.00000V ? ( the 34401 can go to 1.2 milion count , no more )
 

Offline robrenz

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6.5 digit multimeter is the bare minimum for digital work. I recommend a 8.5 for analog.

Fortunately, for my low level analog work my 6.5 digit meter can read to 100 picovolt resolution in statistics mode so I can get by without buying a 8.5 digit meter ;)

Offline eevblogfan

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hey robrenz , didn't you mean 100 nanao volts ?  :bullshit:
 

Offline robrenz

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No I meant 100 Pico volts or 0.1 nano volt.   An 8846A at 100NPLC and low voltage levels has in internal resolution of 100 pico volts. You have to be in statistics mode to see the values.  Actual readings only go to 100 pico volt but the calculated values like average go to 0.1 pico volts

Offline eevblogfan

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WOW , that's low 0_0

do you or any one else know any keithley instrument who does better than that ? , I mean , what is the lowest voltage you can measure ?

and as for the subject , I've asked Why do you really need 6.5 res ? can someone point some data ?
 

Offline ben_r_

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Thankfully I dont work in anything that needs 6.5! I have a brand new 5.5 benchtop Agilent on order and thatll be overkill for me as is. But that OLED display is sooo puuurdy! lol
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Offline madshaman

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when do you really need 6 1/2 DMM ? , I mean the res and/or the accuracy
« Reply #17 on: May 17, 2013, 03:18:58 pm »
Useful for graphing the behaviour of a system over a long period and looking for any interesting behaviour, especially when the change in voltages over time can be extremely small (e.g. A battery bank powering a system with low power consumption).  There even just having the precision is useful (as long as drift is very low and it's monotonic).

It's also nice to have extreme accuracy. I haven't gotten this far, but I have all my bench meters networked to my PC over gpib and although there's existing software, I plan to write generic software where I can characterise circuits and components' DC properties very accurately.

That means eventually I'll need a set of calibrated references to calibrate my meters from.

Off-topic, but does anyone have any good pointers or links to anyone who maintains a calibrated lab at home (potentially even NIST traceable) and what procedure is easiest and what is required?
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Offline ddavidebor

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when do you really need 6 1/2 DMM ? , I mean the res and/or the accuracy
« Reply #18 on: May 17, 2013, 03:27:56 pm »
You need a 6.5dmm for calibrate 5.5digit ones, a 5.5 digit for calibrare 4.5digit ones
Davide Bortolami,
Fermium LABS srl
 

Offline KedasProbe

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It may be nice to have if you want to check 16 bit DAC/ADC.
Or notice other small changes.
Not everything that counts can be measured. Not everything that can be measured counts.
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Offline ftransform

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I actually sometimes turn down the digits on my meters because they annoy me and I don't need them. For alot of stuff that I found myself doing I basically need a LED probe for 90% of the work and then 4.5 digits is nice for final details.

Remember that this forum has a slight hard on for metrology.
I think that 6.5 digits is nice when working with an analog front end, like determining coefficients for a gain stage.

My original chip sources 10mA, while the 74HC04 can source 16mA.....WTF
« Last Edit: May 18, 2013, 02:20:03 am by ftransform »
 

Offline ddavidebor

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when do you really need 6 1/2 DMM ? , I mean the res and/or the accuracy
« Reply #21 on: May 17, 2013, 07:22:03 pm »
Yeah i love to put in 4.5 digit with a hight plc, so the read are super stable and sufficient slow to read.
Davide Bortolami,
Fermium LABS srl
 

Offline robrenz

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Remember that this forum has a slight hard on for metrology.

