Reference PCB for Calibrating DMM's - Page 1
Forum section: electronics / testgear

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 24, 2022, 11:34:16 am
content: I ran across this on Amazon for under $13:https://www.amazon.com/Viupolsor-Reference-Resistor-Calibration-Multimeters/dp/B0BKPHPHZQIf I bought this, I am wondering if I could take it to my local electronics store and have them (or pay them) to record the results for voltage and resistances onto the pcb. Or perhaps someone here sells those boards with values they recorded from a very good calibrated meter?I just need a reference to see which of my meters is off.. the 8251A or the UT61E. I want to see how close both are to actual values. I also need a reference in case I need to calibrate a meter.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 24, 2022, 11:48:27 am
content: as you wrotethey are mostly fineBUTyou need them properly certified with the voltages "read values" written on them, some peoples check them with 3458 meters, with proper warm up times etc ...Since there is no value markings i would still have doubts ....you have lm399 or ltz 1000 projects here on eevblog (but costly)

author: dophuc
author_rank: Contributor
timestamp: November 24, 2022, 12:01:04 pm
content: You should buy a referenced circuit board with a value on itExamples of similar products from Aliexpresshttps://www.aliexpress.com/item/1005002270593558.html?spm=a2g0o.order_list.0.0.21ef1802t7vwu4

author: Kleinstein
author_rank: Super Contributor
timestamp: November 24, 2022, 12:25:20 pm
content: Similar boards are offered in number variants. Many of them come with written values for the actual values, e.g. measured with a good quality meter. While many of those reading are legit, there also cases with just arbitrary numbers. For the AD584 reference there is no need for a 8 digit meter - a proper calibrated 34401 would be good enough. Even the recycled reference chips can initially show quite some drift.These boxes are OK for a simple check of handhelp meters. They don't replace a proper calibration if required. A point is a lack of reliabilty, like the small the chance to have fake numbers writen on them.With the relatively high resolution 8251A one may be able to check the ratio and this way has a good chance to detect more random fake values.The boxes would mainly check 1 of the voltage range and maybe the resistors.Even without good values to start with one can still check for excessive drift and use it as a stable source to compare 2 meters.For the shown PCB from amazon the somewhat special battery type could be a slight weak point.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: November 24, 2022, 02:10:25 pm
content: Quote from: JenniferG on November 24, 2022, 11:34:16 amyour GW Instek GDM-8251A is a 5½ digit multimeter, with a DC voltage accuracy of 0.012%. assuming it has not suffered some major mishap in the past, i would consider itfarmore likely to be reading accurately than the UT61Einitially, you might like to buy something like a MAX6350 reference IC, which is available with an initialfactory calibrationto within +/-0.02%. for an example see:https://geoffg.net/Voltage_Reference.htmlshowing just the single DIP IC placed in a box, you can't get much simpler. the MAX6350's 5v output sits near the middle of the range of your 8251A, which is probably a good choice of calibration point. cost: around us$20. you can also pick up a 10k 0.01% resistor for less than us$10 (both prices from digikey, farnell will no doubt have similar). this will provide you with some certainty of your meter's performance on DC volts and ohms.cheers,rob :-)

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 24, 2022, 02:22:10 pm
content: Quote from: robert.rozee on November 24, 2022, 02:10:25 pmOh this is great thanksI'll do that. I didn't know there were 0.01% resistors.. I guess they have to hand test those? the Max6350 sounds cool. Yeah I am just curious where both meters sit right now. I've never compared them to a reference voltage or resistor measured with a calibrated meter.

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 24, 2022, 02:47:56 pm
content: I'm seeing only 0.04% accurracy/ tolerance on Mouser and Digikey for the Max 6325. But on Maxim's site I believe they say 0.02% on datasheet.

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 24, 2022, 02:55:55 pm
content: Regarding the 0.01% 10K resistor. I am seeing them for more like $20 on Mouser. Then of course with the shipping it becomes $30 with tax lol.I found a wire wound (Vishay) , would that be better than say metal film for a reference resistor? Seems like it would.https://www.mouser.com/ProductDetail/Vishay-Mills/MR10610K000TAE66?qs=HsLnPBGFaK%2FFD5gZVigOWw%3D%3D

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2022, 03:49:40 pm
content: Yes, close tolerance wirewound resistors are always more stable than metal film, and usually also the equivalent Vishay foil resistors in the longer term too.EDIT: I should maybe qualify my definition of 'stability'. You can get metal film resistors down to about 3ppm/'C temperature coefficient. I just noticed that the linked resistor is 30ppm/'C. What I am referring to is long term stability, ie. the resistance change over time, when maintained in a reasonably stable thermal environment (and the likelyhood of it returning to the same resistance value after a temperature excursion).

