'Affordable' Reference Standards to Have On-Hand in a Electronics Lab

[mawyatt]

Would be very interesting to see what other folks find wrt this squarewave concept.

If only there would have been another topic where the same solution and problems with it were already discussed by the same people.

This would be in reference to someone with an HP3458A!!

Best,

[mawyatt]

..... It probably puts some of us in mind of a certain controversial 10V reference by an ex-member though. He made extravagant / magical claims (2ppm) for a bodged together matrix board,copper foil, and dodgily soldered unit that he still sells on ebay to this day for >$1200 (do a forum search for 'D-105' if you're interested in the sorry tale).

Just a matter of not calling a spade an earth inverting horticultural implement. You don't need it.

Remember that "Devine Intervention" type reference, with the magical thermal equilibrium protoboard construction, "Heavenly Derived" thermal compensation diode and "Majestic Thermal Pipe". Reading those posts was entertainment for the evening, of course along with ones favorite brew

Best,

[deepfryed]

Thanks to you fellas I wasted the evening reading that thread but I did have a good chuckle. He's still selling the divine device for over $1000 now 

[J-R]

While it's true that Kelvin, Kelvin-Varley and Hamon dividers, various bridges and a null meter could be used to accomplish many DMM calibration steps, and this method is frequently used in older calibration procedures, the problem remains that you need to acquire or construct all that equipment which is not trivial or cheap.  I also consider it to be more prone to errors/mistakes, as you're not using another calibrated DMM to verify the outputs before setting a calibration point.

So a solid solution still is to just buy a DMM that you can afford to have calibrated regularly.


Back to references, DMMCheckPlus.com & VoltageStandard.com surely are the lowest-end worth considering.  Both of these companies have good reputations and have many public reviews.  It defeats the entire concept of a reference/standard to have something that is surrounded by unknowns.  Buying these eBay references is worse than not having them because it lulls people into a false sense of security such that they might adjust their equipment, and it wastes their money in the process.

I see our eBay-er has added the "NEW PRECISION MULTIMETER CALIBRATION LAB" listing for approximately $110.
For under $175 delivered, you can get a new Brymen BM235 from Welectron with an ISO calibration, which is going to cover all of the DMM's functions and ranges, not just a small sub-set.  Under $275 for the BM789 which is a 50,000 count DMM.

[alm]

While it's true that Kelvin, Kelvin-Varley and Hamon dividers, various bridges and a null meter could be used to accomplish many DMM calibration steps, and this method is frequently used in older calibration procedures, the problem remains that you need to acquire or construct all that equipment which is not trivial or cheap.  I also consider it to be more prone to errors/mistakes, as you're not using another calibrated DMM to verify the outputs before setting a calibration point.

I linked to the mini metrology lab tutorial which builds these devices quite cheaply. Sure, it won't give you standards lab level performance, but neither will give any DMM. That you trust a DMM that has been calibrated in the past more than standards and bridge-circuits that can be verified before every important measurement is your opinion. I trust my standards over anything else in my lab. For me DMMs are just ratio devices, like the old dividers, to measure the ratio between an unknown value and a reference. We can agree to disagree there.

I see our eBay-er has added the "NEW PRECISION MULTIMETER CALIBRATION LAB" listing for approximately $110.
For under $175 delivered, you can get a new Brymen BM235 from Welectron with an ISO calibration, which is going to cover all of the DMM's functions and ranges, not just a small sub-set.  Under $275 for the BM789 which is a 50,000 count DMM.

Okay, so now you have a DMM that was calibrated on all ranges and functions, and you have another DMM that you want to verify on all ranges and functions. What now? You still need sources for all ranges and functions. I don't see how buying yet another DMM solves that.

[bdunham7]

You still need sources for all ranges and functions. I don't see how buying yet another DMM solves that.

Actual standards worthy of the name are one thing, but at this level for sources you can get an FY6x00 AWG and a few select non-precision test resistors and perhaps a few PP capacitors. 

[alm]

Actual standards worthy of the name are one thing, but at this level for sources you can get an FY6x00 AWG and a few select non-precision test resistors and perhaps a few PP capacitors.