I never noticed that, thanks for pointing that out. ;D

Offline saturation

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You can easily check the health of certain batteries like NiMH or SLA if you check the output voltage at the max digit resolution.  You can see in real time self discharge rates on the LSD, be it uV or 10s of uV.
Best Wishes,

 Saturation
 

Offline free_electron

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6.5 digit multimeter is the bare minimum for digital work. I recommend a 8.5 for analog.

bullshit. for digital work you only need a 1 digit 7 segment common anode display where segments b and c can be stuck in on position work. the others can be broken. the duality of the binary system guarantees that , if it is 1 it is not zero and vice versa.

segments ba and c are always on. segments a,d,e and f are connected together and go to a wire. touch a logic zero with that wire and segments a,b,c,d,e and f will light showing '0'. leave that wire floating or logic high and only segment b and c will light showing a logic '1'.

there you have it a digital binary logicel level meter using exactly 1 7 segment common anode display and 1 resistor.

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Offline PA4TIM

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Home build Vref based on LM399 connected to my Fluke 720 . 100nV resolution
(got 2 x 7,5 digit, 2 x 6,5 digit and 1 x 5,5 digit and a bunch calibrators
« Last Edit: May 18, 2013, 12:23:48 am by PA4TIM »
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Offline babysitter

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To Show off:-), for low Level stuff, physiological Signals, long-term Experiments and in troubleshooting.

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Offline onlooker

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Quote
Quote

    6.5 digit multimeter is the bare minimum for digital work. I recommend a 8.5 for analog.

bullshit. for digital work you only need a 1 digit 7 segment ...

I always thought the "6.5..8.5" line is just for kidding around. Now it got a serious reply. Is this a higher level of kidding around?
« Last Edit: May 18, 2013, 12:44:15 am by onlooker »
 

Offline c4757p

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I always thought the "6.5..8.5" line is just for kidding around.

:-+ Yep, I thought it was a joke too.
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Offline robrenz

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The specs I stated in my posts were correct but I was just playing along with what I thought was a joke. ::)

Offline eevblogfan

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not when the LDS are jumping around  :scared:
 

Offline c4757p

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not when the LDS are jumping around  :scared:

Most Mormons I know don't do much jumping...
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Offline eevblogfan

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when the ripple is above certain amount, there is DC variation , when the NPLC is pretty bad , you don't get enough averaging , resulting to instability of the LSD(s)

while the 3478A preforms well in that regard 
 

Offline AlfBaz

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Speaking of metrology hard-ons
What sort of setup should you have to take small measurements or observe small changes in voltage, current and/or resistance.

For example what features should you enable/disable in the meter?

What about test leads? Keep them short? shielded? guarding? avoid certain insulation? (whats that term where movement of the lead creates noise).

I've seen pictures where BNC to banana adaptors are used in conjunction with coax cable or heavy external braiding.
What about environmental control such as temperature, humidity and perhaps even barometric pressure?

 

Offline AlfBaz

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(whats that term where movement of the lead creates noise).
Just found it. Triboelectric noise
 

alm

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The Keithley 'Low level measurements handbook' contains answers to some of these questions, for example which types of insulation are best for various applications. It will also explain why those primitive 6.5 digit DMMs are not the best tool for some low-level measurements ;). I think it's available for free from their website, unless the evil Danaher group made them pull it.
 

Offline AlfBaz

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The Keithley 'Low level measurements handbook' contains answers to some of these questions, for example which types of insulation are best for various applications. It will also explain why those primitive 6.5 digit DMMs are not the best tool for some low-level measurements ;). I think it's available for free from their website, unless the evil Danaher group made them pull it.
Thanks alm, I already have a copy, just forgot about it  :palm:
 

Offline quantumvolt

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Given time I guess one can find many situations where you need some resolution / accuracy. I searched the web for 'lab experiment 6 1/2 digit dmm'. Most lab guides from universities were junk - like measuring an AA cell or a 1kOhm resistor.

But one of them was kind of interesting: A Thermocouple producing (-0.059, 0.000, 0.059) mV for  (-1, 0, +1) degrees Celsius.

http://www.omega.com/temperature/z/pdf/z206.pdf

Scroll down the website - it seems to be serious industrial sensor stuff. If you want  to discern 0 degrees from -0.1 and +0.1 you should be able to detect 0.0059 mV (if you can linearly interpolate for the junction). Practically imo I would then demand to be almost sure that my meter measured real 0 to between -0.003 and 0.003, real 0.0059 to between 0.0031 and 0.0090 etc. The limits here are not important (imo), but basically I am asking to measure 0.003 mV on a 20 mV range. Well 0.003/20=0.00015 which is 0.015% of full scale.

