Too much GAS - Low cost voltage reference? - Page 1
Forum section: electronics / testgear

author: Bakaman
author_rank: Contributor
timestamp: October 11, 2021, 09:27:28 am
content: Hi Folks,Despite having a bad bout of gas (gear acquisition syndrome), I'm determined not to buy any big expensive gear that I don't really need - none of my gear is calibrated because it is only used for reference/experimenting, and is most usually used for logging dynamic changes in things.Back to the point, I have reasonable meters like the Fluke 289, 89IV, BM867 etc, and can cross check their relative performance comparatively to make sure that none have gone faulty. I know that I can buy all manner of calibration sources including things like Time Electronics' stuff, but it would be used so infrequently and take up so much room that I'm thinking about a single voltage reasonable precision reference for independent spot checking. I'm not talking crazy accurate, maybe 0.1% would do. There are a load of '10 bucks' voltage reference bare boards that purport to do 2.5/5/7.5 and 10V from China.I'm wondering if anyone has experience/thoughts as to whether that would be a reasonable option. With a little case, it could live in a drawer for yearly exercise without grabbing space or problems. Alternatively, I could use one of the low cost references, maybe with an Opamp and make my own on some veroboard.Perhaps there are better options that you know about that could achieve the same sort of thing but in some better way? I reiterate that I don't want or need a full blown calibration source no matter how good value it would be, just a reasonable, low cost but dependable single voltage would do the job. [and I do appreciate that low-cost and dependable don't always come together, especially from China IME].Here's a point as an example of some QC issues, in the box with a meter that a bought, came an extra thermocouple probe - 'oh that's a nice freebie' I thought, only to realise that one jack wouldn't plug in.Why? Cheap solid 4mm jacks, which are split at the ends in quadrants to allow it to compress/give. Except one jack was split and other wasn't. How did that oh-so-obvious and highly visible quality problem get through? (rhetorical)

author: Fungus
author_rank: Super Contributor
timestamp: October 11, 2021, 10:43:30 am
content: Quote from: Bakaman on October 11, 2021, 09:27:28 amI've got one of those and it appears to work perfectly. OTOH there was a thread on here where somebody bought two of them and got the exact same calibration report sheet for both of them. Caveat Emptor.With that array of meters you should be able to make your own calibration sheet though. My Fluke 187 and BM857 agree to with a digit on everything I've tried them on.You can also build your own references: Google "ref05" (for example). I've built one of those using a 0.05% accuracy chip. All you need is the chip, a 9V battery and a capacitor or two. FWIW mine measures about 5.0016 volts without any trimming.You can also go on ebay and search for "vishay 0.01% resistor", see what turns up. Get a couple of values that are near the upper end of the autoranges on your meters, eg. 4.7k, 470R.For current references you can easily buy chips in the uA range, eg "ref200". The mA ranges are slightly trickier to do, I made one that uses a ref05 chip and a 500 Ohm 0.01% resistor. The trick is to float the ground pin of the ref05 and tie it to the 500 Ohm resistor in the middle of a resistor divider (or something like that, I don't remember the exact details offhand). The chip will force exactly 5V across the 500 Ohm resistor and that gives you exactly 10mA of current in the circuit.

author: Bakaman
author_rank: Contributor
timestamp: October 11, 2021, 11:01:24 am
content: Thanks Fungus, useful advice.

author: rodpp
author_rank: Frequent Contributor
timestamp: October 11, 2021, 03:31:40 pm
content: Some years ago I bought one DMMCheckPlus and it is a good device to check the handheld multimeters accuracy.When I bought it was produced by Doug, and he was very helpful. I'm not sure, but it seems that Doug is not the owner of the DMMCheckPlus anymore.The basic specs are:5Vdc - 0.007%1mAdc - 0.1%5Vac (rms square wave) - 0.1%1mAac - 0.2%100R, 1K, 10K, 100K - 0.1%, 10ppm precision100Hz - 0.02%more details here:https://dmmcheckplus.com/technical-informationPrice: USD 125Its price is one order of magnitude over the USD 10 chinese reference, but has its advantages.That device is powereded to age for a minimum of 250 hours before calibration. And the main advantage: it is calibrated against NIST certified equipments and sent with the measurements write in a paper, like a homemade call report with values.

