interested in hobbyist voltage reference standards?

[slburris]

Hi,

I'm using the cheap Malone voltage standard for my home lab:

http://www.voltagestandard.com/Home_Page_JO2U.html

It's supposed to be 0.01% accurate.

I know about the more expensive Malone standard:

http://www.voltagestandard.com/New_Products.html

as well as the Geller Labs standard:

http://www.gellerlabs.com/SVR%20Series.htm

Anyone have opinions about these standards?  Know of any better
ones that a hobbyist could afford?

I'm always looking for better standards :-)

Scott

[Simon]

so whats the point of this post other than to show us something only someone with more time on their hands than is good for them would worry about ?, i'm not sure how stopping the ship from getting stress by putting slots round it helps, any ideas ?

[slburris]

Well the point is I have a Fluke 8505A bench meter which is specified to
be accurate to 0.001% +- 1 count.  It's quite expensive to send this
equipment out to a calibration lab, so I'm exploring what's state of the
art for home use. 

If no one else is interested in such things, that's OK, I'll go find someone
else to ask.

As for the slots and mechanical stress, I have no idea how important
that it and how well the slots work.  I'd think keeping everything at
some constant temperature would be more important.

Scott

[DJPhil]

Anyone have opinions about these standards?  Know of any better ones that a hobbyist could afford?

I think you've covered what I know and more, but I'm the least knowledgeable one here regarding voltage references aside from some of the history. I asked my physics teacher once how it was that mankind agreed on standards, and wound up doing a science project based on the Weston Cell. I'm glad we have better now, those things are fickle.

so whats the point of this post other than to show us something only someone with more time on their hands than is good for them would worry about ?

Aw, come on now. I'm sure most of us here can relate to having a technical passion, if not a meticulous obsession. Geek solidarity!

Well the point is I have a Fluke 8505A bench meter which is specified to be accurate to 0.001% +- 1 count.  It's quite expensive to send this equipment out to a calibration lab, so I'm exploring what's state of the art for home use.

Assuming you're not worried overmuch about keeping your unit documented as traceable you might be able to find a person, company, or university nearby that would allow you to check your Fluke against a regularly calibrated unit they have. This is what I'd try, as you might make some interesting friends along the way.

This topic actually came up not that long ago, and we had a visit from Joe Geller. I had to look up what a proton magnetometer was.

[saturation]

Hello, and thanks again for your fine work on the Uni-T, I have more comments but its time to say goodnight soon!

There's another thread here on the same topic:

https://www.eevblog.com/forum/index.php?topic=601.msg7082#msg7082

I think what you ask is a very important question to anyone without access to a calibration reference standard.  Calibration, and adjustment if need be, is like sharpening your knives, thinning your soldering iron, etc., it keeps your tools in shape.  Sure, you can work with a dull knife, up to point, just as you can with a cheap inaccurate DMM.  

https://www.eevblog.com/forum/index.php?topic=323.msg7053;topicseen#msg7053

I brought up the topic in this post, in relation to discussing what you are now doing, calibrating or checking your new DMM's accuracy and precision, and later its reliability [ i.e., accuracy and precision over time period, like months to years.]  Your high end Fluke probably gives its specifications tied to months or days from turn on, to reflect its reliability [ handheld's usually 'refer' to its reliability by specifying a recommended calibration cycle in 1-2 years].

The only other option to buying these references is making your own, see the link on my post, using a voltage reference chip.  The band gap variety are very stable, but to be sure, you still need to reference that chip to a calibrated meter, so the Geller style approach I think is the best for now.  It would cost about $10-20 every calibration cycle or so to recalibrate your references, $5 as Geller or the other's charge, and the rest for postage.

Another approach is to reduce your cost is buy 2 4 months apart, Geller's is the most accurate so far.  Since they are roughly stable for about 6 months, before the time is out for one reference, you can calibrate check it against the one that is still within cycle and vice versa.  If your high end meters are also calibrated checked against these standards, you can go theoretically for some time before sending it back for recalibration, as you can cross reference each high end reference and meters, against each other.

