Easy way to test the calibration of a DMM (Fluke 45)?

[shapirus]

Correct, but what's the worst thing that could possibly happen? That you have to return it?

That you end up with N+1 devices about which you're not certain as to whether they're up to spec. Returning it and ordering another does not solve the problem of needing a known-good meter to perform the initial sanity check of the reference.

[Fungus]

Correct, but what's the worst thing that could possibly happen? That you have to return it?

Returning it and ordering another does not solve the problem of needing a known-good meter to perform the initial sanity check of the reference.

That's why we all own several meters. 

My BM857s and my Fluke 187 both agree (to within a digit) with what's written on the sticker.

Neither have been calibrated, but what are the odds of that?

(that they agree with each other and with the randomly-purchased reference to three or four decimal places)

[Fungus]

If you only own a single meter and there's a disagreement then it doesn't help, but that doesn't apply to anybody here...

If it agrees then it's 1000:1 that it's just dumb luck.

[shapirus]

If you only own a single meter and there's a disagreement then it doesn't help, but that doesn't apply to anybody here...

If it agrees then it's 1000:1 that it's just dumb luck.

In the OP's situation, if the reference agrees with one of the meters, but not the other, then it's very likely, or, maybe, likely enough, that the one that it agrees with is the good one, however the chance that both the reference and that meter are off is not zero.

If, however, both meters don't agree, then we end up with three untrusted devices instead of just two :).

But yeah your point is valid. Once again, know the limitations and know what you're doing, and you can get useful results out of any crap.

[KungFuJosh]

"Or spend a lot more money, build a ref yourself, and still need to send it out for calibration (after hundreds of hours of runtime to confirm stability)."

Or, spend $5! and not send it for calibration, nor wait hundreds of hours (a ridiculous suggestion) because it will immediately tell you which of the two multimeters has the 20 mV discrepancy. Then, you know which multimeter might actually be functioning, and you can send it for a cal, at less than the cost of a calibrated reference.

Not a ridiculous suggestion as thermal regulation and drift over time are things that exist. There's a reason some people leave their DMMs on 24/7.

This is the part you don't seem to understand: if the POS $5 ref you buy agrees with one of your meters, it doesn't tell you anything without valid calibration. 2 items agreeing only matters if the results are repeatable on calibrated devices. 2 devices can be equally out of spec. If both meters agree with the ref, then you might be lucky. As was mentioned, most of those $5 refs come with bullshit cal sheets that are copied over and over again and are completely useless.

[shapirus]

As was mentioned, most of those $5 refs come with bullshit cal sheets that are copied over and over again and are completely useless.

"Most" is either ambiguous, or has to be proved. Since this thread has a slight hint of metrology, I would reword it like so: there is a significant number of bogus cal sheet reports on certain types of those ref boards. At the same time, many appear to be legit, but none seem like they can be traced to an instrument with a valid calibration certificate.

[Fungus]

As was mentioned, most of those $5 refs come with bullshit cal sheets that are copied over and over again and are completely useless.

"Most" is either ambiguous, or has to be proved. Since this thread has a slight hint of metrology, I would reword it like so: there is a significant number of bogus cal sheet reports on certain types of those ref boards. At the same time, many appear to be legit, but none seem like they can be traced to an instrument with a valid calibration certificate.

There's many threads discussing these references, eg.
https://www.eevblog.com/forum/metrology/are-cheap-ad584-units-worth-it/
https://www.eevblog.com/forum/testgear/cheap-chinese-ad584-voltage-reference-legit-cal-data-let_s-find-out!/
https://www.eevblog.com/forum/metrology/cheap-ebay-ad584-voltage-references-my-experiences/

[shabaz]

"if the POS $5 ref you buy agrees with one of your meters, it doesn't tell you anything without valid calibration"

I'm almost speechless that you can't engineer a way to determine a 20 mV discrepancy without "valid calibration".
It's really not rocket science.