I have done that. It's... interesting with bench meters (which is what the topic started with). But the argument was that a DMM gives you all the ranges and functions, as opposed to a cheap 'calibrator' that will just test a few functions and ranges. But in my experience getting the sources to compare multimeters is actually the hard/expensive/bulky part. Especially high voltage DCV, ACV beyond what a function generator can produce, and AC current.

[bdunham7]

But in my experience getting the sources to compare multimeters is actually the hard/expensive/bulky part. Especially high voltage DCV, ACV beyond what a function generator can produce, and AC current.

If you're doing informal verification, you can live with what you have.  If you want to do an actual calibration with adustment on even a simple handheld, then you need a lot of stuff.  There are piles of such stuff in my office (working) and in my garage (awaiting repair). The better handhelds are actually more difficult in some ways than bench meters because they need exact nominal sources while typical decent bench meters only need stable and accurately known sources.

But yes, actually coming up with 750VAC @ 50kHz is a challenge.  The solution is in my garage and weighs a few hundred pounds.

[Majorassburn]

So a solid solution still is to just buy a DMM that you can afford to have calibrated regularly.

Back to references, DMMCheckPlus.com & VoltageStandard.com surely are the lowest-end worth considering.  Both of these companies have good reputations and have many public reviews.  It defeats the entire concept of a reference/standard to have something that is surrounded by unknowns.  Buying these eBay references is worse than not having them because it lulls people into a false sense of security such that they might adjust their equipment, and it wastes their money in the process.

I see our eBay-er has added the "NEW PRECISION MULTIMETER CALIBRATION LAB" listing for approximately $110. For under $175 delivered, you can get a new Brymen BM235 from Welectron with an ISO calibration, which is going to cover all of the DMM's functions and ranges, not just a small sub-set.  Under $275 for the BM789 which is a 50,000 count DMM.

Hi. Thanks for referring to my amateur calibrators on eBay. It gives me the opportunity to clarify why I think they offer some value to a certain segment of the market that professional, calibrated, expensive standards and DMM's just can't serve.

!.  Some might interpret your comments, "Buying these eBay references is worse than not having them because it lulls people into a false sense of security" as unfounded because:  a) The comment assumes that any such buyer is ignorant as to the realities of any calibrator's limitations and, therefore, b)  Such a buyer should not, under any circumstances, be playing with anything that runs on batteries, is plugged into the wall or, is powered by alternate energy sources of any kind! 

2.  Your suggestion that buying a 50,000-count Brymen for a hundred or so more is an alternative but really impractical because you still lack a stable source for the accurate comparison of various DMM's, without regard to the actual calibrated accuracy of that source.

I think what you may have missed here, probably because I failed to emphasize it earlier, is the point that possessing, for example, an AC Sine Wave Calibrator or a 10 Volt Reference or a Precision Resistor module gives one the ability to use IT as the source for comparison of the various DMM's attached to it so that you can note the varied readings of your attached DMM's.

As an example, while possessing a NIST-traceable, calibrated, multi-thousand-dollar 10V reference, your attached Brymen and Agilent and Keysight, etc. 10-12-digit DMM's are ALL going to disagree as you move toward LSD's. So, which one do you end up loving and which one finds a new home? 

My eBay amateur 10V Reference, 0.05% basic accuracy, trimmed to 0.0001% accurate, Multi-Output reference module will allow you to do the same thing but, for only 50 bucks! And, you get TWO other useful and usable reference outputs for the 2-6V ranges thrown in! Whatta Deal! 

Granted, you cannot trust my eBay references to do much more than get you in the ballpark. BUT, after you buy them, you can still afford to make your mortgage payment! 

I think the overall message in this discussion is that, short of possessing a slew of expensive calibrating standards AND incredibly expensive calibrating equipment AND the experience and ability to re-calibrate various DMM products that allow such re-calibration, none of us can really calibrate ANYTHING to perfection. And, I also acknowledge the fact that most lower-cost meters cannot be "calibrated" at all, in the traditional sense. No knobs or dials or 'twisties' inside. They can only be verified, right?