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 24, 2022, 05:10:10 pm
content: What about these 15 cent cement resistors from Tayda?https://www.taydaelectronics.com/resistor-10k-ohm-5w-cement.htmlSince they are cement resistors, does this mean they are wirewound? Wirewound resistor can't drift over the years right?It's 5% tolerance, but since it doesn't change, all I'd need is for someone with a calibrated scope to measure it for me and write it on teh cement in sharpieIt's freaking me out Mouser's prices.. I really never want to buy anything from them because they charge minimum $7.99 shipping.. When it only costs $3.50 to ship with tracking USPS.

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2022, 05:38:36 pm
content: No. Precision wirewound resistors are going to be using specific alloys (Manganin, Constantan etc) and minimal stress windings. The only real purpose of one of those bog standard wirewounds is to dissipate heat, so no real care in construction.Yes, wirewound resistorscanstill drift. Changes in composition (you specifically don't want to dissipate heat in a precision resistor), Winding stresses (if the wire gets too tight on the former it will stretch and get thinner!) etc. NOTHING stays constant forever, but with close attention to construction in precision parts, you can minimise it. The mouser resistors aren't precision in the 'laboratory resistance standard' sense, they are just 'good'. The Tayda are neither in this context.Cement resistors transition from wirewound to film at some arbitrary value, depending on size (the wire gets too thin and hard to wind at some (price)) point.

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 24, 2022, 05:45:26 pm
content: Quote from: Gyro on November 24, 2022, 05:38:36 pmThanks, constantly learning in this hobby; having funI think what I am going to do is take some Royal Ohm 1 cent metal film resistors (1%) , say like 3 of them, to my local electronics store and see if they'd kindly measure them for me on a calibrated scopeThen perhaps go there every year with the same resistors to see how they drift and to update the values if changed. I need some local electronics buddies -- I'll have to look into that.. can share resources and ideas to save money, etc._ I wonder if I should bring my probemaster leads if that makes a difference.. I imagine using the same leads on both unit and the calibrated unit would help?

author: ci11
author_rank: Regular Contributor
timestamp: November 24, 2022, 05:57:22 pm
content: Getting components one at a time may still leave you wondering if other measurements and values are within spec, and perhaps more importantly, whether the specific ones you received from the likes of Mouser are actually on the mark. Calibrations are expensive at labs because the calibrators they use are very expensive, and for good reason. One alternative for hobbyists is be the DMMCheck plus and there are others. Many DMM users like the DMMCheck as a quick and easy way to know if their meter is good to go or needs work. It's not inexpensive but it is worth mentioning Just Google "DMMCheck Plus".

author: bdunham7
author_rank: Super Contributor
timestamp: November 24, 2022, 06:18:16 pm
content: Quote from: JenniferG on November 24, 2022, 11:34:16 amI've yet to see an electronics store that has a calibrated meter on hand. Actually, it is getting hard to find a retail electronics store of any kind these days.The real enemy of your plan is tempco. The temperature coefficient of typical parts causes their values to move around and this will probably be the main source of error in this sort of endeavor. I doubt the $13 reference will be excellent in this regard. The typical good-quality entry-level calibration checker is the DMM Check, which goes for $100 or so.These are all OK forcheckingyour DMM, but would not be suitable for actually correcting the calibration. That requires a much more comprehensive set of precise stimulus voltages. In the case of the GDM-8251, at least with mine, I found that the entire tolerance specification (and maybe a bit more) was taken up by the in-range (+/- 5C) tempco of the unit. IOW, it wasn't really stable enough to benefit from calibration. I'm not saying you should chuck it in the trash, but just keep your expectations reasonable!I'd say that if you don't want to spend the money on a DMM Check right now, get the $13 unit and have someone check it for you. Are your two meters not in agreement on things that you measure? If so, how much are they off?