[shabaz]

Really ugly prototype, but it seems to indicate that even an unrecommended layout of the REF50xx chip will perform for the use-case being discussed.

I didn't have many through-hole parts, so I've used SMD capacitors with wires soldered on.
The parts used are:
REF5045AID (stored in an unsealed packet with humidity indicator indicating humidity is present, and the purchase date on the sticker is Dec 2014, i.e. 9 years old).
Input capacitor: 22uF, TPSB226K010R0700
Output capacitor: 4.7uF, TPSA475K010R1400
Noise reduction capacitor: 1uF, TPSA105K035R3000
Input supply: 9V PP3 battery

Just a quick measurement with a Fluke 87V was 4.503V, so it certainly appears to be a good result, but I'll measure with a DMM6500 tomorrow (and will also try some more samples of that part, plus some REF5020AID.

[KungFuJosh]

As was mentioned, most of those $5 refs come with bullshit cal sheets that are copied over and over again and are completely useless.

"Most" is either ambiguous, or has to be proved. Since this thread has a slight hint of metrology, I would reword it like so: there is a significant number of bogus cal sheet reports on certain types of those ref boards. At the same time, many appear to be legit, but none seem like they can be traced to an instrument with a valid calibration certificate.

Most by definition means more than 51%. I was being generous since from the numerous threads and examples I've seen, only one had a seemingly valid cal sheet. I also have purchased a bunch of these, and know from experience most of them are not worth the $2 shipping fee. I also mentioned the $20 range, because this ~$22 example was the best of all the shitty refs I bought from any only retailer for less than $100: https://www.aliexpress.us/item/3256804431053626.html

ETA: Another reason I don't like non-calibrated sources is because I find it pretty easy to have multiple DMMs disagree with a ref that one DMM might agree with. Hence the need for calibration.

[Fungus]

REF5045AID (stored in an unsealed packet with humidity indicator indicating humidity is present, and the purchase date on the sticker is Dec 2014, i.e. 9 years old).

Humidity doesn't really matter if you're hand-soldering. It's more for ovens.

And if you're putting it in a socket, then... 

[Fungus]

Most by definition means more than 51%. I was being generous since from the numerous threads and examples I've seen, only one had a seemingly valid cal sheet. I also have purchased a bunch of these, and know from experience most of them are not worth the $2 shipping fee.

Seems to me like somebody here could buy a bunch of them, calibrate them properly, and resell them.

[CatalinaWOW]

Lots of trash talking here, backed with pedantic accuracy.

Yes, you do have to have traceability to assure accuracy, and history to assure accuracy over time.

I will be the first to admit that my array of meters all have long outdated calibrations and my similar array of sources are mostly worse, although I do have actual paperwork on my DMMcheck.  My meters are stable to several parts per million compared to the USA cal club reference, which is not formally calibrated, but TiN thoroughly evaluated it against multiple calibrated 3458s.  I can't truly claim that my equipment is calibrated or truly traceable.  But I am actually pretty confident that I know how good they are and feel that they do meet their specs (5.5 digit in several cases, 6.5 in one case).

 Maybe I got lucky on my cheap chinese reference, but it is stable relative to the other references, and the values on its sticker are in within a couple dozen ppm of the observed value. 

Labeling these things as junk in the context of a 3.5 digit meter is extreme.  Without additional measurement and calibration they are inadequate for much higher accuracy meters, and perhaps few of them are useful even with additional care.  So maybe in that context they are junk.

Perhaps the right way to ask the question is: If your budget is ~$20 dollars are they as good as anything else in the price range?  Are you better off with nothing than with one of them?  Are you better off using a pile of fresh alkaline batteries than with one of them?  Do you really believe that more than half are not within parts per 1000 (or even 10,000) of their stated output?

[J-R]

The higher-quality references from Doug/Russ/Ian don't have any reports of fake calibration certificates.  They list out the specific calibrated equipment that was used and when they were last calibrated.  All three have been active here on the forums and many reviews have been published about their products.  (I use their names because we have them and also it's easier than pasting a huge line of URLs.)