Its one reason its not unreasonable to own several DMM of the same class, as you may use them to monitor different parts of a design simultaneously, and also serve to check their calibrations against each other.  Of course, owning several DMM of the FLuke 8505 class, if you really have little use for such, is more costly that getting a Geller.

Finally, in my experience quality meters may not drift at all, and you find how a particular device functions only by experience, once you get a feel for it, a true calibration cycle can be prolonged.

Hi,

I'm using the cheap Malone voltage standard for my home lab:

http://www.voltagestandard.com/Home_Page_JO2U.html

It's supposed to be 0.01% accurate.

I know about the more expensive Malone standard:

http://www.voltagestandard.com/New_Products.html

as well as the Geller Labs standard:

http://www.gellerlabs.com/SVR%20Series.htm

Anyone have opinions about these standards?  Know of any better
ones that a hobbyist could afford?

I'm always looking for better standards :-)

Scott

[saturation]

Every engineer to technician is also part metrologist, to varying degrees!  Measurement, and the science of getting it done right, is at least part of everything we do.

Aw, come on now. I'm sure most of us here can relate to having a technical passion, if not a meticulous obsession. Geek solidarity!

This topic actually came up not that long ago, and we had a visit from Joe Geller. I had to look up what a proton magnetometer was.

[slburris]

I've looked at the Geller labs reference before, and it's "only" accurate to
0.005% over 6 months.  The new more expensive Malone reference
is can do better, 0.0025%, but both of these are still more then double what
the Fluke 8505A is rated for.

I suppose what you are saying is keep sending them back for recalibration
(or at least remeasuring with their 8.5digit calibrated multimeters) in order
to hide their drift over time.  Yeah, I guess that would work.

In general, I'm finding my Fluke bench multimeters to have excellent
stability.  I've also got a Fluke 8502A meter which agrees with the 8505A
within +- 2 digits.  So they are either stable or drifting together :-)

What's frustrating is this problem is solved for frequency.  Just get a GPS
locked frequency reference and you have Cesium clock stability for < $200.
All I need is someone to make a cheap Josephson Junction voltage standard
that I could feed the GPS clock to, and I'd be set, see:

http://assets.fluke.com/appnotes/Calibration/Josephson-Voltage-Standard-in-a-working-Calibration-Laboratory.PDF

Maybe in 20 years I'll be able to buy this on Ebay!

I've heard that the Linear Technology LTZ1000 reference has extremely
low drift.  Found someone selling 7voltish references with this part:

http://www.maxmcarter.com/vref/

Scott

[Simon]

hm if your looking to calibrate a meter I'm not sure just firing 5 volts at it only will be a good test to say it's ok, you should really test a number of voltages, if possible trying to engage each range (as different internal circuitry will get used on each range), of course if you just want to do a quiky on it to satisfy yourself go ahead

standards ? what I never figured out was how could we have known the speed of light hundreds of years ago ? or did the definition of a metre being 1/299'999'999.something of the distance light travels in a second come later ?

[DJPhil]

standards ? what I never figured out was how could we have known the speed of light hundreds of years ago ? or did the definition of a metre being 1/299'999'999.something of the distance light travels in a second come later ?

That's where the story gets interesting, I love science history. My memory of the speed of light tests is limited to Focault's gear wheel gizmo. I'd forgotten about the astronomy that had been done prior. I'm almost certain that the meter would have come first, as it is much more useful for everyday measurement. In Focault's time, the standard was a platinum bar that resided in Paris. In the 60s we managed to improve the accuracy by referencing the meter to the speed of an atom of krypton sneezing, finally improving on the bar standard a thousandfold by referencing length to the speed of light. This is the present state of affairs, with a small addendum by the International Committee for Weights and Measures (CIPM) recommending this definition be restricted to "lengths which are sufficiently short for the effects predicted by general relativity to be negligible with respect to the uncertainties of realisation."