author: rodpp
author_rank: Frequent Contributor
timestamp: October 11, 2021, 03:52:02 pm
content: It seems Doug still sells voltage references, not the DMMCheckPlus, but better and cheap ones if you are looking for Vdc references only:https://voltagestandard.com/The Vdc reference with 0.01% accuracy costs USD 31.And with 10Vdc 0.002% costs USD 86.Both aged for 250hours and calibrated against NIST certified 3458A.

author: bdunham7
author_rank: Super Contributor
timestamp: October 11, 2021, 04:24:26 pm
content: I'll second the recommendation forhttps://voltagestandard.com/, Doug is very conservative about his claimed specifications. DMMCheck obviously has a lot more features but not a super-accurate 10V option.However, I'd also observe that if your Fluke 289 and 89IV both agree on something, the chance of them both being significantly wrong is very, very low.

author: iMo
author_rank: Super Contributor
timestamp: October 11, 2021, 06:09:25 pm
content: Quote from: rodpp on October 11, 2021, 03:52:02 pmAny ref sources made of ref chips in epoxy packages are fluctuating based on the weather..For example an LT1021-10 in epoxy package let unpowered on the shelf for a year or two will drift +/- 1mV easily.

author: Fungus
author_rank: Super Contributor
timestamp: October 11, 2021, 06:17:05 pm
content: Quote from: bdunham7 on October 11, 2021, 04:24:26 pmBut it's such fun to build your own!Quote from: bdunham7 on October 11, 2021, 04:24:26 pmYep. This also makes it much easier to build/calibrate your own references.PS: His BM867 doesn't count? Is it only the Flukes that can be accurate?

author: bdunham7
author_rank: Super Contributor
timestamp: October 11, 2021, 06:27:46 pm
content: Quote from: imo on October 11, 2021, 06:09:25 pmThe referencesrodppand I are referring to are indeed LT1021-10 based and they exhibit no such issues. I haven't left mine off for years, but it has sat for months and has no discernable long-term drift. It does have predictable and documented temperature variation, but that is single-digit ppm over normal room variance. Doug does say that he selects them, but I have no idea of the accept/reject rate. I think he also uses a conformal coating now, but mine does not have this.

author: J-R
author_rank: Super Contributor
timestamp: October 12, 2021, 01:03:05 am
content: I'll throw out another plug for Ian's PDVS2mini, if you can afford it:https://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-2-mini-detailI've had mine for over 18 months and it's still spot on. The ability to easily select any voltage between 0-10V has been great. It's what I use to check my 6.5 digit meters, among some other uses.As for DMMCheck Plus and Doug's references, both of course can't compete with the PDVS2mini, but they are great primarily because you can mail them in to be calibrated/adjusted for a reasonable fee plus receive the calibration values.In addition to the above products, snagging a few mOhm precision resistors is a great addition since you can use them for shunts/4-wire tests.

author: bdunham7
author_rank: Super Contributor
timestamp: October 12, 2021, 03:12:35 am
content: Quote from: Fungus on October 11, 2021, 06:17:05 pmThey probably can, but I don't have decades of experience with them to draw on to make any statistical estimations as to the likelihood that one would be off.

author: Fungus
author_rank: Super Contributor
timestamp: October 12, 2021, 05:03:17 am
content: Quote from: bdunham7 on October 12, 2021, 03:12:35 amMy BM857 agrees with my Fluke 187 down to a digit or so. Both 50,000 counts.Maybe Bakaman can add his experience.

author: Bakaman
author_rank: Contributor
timestamp: October 12, 2021, 11:34:18 pm
content: An interesting discussion, I didn't realise that so many options were out there, and some of you guys are desperate to keep my GAS bubbling awayBut I absolutely don't need absolute (or very high) precision, in fact you could argue that I need no precision at all - with multiple meters out there, it is highly unlikely that they'll all drift the same way and by the same amount. I think that I'm with Fungus on this one - but my GAS is already starting to bubble (grrrr).As to relative performance of my Flukes and the BM867 I'll drop you an update in due course. I've been offline writing my a paper for Uni, and I've just submitted it.Thanks in advance,Brian.

author: J-R
author_rank: Super Contributor
timestamp: October 14, 2021, 12:01:45 am
content: The "odds" are in your favor if multiple DMMs agree, but if they don't you can start a DMM religion where you believe in one or the other!Just get the DMMCheck Plus. With every option it's only $190+shipping. Re-calibration is free the first two years and only $20 each time after that.