author: coromonadalix
author_rank: Super Contributor
timestamp: November 24, 2022, 06:20:05 pm
content: Quote from: JenniferG on November 24, 2022, 05:45:26 pmexcuses me 1% resistor as a reference is pure #$##^ you need at least 0.0xx percent precision resistors i have 0,0014% ones from an old calibrator, even theses will be affected by temperatures and or humidityprecision stuff like, meters have warm up times, precision references too, precision resistors etc ... you are venturing in a different world when you talk about precision referencesan 6.5 digit meter cost me 160$ CAD, i had 3x oldies hp 34401A meters, and now i've switched to more modern stuff, it cost a little bit more ...i have 300,000 count Gossen Metrawatt meters, same price here 160$ cad for each ...I mean like others you need some references points before thinking doing some precisions stuff, grab an 5.5 digits or more meter and make it calibrated and keep it clean and stabilized in temperature and humidityThat's why people love old meters like the 34401a, when working perfectly they have "aged" very well and got pretty stable over the years, one i had was almost 25 years and was pretty close to "calibrations standards" even calibrated 10 years after the last calibration stickerYou could buy a precision reference divider network like the ones we find in meters, they can go up to 20-25$, they will be way better than the 1% precision idea

author: wasedadoc
author_rank: Super Contributor
timestamp: November 24, 2022, 06:26:51 pm
content: Quote from: JenniferG on November 24, 2022, 05:45:26 pmThink again. Scopes are rarely if ever calibrated to anywhere near the accuracy of a decent meter. And as for a local electronics store having one calibrated and maintaining its calibration - more chance of seeing pigs fly overhead.

author: tautech
author_rank: Super Contributor
timestamp: November 24, 2022, 06:47:10 pm
content: This is the proper tool if you are serious about a reference check:https://dmmcheckplus.com/

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2022, 06:50:18 pm
content: Quote from: wasedadoc on November 24, 2022, 06:26:51 pmYes, a typical DSO is 8 bit resolution, an expensive one 12 bits, I even have an old picoscope ADC216 that manages a whole 16 bits up to 333ksps. Your local ectronics store will have the 8 bit - a $3 DMM [Edit: or Harbour Freight giveaway in the US?] has better resolution and accuracy.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 24, 2022, 08:41:16 pm
content: The cheap boards are no way near as good to calibrate the GDM8251 (5.5 digit meter). For calibration one tends to need a significant better reference / meter.The cheap references from China are at best a quick spot check for one maybe 2 ranges and over time a crude test for dirft. Usually the DMM would still be more stable than the ref., but as long as the result is good, there is no need to ask which one is drifting.One may (still have to check if the ranges line up OK) use the 5.5 digit meter together with an only somewhat stable source (but larger range to check most ranges) to check a 3.5 and maybe 4.5 digit handheld meter, at least in some ranges. The AC part can still be tricky and so is a suitable AC source.

author: alm
author_rank: Super Contributor
timestamp: November 24, 2022, 10:51:01 pm
content: If you want to adjust one of the meters, then you also need to study if you can adjust just a single range at a time, or if you need to adjust all ranges before you can save. Even if you are forced to adjust just the DCV ranges, that still means having a stable 100V and 1000V (or whatever the correct points are). For meters where you can adjust a single range at a time this is no problem. At least some Keithley bench DMMs require adjusting all ranges and functions before it will save the adjustments. So having a reference to adjust a single range doesn't do you much good then.

author: artag
author_rank: Super Contributor
timestamp: November 24, 2022, 10:59:29 pm
content: Quote from: JenniferG on November 24, 2022, 05:45:26 pmDo you have a hackspace near you ? A good place to find electronics buddies, and maybe even a high quality DMM.

author: J-R
author_rank: Super Contributor
timestamp: November 24, 2022, 11:13:56 pm
content: As mentioned, those are some decent references for the hobbyist, but stock has been a major issue this year. All of these are out of stock currently:https://voltagestandard.com/pentarefhttps://dmmcheckplus.com/technical-informationhttps://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-2-mini-detailThe DMMCheck Plus is easily the best bang for the buck because of the sheer number of references it provides, and you just mail it back for yearly calibrations. Use it to calculate other values that may be necessary to check other ranges of your equipment.I'd really push for only using references that can be traced to calibrated equipment. Otherwise you're punching fog...