This is what happens when your television diet doesn't often stray from the science channel and James Burke reruns for a decade.

[cybergibbons]

The calibration procedures I have seen for expensive meters seem to involve testing everything in decades - 0.9V, 9V, 90V, 900V etc.

Whenever I've used a meter in an industrial application, we've had a test station which generally has 12V, 24V, 50V, 110V, 240V, and 415V, along with a wire and a 100 ohm resistor for continuity tests. Though you aren't really calibrating in this situation, more just checking that the meter is working at all.

[alm]

Calibrating/checking a 3.5 digit .3% meter (as you might use in industrial applications, that 24V supply probably has a tolerance of 5% or so) is much easier than calibrating a 6.5 digit 25ppm meter. For calibration / performance verification, your reference usually must be an order of magnitude or so more accurate. For example, in the HP 3468A calibration procedure, HP recommends that the reference multimeter they recommend (either 3455A or 3456A, don't recall) is calibrated shortly before the calibration, because the 90 days accuracy is not good enough to be significantly better than the 3468A.

[saturation]

For completeness sake, you are absolutely right and I'm wrong.  Although what calibration consists of varies from device to device, in most cases it does require more than measuring the Vdc of just one range, or a single measurement on any range.

However, in many lesser DMM, the calibration can be very easy [ if they provide that type of documentation at all ] as measuring only Vdc but those are typically low end DMMs.  

I've corrected my post to use a better term!

The calibration procedures I have seen for expensive meters seem to involve testing everything in decades - 0.9V, 9V, 90V, 900V etc.

Whenever I've used a meter in an industrial application, we've had a test station which generally has 12V, 24V, 50V, 110V, 240V, and 415V, along with a wire and a 100 ohm resistor for continuity tests. Though you aren't really calibrating in this situation, more just checking that the meter is working at all.

[Simon]

standards ? what I never figured out was how could we have known the speed of light hundreds of years ago ? or did the definition of a metre being 1/299'999'999.something of the distance light travels in a second come later ?

That's where the story gets interesting, I love science history. My memory of the speed of light tests is limited to Focault's gear wheel gizmo. I'd forgotten about the astronomy that had been done prior. I'm almost certain that the meter would have come first, as it is much more useful for everyday measurement. In Focault's time, the standard was a platinum bar that resided in Paris. In the 60s we managed to improve the accuracy by referencing the meter to the speed of an atom of krypton sneezing, finally improving on the bar standard a thousandfold by referencing length to the speed of light. This is the present state of affairs, with a small addendum by the International Committee for Weights and Measures (CIPM) recommending this definition be restricted to "lengths which are sufficiently short for the effects predicted by general relativity to be negligible with respect to the uncertainties of realisation."

This is what happens when your television diet doesn't often stray from the science channel and James Burke reruns for a decade.

So thats why it's not 1/300'000 of the distance light travels in a second

[saturation]

Again, if you'll are interested in why the standards are vital and how they are calculated, particularly for commerce -- trouble brews when the pump only puts out 1/2 gal of gas and charges for a full because their pumps were not calibrated -- you can read about it at the NIST website or a summary here:

http://en.wikipedia.org/wiki/Metrology

NIST differs from 'weights and measures' crowd in that they are pure science and measurement accuracy, no enforcement or binding rules and law.

As Alm mentions, when you get into it, metrology includes a probability component to accuracy due to drift and other uncertainties, so getting a value today, is no guarantee it will be that way when you need to measure it.

In many cases, calibration is more so you can pass the buck, its 'SOP' or accepted practice.  If you did the right thing and a production run is off because you measured 1V when it should have been 10V, nearly impossible as that seems, it won't be your fault, but the documentation of your instrument will try to pass the buck.  So in the end, the final measure is a cross referencing of several instruments, all presumed accurate, against each other at the time of measurement.