The 34401A has basic DC accuracy +-0.0035% which is less than 5 times what is needed. If the Agilent is out of calibration it might very well not be able to do this measurement.

My quasi-math might be flawed, but I am sure some forum members will correct or refine what I say. Also one should may be discuss resolution and accuracy separately. I'll leave that to people more knowledgeable than me (i.e. less lazier :=\). Here is the link to the lab where they state 'Thermocouples generate very small voltages, which must normally be amplified to be read by a DAQ or oscilloscope, although the 6-1/2 Digit DMM has sufficient range.' Then they go on putting a DC amplifier in front anyway? :scared: Makes me wonder how many digits and what accuracy you need to be reasonably sure that the offset for the DC amplifier is neglectable in this case ...


Edit: Link: http://www.eng.hmc.edu/NewE80/TemperatureLab.html
« Last Edit: May 19, 2013, 11:30:20 am by quantumvolt »
 

Offline robrenz

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Much easier with a RTD if it covers your temp range.  Some temp gages alone that can measure to .001 deg C resolution cost more than the 8846A

Offline saturation

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Concur, the Keithley manual is great summary and a must for everyone. If you work with such signals regularly its pays say to get an electrometer vs a high digit DMM, at some point its the only way to work.  For e.g. an electrometer has teraohm input impedance, good DMM like the 3456a has gigaohms, but even then it may load some systems too much.  The Agilent 1252a HH DMM can resolve 1 or 10uV I believe, and  has >1G ohm ohm input impedance at under 1V scale, 10+ megaohm otherwise, and beware, 1 megaohm in dual display mode!  Its low voltage capability is a reason I grabbed it while it was still being made.



Such work is purely analog.  Detecting tiny voltages in organic systems [ e.g. nerve potentials] or materials [ piezo, thermo electric etc.,] are all the realm of low power signals requiring very high input impedance.  For materials labeled as high in the Keithley graph, a good DMM works very well [e.g. the nerve of an octopus is fairly large versus a single axon in rats].

Likewise, detecting low variation in an otherwise 'normal' signal like a NiMH self discharge voltage, can only be detected if you have a high precision DMM, that is looking at uV variations within 1V signal: 1.000 000VDC.  In many modern designs, you can check the quiescent power drain of 'smart off' devices that are really never power off mechanically using the LSD of your DMM.




The Keithley 'Low level measurements handbook' contains answers to some of these questions, for example which types of insulation are best for various applications. It will also explain why those primitive 6.5 digit DMMs are not the best tool for some low-level measurements ;). I think it's available for free from their website, unless the evil Danaher group made them pull it.
« Last Edit: May 19, 2013, 01:49:23 pm by saturation »
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Offline madshaman

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when do you really need 6 1/2 DMM ? , I mean the res and/or the accuracy
« Reply #40 on: May 20, 2013, 03:12:37 pm »




Home build Vref based on LM399 connected to my Fluke 720 . 100nV resolution
(got 2 x 7,5 digit, 2 x 6,5 digit and 1 x 5,5 digit and a bunch calibrators

1) Jealous
2) Do you keep one of your 7.5 meters as a master reference and do-you/how-to-you keep all your meters calibrated around your home-built vref?
3) Have you had your vref calibrated to a NIST traceable standard?

Benchtop-wise, I have one 7.5 digit meter, 4x6.5 digit meters, and the only decent handhelds I have are two Brymen 867s.

I would really like to have all my meters dead-on accurate as well as precise, but I don't really know all I need and how to build myself a calibration schedule and procedure.

(All my meters track most values down to the LSD together, so they're close to each other, but I have know idea how close to a reference they are).