I do believe that there is some meaningful value to a lower-cost DMM owner in being able to access a $30 sine wave source that can put out a non-fluctuating, usable 100Hz, 0-6VACrms, Lo-Z voltage source to at least verify that when you stick the probes into a 120VAC wall socket and it bounces around and finally reads 124.2VAC, that you're reasonably sure the damned thing is reading "close enough for guvmint work". Same for the other "calibrators" I offer.

Two other suggestions:  1) Having access to low-cost but properly-presented references like the ones I offer is better than having nothing at all. And, it's much cheaper than buying a calibrated multi-thousand-count DMM to compare that $30 do-all, Shanghai-Special to.

2) I agree that the DMM-checkers you referred to certainly are excellent quality and trustworthy but, they, too are limited in capability as initial verifiers of functionality. And, they may also mislead the lesser informed DMM owner into a false sense because 'DDS-ing' a square wave and grinding off its inconvenient edges to APPROXIMATE a sine wave has its own inherent faults, as well.

I think we all agree that lesser-quality DMM's may handle DDS-ed and AC square waves, no matter how many ppm's accurate they may be, differently and produce somewhat varied 'readings' as to the accuracy of the attached DMM's AC range really are. Just hook up a gaggle of $30 DMM's to a DMMCheck-generated, bi-polar, square wave output and you'll get a wildly different reading from each one. 

So, in summary, I invite your further suggestions and constructive criticisms of my efforts so that I can continually improve my amateur low-end offerings. Thanks in advance.

[J-R]

Of course the odds are extremely high that an amateur will be ignorant of your calibrators' limitations.  Also, your listings are filled with confident exposition which will surely tickle the ears of potential newbie shoppers...

Short-term stability is far easier to obtain than accuracy, so having a calibrated DMM in general is still more usable than having a reference standard.  To compare the calibrated DMM to another you only need to check the readings at the same moment, which is extremely trivial and can be done with equipment found on most electronics benches.  And you can check many ranges, not just the ones your calibrators cover.

The VoltageStandard.com 0.001% 10V reference is only $130, not thousands.  The individual TC graph is particularly a great value-added feature.

There is no issue with DMMs and the references disagreeing, because at minimum you have published accuracy specs and temperature variations to consider.  I can check the calibration data for my calibrated DMMs and references to see the reasons for any displayed discrepancies.

I've repaired and calibrated/adjusted about a dozen DMMs in the last few years and it's really not that hard if you have another, better calibrated DMM to use.

Ultimately it's just illogical for someone to buy $110 worth of calibrators to check only a few ranges/functions on a $30 DMM.  If you want to know if your cheapie DMM is working correctly, it's trivial to do this with things you can find around your house.  If you want accuracy, then you would not be putting all your trust in a $30 DMM, but instead you would go the route I presented which is buy one or more calibrated DMMs, such as from HPAK/Fluke/Brymen/etc..  And it's just not that expensive to go this way.


If you are serious about selling references, then I would suggest at minimum:
- obtain a quality 6.5 digit DMM and have it calibrated yearly
- use proper PCBs ($1-$2 per board from any of the popular PCB makers?)
- at least add stand-offs or feet to the bottom of the PCB
- perform long-term tests/characterizations of your calibrators, burn-in, drift etc.
- provide a calibration printout with each unit sold (values + environmental data)
- offer a recalibration service
- ship some samples to various Youtubers for review

[alm]

Short-term stability is far easier to obtain than accuracy, so having a calibrated DMM in general is still more usable than having a reference standard.  To compare the calibrated DMM to another you only need to check the readings at the same moment, which is extremely trivial and can be done with equipment found on most electronics benches.  And you can check many ranges, not just the ones your calibrators cover.

For 3.5-4.5 digit DMMs yes, but this topic started off with bench meters (which I interpret as 4.5-6.5 digits) and 50 ppm uncertainty. At that level finding a sufficiently accurate DMM or stable source get difficult. I've tried comparing 6.5 digit DMMs against bench supplies, function generators and metal film resistors, and trying to find correlation between the signals.

What equipment do you find on the average bench to generate > 90 Vdc, > 100 mA AC or > 20 Vrms AC, particularly something like 100 V 50 kHz or 1000V 100 Hz? I really struggled with this, and have done things like adjusting a 300 Vac range at 7 Vrms 1 kHz because that was all my function generator could produce. Eventually I got a calibrator that can do it, but that's not exactly common lab equipment.