James Burke?  I haven't heard that name in a while.  The first Connections is a classic, I don't think any science show since that can match its depth and breath.

standards ? what I never figured out was how could we have known the speed of light hundreds of years ago ? or did the definition of a metre being 1/299'999'999.something of the distance light travels in a second come later ?

That's where the story gets interesting, I love science history. My memory of the speed of light tests is limited to Focault's gear wheel gizmo. I'd forgotten about the astronomy that had been done prior. I'm almost certain that the meter would have come first, as it is much more useful for everyday measurement. In Focault's time, the standard was a platinum bar that resided in Paris. In the 60s we managed to improve the accuracy by referencing the meter to the speed of an atom of krypton sneezing, finally improving on the bar standard a thousandfold by referencing length to the speed of light. This is the present state of affairs, with a small addendum by the International Committee for Weights and Measures (CIPM) recommending this definition be restricted to "lengths which are sufficiently short for the effects predicted by general relativity to be negligible with respect to the uncertainties of realisation."

This is what happens when your television diet doesn't often stray from the science channel and James Burke reruns for a decade.

hm if your looking to calibrate a meter I'm not sure just firing 5 volts at it only will be a good test to say it's ok, you should really test a number of voltages, if possible trying to engage each range (as different internal circuitry will get used on each range), of course if you just want to do a quiky on it to satisfy yourself go ahead

standards ? what I never figured out was how could we have known the speed of light hundreds of years ago ? or did the definition of a metre being 1/299'999'999.something of the distance light travels in a second come later ?

[slburris]

Here are the two higher end meters I have.  It's a bad disease when
you start collecting this stuff.  I've occasionally seen non-working
8.5digit Datron meters on ebay, and I find myself wondering if I could
fix them, then of course the calibration issue becomes much worse!

[DJPhil]

James Burke?  I haven't heard that name in a while.  The first Connections is a classic, I don't think any science show since that can match its depth and breath.

A quick aside on Mr. Burke. There's a fellow on youtube who's posted just about everything he's done. It's a bit unorthodox as youtube channels go, so be sure to skim the directions. Top quality science history programming, and a wonderful dose of humor to boot.

[saturation]

You are not alone; I have a mind to do the same.  Much reference quality equipment, if working, costs as much as new low end gear.  One need be aware some may be difficult to repair if not working .

This fellow does so but with a focus on atomic clocks and support equipment; a lot from eBay.




http://www.leapsecond.com/


but on electronics per se:

http://www.kennethkuhn.com/hpmuseum/

Here are the two higher end meters I have.  It's a bad disease when
you start collecting this stuff.  I've occasionally seen non-working
8.5digit Datron meters on ebay, and I find myself wondering if I could
fix them, then of course the calibration issue becomes much worse!

[saturation]

To quote his last comments on the last episode:

"Lastly, if the entire modern world is built from these interconnected innovations, all increasingly maintained and improved by specialists who required years of training to gain their expertise, what chance does the average citizen without this extensive training have in making an informed decision on practical technological issues, such as the building of nuclear power plants or the funding of controversial projects such as stem cell research? Furthermore, if the modern world is increasingly interconnected, what happens when one of those nodes collapses? Does the entire system follow suit?"

http://en.wikipedia.org/wiki/Connections_%28TV_series%29

I spent years tracking for Connections 1 on recorded tape to use in classes and what existed was poor compared to what I had: a pristine PBS broadcast version I made on betamax.  I closed my class with Burke's final episode.  It wasn't until the late 2000 that it was released on DVD!  Its still commands a premium price, $100 for the DVD series, compared to under $50 for many multidisc bluray BBC Attenborough nature series.




Sorry, no more off topic hijacking threads!  Back to voltage references.

James Burke?  I haven't heard that name in a while.  The first Connections is a classic, I don't think any science show since that can match its depth and breath.

A quick aside on Mr. Burke. There's a fellow on youtube who's posted just about everything he's done. It's a bit unorthodox as youtube channels go, so be sure to skim the directions. Top quality science history programming, and a wonderful dose of humor to boot.