Total side note: is it obscenely difficult to set things up so your own home lab can procude NIST traceable calibration certificates itself?  (as a non industry hobbiest, I know nothing or little avout this)
« Last Edit: May 20, 2013, 03:17:19 pm by madshaman »
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Offline saturation

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At some point you need to reference it against a standard.  But the big question you have is can you create a standard that has no other reference but itself, like the JJ is for volts in the world?

JJ compare themselves against frequency standards, and making conversions is the key.  In this paper, you see 2 separate JJ setups compared against a single microwave source and shows the degree of agreement.

http://iopscience.iop.org/0026-1394/31/1/007

Since you cannot do a similar task with semiconductor references, what you can do is the old-style metrology methods.  To make a long story short you need to track the drift of your reference vs time.  The curve will show cyclic variations with climate, and a long term variation over its mean.   You can then reference your measurement against itself, to the past, and knowing at the zero point, you were in calibration against the standard, estimate the variation from the true value.  This creates a level of certainty and thus, your standard is now expressed both as a value, and a degree of certainty; what is purely physical now becomes partly statistical.

Referencing the photo, you cannot use a reference so young that it drifts in mean as is A, either up or down in value.  You want drifts like b.  Choose a type of reference design that reduces the width of c.  Your stable reference should give a mean value of d and the certainty is effectively the width of c.

The X axis is time, the Y is volts.



The graphic is a simplification, the real data can be hard to decipher until you amass enough data to see the forest from the trees.  An example of real data, taken from a volt nuts post.



..
I would really like to have all my meters dead-on accurate as well as precise, but I don't really know all I need and how to build myself a calibration schedule and procedure.

Total side note: is it obscenely difficult to set things up so your own home lab can procude NIST traceable calibration certificates itself?  (as a non industry hobbiest, I know nothing or little avout this)
« Last Edit: May 21, 2013, 01:13:56 pm by saturation »
Best Wishes,

 Saturation
 

Offline madshaman

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when do you really need 6 1/2 DMM ? , I mean the res and/or the accuracy
« Reply #42 on: May 21, 2013, 04:52:15 pm »
Thanks saturation, that gives me a good idea of the general process.

Given that I'm unlikely to buy references with calibration history or will more likely roll my own (money), I take it I start with getting them calibrated often at first and establish and log their long-term drifting behaviour.  If I calibrate my other instruments against these references, I log this action.

Then, at any point I can extrapolate both any instrument's expected drift at the time a measurement was taken and also a certainty metric?

So in essence, keeping a well calibrated lab is mostly documentation/logging (and the ability to extrapolate from the collected data) ?
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Offline saturation

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You're welcome, and yes, that's exactly right.  Also the "c" part of the graph can be kept very narrow by controlling the climate, that's a key factor in variation, temperature is the easiest, humidity somewhat and pressure difficult. Metrology labs keep very detailed careful records, but today, its all automated.  Most engineers will look at the data and analyze it for quality assurance, as practically, most labs will do annual comparisons against national lab standards anyway to meet ISO procedures, which you won't have to and can rely on statistics.   Frequent comparisons to a standard reduces uncertainty to a lowest possible value.

Its not difficult work, but one has to be 'anal' about doing collecting data.

For my lab, I stagger the calibration dates, so they don't expire at the same time, and so in theory the 'calibrated' instruments check the uncalibrated ones like a round-robbin, eventually they all 'calibrate' themselves.


Thanks saturation, that gives me a good idea of the general process.

Given that I'm unlikely to buy references with calibration history or will more likely roll my own (money), I take it I start with getting them calibrated often at first and establish and log their long-term drifting behaviour.  If I calibrate my other instruments against these references, I log this action.

Then, at any point I can extrapolate both any instrument's expected drift at the time a measurement was taken and also a certainty metric?

So in essence, keeping a well calibrated lab is mostly documentation/logging (and the ability to extrapolate from the collected data) ?

Best Wishes,

 Saturation
 


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