Side note, Doug has some budget $35 single-voltage references that could be of interest to some: https://voltagestandard.com/01%25-voltage-references


It's true that not everyone needs the same level of performance from their test equipment, but these random unknown references really are bad no matter how you look at it.  The technical shortcomings have already been well discussed (and perhaps somewhat ignored), but a different point is that you are frequently contributing to people who would be totally fine scamming you wherever possible.  Instead, send your money to individuals who are not really doing it for the money but for the love of the hobby.  Win/win for everyone.


And again my main point about why these cheap references are "dangerous" is that they have specific voltages printed on them and are sold as "references" so buyers apply additional weight to them.  What ratio of buyers do we think have adjusted their DMM based on the "reference" compared to adjusting the "reference" based on their DMM?

[tggzzz]

"Or spend a lot more money, build a ref yourself, and still need to send it out for calibration (after hundreds of hours of runtime to confirm stability)."

Or, spend $5! and not send it for calibration, nor wait hundreds of hours (a ridiculous suggestion) because it will immediately tell you which of the two multimeters has the 20 mV discrepancy. Then, you know which multimeter might actually be functioning, and you can send it for a cal, at less than the cost of a calibrated reference.

When responding please use the "quote" button, and proceed from there. Doing that reduces ambiguity and increases comprehension.

Why spend $5? I'll sell you one for $0.50.

[CalibrationGuy]

Everything is relative and it all begins with your requirements. My clients produce military and space hardware. When you're flying in an aircraft, I'm pretty sure you wouldn't want it serviced or tested by people who have uncalibrated equipment of unknown origin.

Having said that, let's see what we could do with a minimal budget.

As suggested above, we could use a AA battery for the 1 volt range. A new battery would be in the 1.5v range. A 9 volt battery could test the 10vdc range. Put 10 9v batteries in series to test the 100vdc range. You could even select 10 resistors that are the closest match and build a divider circuit to test the millivolt range. Plug the meter into the socket and measure 120vac, but observe all cautions when working with line voltage.

By being creative with a few resistors and batteries, you can accomplish a lot. Remember that ratios and relative measurements can get you very close to your goals.

TomG.

[shapirus]

Side note, Doug has some budget $35 single-voltage references that could be of interest to some: https://voltagestandard.com/01%25-voltage-references

Unfortunately he doesn't.
All the products in the "shop" section have been out of stock for a good while.

[KungFuJosh]

Plug the meter into the socket and measure 120vac, but observe all cautions when working with line voltage.

I don't know anybody with wall voltage that's reliably on point, or stable throughout the day. I have to use a variac when I do circuit testing.

[CatalinaWOW]

Everything is relative and it all begins with your requirements. My clients produce military and space hardware. When you're flying in an aircraft, I'm pretty sure you wouldn't want it serviced or tested by people who have uncalibrated equipment of unknown origin.

Having said that, let's see what we could do with a minimal budget.

As suggested above, we could use a AA battery for the 1 volt range. A new battery would be in the 1.5v range. A 9 volt battery could test the 10vdc range. Put 10 9v batteries in series to test the 100vdc range. You could even select 10 resistors that are the closest match and build a divider circuit to test the millivolt range. Plug the meter into the socket and measure 120vac, but observe all cautions when working with line voltage.

By being creative with a few resistors and batteries, you can accomplish a lot. Remember that ratios and relative measurements can get you very close to your goals.

TomG.

Those battery combinations can hit the right voltage ranges.  But my observations indicate that new batteries have variability in their output of a few parts per thousand.  Is that good enough?  Only the user can say.

[J-R]

Side note, Doug has some budget $35 single-voltage references that could be of interest to some: https://voltagestandard.com/01%25-voltage-references

Unfortunately he doesn't.
All the products in the "shop" section have been out of stock for a good while.

That's common for small, one-man shops, just check back later.

[Fried Chicken]

Plug the meter into the socket and measure 120vac, but observe all cautions when working with line voltage.