[Rhythmtech]

For all intents and purposes, you can perform calibration checks on your meters to a lower accuracy if you are most interested in doing a "go / no go" type check.  If any drift does show up during a reduced accuracy check you would be able to say for certain there is an issue with the meter, which may give you an idea of its condition. Otherwise, you are able to verify the meter is staying within the reduced spec.


In looking through the manual for the Fluke 8505a, it appears to me that to calibrate it to those specs would require an environmentally controlled lab in order to use reference standards that would sufficiently exceed the accuracy of the 8505a.  For most units I am familiar with, 4 times better is a general rule of thumb, however in some cases it may not be possible. If you come across a hobbyist voltage reference claiming to be more accurate than the 8505, I would question how they determined the accuracy.

In this app note by fluke, they refer to using the 8508 as a reference meter for calibration instead of a Kelvin-Varley voltage divider and a method of using PWM in voltage references.  From reading the app note, I believe the conclusion is that there is a threshold for the accuracy of an absolute reference such as the hobbyist standards we've been looking at. The app note throws a bunch of other measurement terminology and unreferenced calculations which can make it hard to follow, but there is some good info in there.

My opinion - An 8505a to be calibrated to manufacturer specs would need to go to a calibration lab, but if you only really need to rely on ~.01% accuracy you should be able to take data and track how the meter acts over time successfully at home well enough for a hobbyist. If you are doing stuff at home that requires .001% accuracy in your readings, perhaps you should be looking for grant money for new equipment, calibration costs, ISO 17025 certification, and an environmentally controlled lab.

[Rhythmtech]

Found this - Tektronics precision DC divider
1:10 and 1:100 divider.  If you have a 5 and 10 volt voltage reference this might make a nice addition to give you an easy way to take readings across a .1 V to 10 span and get a multiple point calibration.

[alm]

Note that it's designed for output into a 1Mohm +/- 1% load (average scope input), so the divider is probably designed with that 1Mohm loading in mind. Loading it with 10Mohm (or 10Gohm) will probably produce slightly higher results than specified.

[Rhythmtech]

Note that it's designed for output into a 1Mohm +/- 1% load (average scope input), so the divider is probably designed with that 1Mohm loading in mind. Loading it with 10Mohm (or 10Gohm) will probably produce slightly higher results than specified.

Good point. Any work around ideas? Maybe hand the output off to an unity gain feedback amp?  yeah it just got more complicated than need be....

[alm]

You could put something like a 1.1111Mohm +/- 0.01% (higher accuracy would be better) resistor in parallel with your meters input, if it's exactly 10Mohm on these ranges (and not 11Mohm, 10.1Mohm or 10Gohm on some ranges). I doubt an op-amp buffer will have the necessary <<0.01% accuracy. Anything you add should be significantly more accurate than the 0.01% if you don't want to reduce the accuracy.

[Rhythmtech]

You could put something like a 1.1111Mohm +/- 0.01% (higher accuracy would be better) resistor in parallel with your meters input, if it's exactly 10Mohm on these ranges (and not 11Mohm, 10.1Mohm or 10Gohm on some ranges). I doubt an op-amp buffer will have the necessary <<0.01% accuracy. Anything you add should be significantly more accurate than the 0.01% if you don't want to reduce the accuracy.

It's difficult finding reasonable .01% resistors without custom ordering let alone better. I was thinking like a chopper might do the trick, but I don't have a whole lot of experience in that area. Feel free to enlighten me.

[alm]

At some point it might just be easier to design your own divider (it's probably just a bunch of resistors, although some care might be taken to decrease thermal gradients and temperature coefficients), or search for a divider designed for the impedance of DMM's. I doubt that you could get this kind of accuracy from an op-amp without tricks (what will you use as feedback resistors?). Note that both accuracy at the base temperature (23C or so) and the temperature coefficient are important.

Of course you don't have to exceed the original specs, but it might be a waste to use a very accurate divider in that case. You can also order some 0.1% resistors for under a dollar each.