[Majorassburn]

If you are serious about selling references, then I would suggest at minimum:
- obtain a quality 6.5 digit DMM and have it calibrated yearly
I already have a calibrated DMM that adequately verifies for this category of calibrators.
- use proper PCBs ($1-$2 per board from any of the popular PCB makers?)
Can't do that and still keep price in line with market demographic.
- at least add stand-offs or feet to the bottom of the PCB
Will do that for new listings. Thanks.
- perform long-term tests/characterizations of your calibrators, burn-in, drift etc.
Have done so but will start to publish that.  Long term will be in hours, not months, etc.
- provide a calibration printout with each unit sold (values + environmental data)
Already do that in packaging.
- offer a recalibration service
Will do that, Thanks.
- ship some samples to various Youtubers for review
I'd rather you evaluated my stuff because you're really serious about calibration. Wanna try?

Thanks for your suggestions. It'd be fun and a real eye-opener if somebody with your enthusiasm, experience and equipment took a look at one of my modules and reported their findings. I'd certainly like to know just how good or bad they are under your critical eye. But, remember that these are for the $30-$100 DMM buyer and selling price minus fees are an overriding consideration, I can't have you demanding 5ppm performance over 5 years with a 5,000 hour burn-in and still offer them for 20 bucks or so, right? 

[bdunham7]

Thanks for your suggestions. It'd be fun and a real eye-opener if somebody with your enthusiasm, experience and equipment took a look at one of my modules and reported their findings. I'd certainly like to know just how good or bad they are under your critical eye. But, remember that these are for the $30-$100 DMM buyer and selling price minus fees are an overriding consideration, I can't have you demanding 5ppm performance over 5 years with a 5,000 hour burn-in and still offer them for 20 bucks or so, right? 

It would actually save you time and money if you just laid out a board in KiCad and had someone produce it for you vs using perf board. 

If you really want a 'review', send me one of your AC modules or whatever you like.  My equipment is all just laying in a heap right now for a few weeks, but I can easily set your module up and log it for a week or so to see what its drift and tempco are like.   I'll send it back when done.  PM me if you want my address.

[alligatorblues]

If you are serious about selling references, then I would suggest at minimum:
- obtain a quality 6.5 digit DMM and have it calibrated yearly
I already have a calibrated DMM that adequately verifies for this category of calibrators.
- use proper PCBs ($1-$2 per board from any of the popular PCB makers?)
Can't do that and still keep price in line with market demographic.
- at least add stand-offs or feet to the bottom of the PCB
Will do that for new listings. Thanks.
- perform long-term tests/characterizations of your calibrators, burn-in, drift etc.
Have done so but will start to publish that.  Long term will be in hours, not months, etc.
- provide a calibration printout with each unit sold (values + environmental data)
Already do that in packaging.
- offer a recalibration service
Will do that, Thanks.
- ship some samples to various Youtubers for review
I'd rather you evaluated my stuff because you're really serious about calibration. Wanna try?

Thanks for your suggestions. It'd be fun and a real eye-opener if somebody with your enthusiasm, experience and equipment took a look at one of my modules and reported their findings. I'd certainly like to know just how good or bad they are under your critical eye. But, remember that these are for the $30-$100 DMM buyer and selling price minus fees are an overriding consideration, I can't have you demanding 5ppm performance over 5 years with a 5,000 hour burn-in and still offer them for 20 bucks or so, right? 

My lab has 4 732Bs, 2 SR104s, a 720A,  a calibrated 3458A, a 34420A, 10 6.5-digit meters, along with many lower precision standards, fixed dividers, a 1.0 ohm standard to 10ppm. I can get repeatable voltage 10VDC measurements to 0.01ppm. But that is quite a project.

So, I'd be willing to give your standard a run through the lab. I'd be interested in just what is available in that price range.

[Conrad Hoffman]

Pretty crummy lab if you don't have a Josephson junction voltage reference.   
FWIW, way back when I did the Mini-Metrology articles, I spent over a year going down to the lab every morning and intercomparing several standards with an ovenized standard cell array. After a year I had reasonable confidence in the performance. This stuff doesn't happen overnight.