I don't know anybody with wall voltage that's reliably on point, or stable throughout the day. I have to use a variac when I do circuit testing.

I noticed this, and it occurred to me that a wall socket will not be accurate, as it will be different behind every transformer, and depending on the copper you have, any resistances along the way.

That said, it can act as a reliable signal source.  ERCOT publishes the current grid conditions, including the frequency to three decimal places.  This should be the same wherever you measure from afaik, but AC is witchcraft.

[mawyatt]

The AC mains are an interesting source.

Here is US the grid frequency can drop during the day with all the heavy loads, but picks up at night and early morning so the average mains frequency over 24 hours is exact.

The voltage can vary and don't think there is any attempt to make the average over 24 hours an exact amount, just attempt regulation at the substations to a fixed amount using tap changes on the large transformers. With the usual large inductive (motor) loads, sometime capacitors are added to improve (tune out the inductive reactance) the Power Factor.

Also the waveform can be distorted from the non-linear loads such as AC to DC conversion, so assuming a pure Sinewave is questionable.

Best,

[shabaz]

I tried a total of five REF50XX samples (4.5V and 2.048V versions), all 2014 era.

Sample 1: 4.50228 V
Sample 2: 4.50254 V
Sample 3: 2.04845 V
Sample 4: 2.04867 V
Sample 5: 2.04852 V

The screenshot shows the graph/stats for about 7 minutes (started about 15 seconds after power-up) for the last sample.

The setup was the worst possible (see earlier photo), and the IC socket is squeezing on the chip. The chip factory trimming and general/mechanical stability are good enough for 10 mV levels of accuracy at a typical room temperature, with no calibration nor burn-in. It's an easy circuit to assemble properly, given the crude set-up performed adequately.

[shabaz]

Trying quoting for the first time:

It's true that not everyone needs the same level of performance from their test equipment, but these random unknown references really are bad no matter how you look at it.  The technical shortcomings have already been well discussed (and perhaps somewhat ignored), but a different point is that you are frequently contributing to people who would be totally fine scamming you wherever possible. 

That works both ways. There's no innocence on either side, with Westerners openly scamming by unlocking functionality that they have not paid for on Eastern test equipment. I don't choose to automatically mistrust every seller based on the occasional bad experience here or there.

Instead, send your money to individuals who are not really doing it for the money but for the love of the hobby.  Win/win for everyone.

Not everyone thinks the same. It is possible to "buy local" as well as to help others across the world, who might actually need the money.

And again my main point about why these cheap references are "dangerous" is that they have specific voltages printed on them and are sold as "references" so buyers apply additional weight to them.  What ratio of buyers do we think have adjusted their DMM based on the "reference" compared to adjusting the "reference" based on their DMM?

Let's guess at an extreme 90% of buyers adjusting their DMMs based on that board. Even if they did, so what? They would have a DMM that was possibly inaccurate by tens of mV at the most? What are the implications:

(a) For a hobbyist, if it affects their projects, it will be a great learning curve on what to trust. If something is worth doing, it's worth doing wrong.

(b) On the other hand, if you're a professional: a qualified engineer may well rely on prototype or 'uncalibrated' equipment when developing products; there's nothing abnormal in that. But equally, that engineer is in every way liable for trusting a $10 device over a calibration if they ignored a process, or were supposed to be working to a standard, or if a body of other professionals would not have done the same for that specific task.

[Fungus]

That said, it can act as a reliable signal source.  ERCOT publishes the current grid conditions, including the frequency to three decimal places.  This should be the same wherever you measure from afaik, but AC is witchcraft.

They make clocks that use the mains as an oscillator and they're very accurate.

Apparently the power companies make great efforts to keep the frequencies stable and even compensate them over the course of the day to keep the total number of cycles per 24 hours correct.

(I'm not sure why they'd do that, was it just so that people could have accurate clocks?)

I don't know what happens in a power cut. Do they have an internal backup oscillator?