author: guymo
author_rank: Regular Contributor
timestamp: October 14, 2021, 07:22:40 am
content: If you don't need super high levels of accuracy/stability then a simple DIY reference would do the trick, wouldn't it?For example:https://www.markhennessy.co.uk/dvm_reference/which makes use of multiple references to reduce initial error.(I believe the author posts here too.)The same site reviews some of the cheap references:https://www.markhennessy.co.uk/ad584_references/

author: J-R
author_rank: Super Contributor
timestamp: October 14, 2021, 09:18:22 am
content: Only having a DCV reference is maybe only 1/5th the battle since your DMM has many other functions that should be checked. DMMCheck Plus has AC/DC voltage/current, 2 frequencies, 4 resistor/capacitor/inductor values. You receive a printout with all the actual values as shown by equipment with calibration certificates that are provided right on the website.The cheap "references" Mark reviewed are a complete waste of time/money in my opinion. "I'm pretty happy with these measurements." Can't say I am. My cheap Korad bench supply provides similar results, within about 1-4mV despite not even having a mV display.The statistical reference is also questionable since he is making changes based on measurements from his DMM, so the experiment is polluted. "...the 12.5 output was higher than expected, so I swapped sample #30 with sample #5." He mentions some equipment with a calibration certificate but really is glossing over that critical concept which is verifying against a known standard. Anyone can put numbers on a page but without something to back them up they are just inventions, to be blunt about it.But as I said before you can play the "odds" game just as well and that is enough for many folks. Hook your DMM to the car battery and it's 12 something, check a package of new AA batteries and they are 1.6 something, check the (US) wall socket and it's 120 something and 60Hz, OK, bam, done, my DMM is calibrated, back to watching cat vids on u-tube...

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2021, 10:26:41 am
content: Quote from: guymo on October 14, 2021, 07:22:40 amI think the eBay ones use second hand AD584s, pulled from old devices. There's no way they could sell them that cheap otherwise.Hey, at least you know they're burned-in.

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2021, 10:35:27 am
content: This is my super-cheap eBay special. The calibration is hand-written, it runs off a 12V battery.The value on the sticker says "5.00407V" @ 21 degrees, measured on a 6.5 digit Agilent 34401A.Here it is on my Fluke/Brymen with about 1 minute warmup:Room temp. is 23 degrees according to my Aneng AN870.Quote from: J-R on October 14, 2021, 09:18:22 amWhen all the stars line up that^ precisely you have to wonder if there's something more to it.

author: myf
author_rank: Regular Contributor
timestamp: October 14, 2021, 03:25:34 pm
content: Hello Fungus,Thank you very much for your photos !Might you tell us the name of this ebay-seller ? There are a lot of sellers and someones are less honest than others.Have a nice day !F. (from France)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 14, 2021, 03:54:00 pm
content: The individual measured values may be OK, and I would give them a more than 50% chance to be OK. The problem is just that you can't really trust it. There are known false numbers around. So the confidence in those numbers is not very high.Maybe with 2 references from different sources the chance are one could check the difference or ratio. With false numbers the chances are low that the ratio would be OK between them. With a 6 digit meter the ratio of the 2.5 / 5 and 10 V may be a resonable test. Usually a 6 digit metes would not need such a reference - makes me think, I could be one of the few exceptions. So maybe take a chance. Even if the values are not valid it would still be a reasonable reference.

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2021, 04:17:48 pm
content: Quote from: Kleinstein on October 14, 2021, 03:54:00 pmYep. As noted on thatweb page, some sellers are printing out the calibration slips and everybody gets the same one.Check how many they've sold, user feedback, etc. Caveat emptor.Quote from: myf on October 14, 2021, 03:25:34 pmSorry, I don't remember. I bought mine three or four years ago.

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2021, 04:37:45 pm
content: At the end of the day, the only real solution is to give in to GAS and get a couple of high-end meters.That way you can build up confidence by comparing them with each other.Here's mine on a lower range (they both autorange at "5" so the 2.5V range is a step down and one extra digit) at about 24 degrees C.The cal. cert. says "2.50167V" for this range:(sorry about the big shadow)Once you own a couple of decent meters that agree down to that level then I figure you're good to go.Either that or find a friend who has some nice gear or works somewhere that does, eg. I compared mine with a 6.5 digit bench meter at the University because I know a teacher there.