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 25, 2022, 03:30:24 am
content: Quote from: wasedadoc on November 24, 2022, 06:26:51 pmSorry I meant calibrated meter.

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 25, 2022, 03:36:51 am
content: Quote from: J-R on November 24, 2022, 11:13:56 pmI'll grab a DMMCheck Plus whenever it becomes available. Thanks everyone! This chip shortage sucks and I wonder when things are going to get back to normal?

author: coromonadalix
author_rank: Super Contributor
timestamp: November 25, 2022, 06:11:23 pm
content: not sure if it will return to a "normal" some are making tons of money selling overpriced stuff, and the fake market has gone upIt is said maybe around 40% of the china electronics may or have counterfeit parts being soldHere at my job we lost tons of $$$ because of this, lots and lots of problems with chip shortages and fakes, some Xilinx 40$ fpga are sold to us 380$ .... huge costs adjustments / redesign / substitutions ... we sell very specialized equipment and had to rise the pricing

author: dophuc
author_rank: Contributor
timestamp: November 26, 2022, 09:15:38 am
content: I usually use LB02A as a device to calibrate DMM or power supply, the accuracy of LB02A is about 0.025% and many functions like: V (1-11V), mA (0-24mA) Ohm (0-2200 Ohm) , ... especially simulates various types of thermoelectric signals Type T, J, K, ...PT100, PT1000...Not extremely accurate but versatile and under $100 (I bought it on Aliexpress for $74 on sale)

author: robert.rozee
author_rank: Frequent Contributor
timestamp: November 26, 2022, 01:45:41 pm
content: i believe the original poster, JenniferG, was just after doing asanity checkon her two meters (one 5½ digits benchtop, the other 4½ digits handheld). i'm picking that the Instek GDM-8251A (benchtop) is far more likely to be reading true.for this sort ofcheckone doesn't need anything too fancy, and a 0.01% resistor of known value and a 5v MAX6350 or similar would be fine. no need to go overboard buying a "DMMCheck Plus" or similar. indeed, for the average electronics hobbyist, a 2% accuracy 3½ digit multimeter is mostly fine!cheers,rob :-)

author: JenniferG
author_rank: Frequent Contributor
timestamp: November 26, 2022, 06:52:05 pm
content: Quote from: robert.rozee on November 26, 2022, 01:45:41 pmYeah the UT61E seems a bit off compared to the GW. But also I've never tested my Gwinstek to see how close it is to be accurate. I bought it used in 2017 off ebay and it's been sitting here; just finally now I've really gotten interested in electronics. It came from the big sale of ITT equipment when they shut down. It doesn't have a calibration sticker on it.

author: bdunham7
author_rank: Super Contributor
timestamp: November 26, 2022, 07:43:15 pm
content: Quote from: JenniferG on November 26, 2022, 06:52:05 pmHow much off? By the specs, at the UT61E has an uncertainty of +/- 51mV, while the GDM8251A is +/- 1.7mV, or 30X better. In real life, I'd expect the UT61E to be better than its specs by quite a bit, the GDM8251A not so much, so maybe 5-10X better. The expectation would be that any small discrepancy (<50mV @ 10V) would be likely mostly due to the error of the UT61E, but a large discrepancy is equally likely to be either one since that would indicate a gross failure on the part of one or the other.

author: mawyatt
author_rank: Super Contributor
timestamp: November 26, 2022, 11:22:24 pm
content: Here's something the OP & others may find interesting.https://www.eevblog.com/forum/testgear/ac-rms-dmm-tests/msg3940957/#msg3940957Been running continuously for a couple years and still producing good results.Edit: Update just checked last night and the device had failed, just completed update & repair on the mentioned thread.Best,

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2022, 11:48:36 am
content: For the UT61E I would expect it to be quite a bit better than the specs when new, but there is a chance that it will drift over time. The tolerace specs are not so much for the initial value but likely to a large part to allow for some dirft for the next 1-5 year (so the meter has a chance to pass calibration even after 4 years). Getting the calibration right from the start is not that difficult and expensive, but keeping the drift low is tricky and costs money.There usually are no seprate 30 days or 3 month specs for these lower rated meters, but expect quite some difference.The older age GDM8251 may already had some dirft from aging. So I would not count on having it really much better than the specs.