Verify or calibrate a DMM - Page 1
Forum section: electronics / projects

author: raptor1956
author_rank: Frequent Contributor
timestamp: August 18, 2016, 02:45:16 am
content: I searched for this and found some old references but nothing I'm looking for so...I have three DMM's, Fluke 85 and Fluke 27, both of which are in the ballpark of 25 years old, as well as a Radio Shack (cat # 22-811) and I'm seeing some differences between them in DC volts. Of course, none of these DMM's are better than 10mV in the volt scale so a dithering of one digit is not unexpected but it got me thinking of a couple questions:1. Can any of these DMM's be calibrated -- I mean adjusted or would a calibration simply provide a table of offsets?2. Assuming that they can not be adjusted what value are the readily available standards from places like Voltagestandard.com (http://www.voltagestandard.com/Home_Page_JO2U.html)? The voltage check is only at 5.000V (+/- 500uV) and I'd think you'd want to check it with reference voltages in at least a few decades.I know I should be looking at a better, and newer DMM with greater accuracy and precision but I can't cough up the several hundred for one at this time. Just looking to see where I'm at.Brian

author: Vgkid
author_rank: Super Contributor
timestamp: August 18, 2016, 03:49:26 am
content: Look for the service manuals for those meters. DC /resistance should be rather easy. AC might be complicated.For the 85 it is in a bundle with the 83/85/87 on Flukes site. Not sure about where to find the 27 on the internet of course. The RS meter should be able to be found as well.85:http://assets.fluke.com/manuals/83_85_87smeng0500.pdf27:http://exodus.poly.edu/~kurt/manuals/manuals/Fluke/FLUKE%2027%20Service.pdfRS:I can only find users manuals, maybe someone will chime in with the oem info.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 18, 2016, 06:14:13 am
content: Quote from: raptor1956 on August 18, 2016, 02:45:16 amCan you tell us how you are testing? Are you measuring a DC power supply simultaneously with all 3 meters? Or are you measuring a 9V battery with meter individually?Can you give us the actual values of each meter over a series of ranges? Say 1V, 3V, 5V, 10V and 15V?QuoteAs raptor1956 mentioned the Fluke service/calibration manuals will tell you what can be adjusted and what reference you will require. Both can be adjusted via pots at the right tongue angle.QuoteYou will be very surprised at how good the Fluke 85 and 27 will hold its calibration over a long period of time. If these two are within 1 or 2mV of each other, that is very good. You would have to spend significantly more to get something better than these two.

author: bitseeker
author_rank: Super Contributor
timestamp: August 18, 2016, 06:37:02 am
content: Unless you need to see very small changes in values (e.g., small variations in voltage) or there are functional problems, you don't need to replace those meters.If you have a very stable power supply that you know to be in reasonable calibration, you can adjust all your meters to that. Otherwise, a single-voltage standard is better than nothing. Set all three DMMs to match that standard, then verify other ranges with a good power supply. The results should be good enough for consistency across devices, keeping in mind the measurement uncertainty of each.

author: Fungus
author_rank: Super Contributor
timestamp: August 18, 2016, 10:08:52 am
content: Quote from: raptor1956 on August 18, 2016, 02:45:16 amThe Fluke 27 certainly can. Google for "Fluke 27 service manual".I doubt that buying a new meter will buy you any more accuracy. Those Flukes keep going and going.Flukes normally use a 2.7V reverence for calibration. 2.7V is hard to find but there's plenty of 2.5V references out there (that's close enough). eg. A REF5025.http://www.ti.com/product/ref5025/description(get the "high grade" one for 0.05% accuracy).

author: MosherIV
author_rank: Super Contributor
timestamp: August 18, 2016, 10:52:19 am
content: QuoteYes, to do a proper calibration you need to have 3 test points per range and the voltage reference needs to be 10 times better accuracy than you Device Under Test.Saying that, try a voltage reference based on the AD584, it outputs 3 reference voltages 2.5V, 5V and 10VThe app note says that you can get 7.5V by shorting the 2.5V to 5VThere are many cheap Voltage Standard available from China based on that device, over in the metrology forum they reckon they use a recovered AD584 but the cheap China knock offs work OK.

author: bitseeker
author_rank: Super Contributor
timestamp: August 18, 2016, 08:52:57 pm
content: Just be aware that some of the references from China have been known to include photocopied "measured" value tables. There was a thread about it somewhere around here.

author: raptor1956
author_rank: Frequent Contributor
timestamp: August 19, 2016, 03:03:13 am
content: Thanks guys, I was able to locate the service manuals and can see what the calibration is for both of them. The Radio Shack unit is, well, not working so scratch that one from the list.The voltage reference is the next thing and ordering one from Voltagestandard or someone else appears the weak spot as you have no way short of having that calibrated to know if the standard you're using is any good. In addition, the range of tests and calibrations the service manual calls for is outside the range offered by any of these online sources that I see.Oh well, I guess the best bet is to spring for an decent 5.5 or 6.5 digit bench meter and compare the DMM's to it, but as I said that purchase is down the road a piece...Brian

author: bitseeker
author_rank: Super Contributor
timestamp: August 19, 2016, 05:31:09 am
content: The ones from voltagestandard are measured using their traceable HP 3458A meter and have more resolution than your DMMs. So, although they don't provide a full range of voltages, will still provide an indication if one or both of your meters are way off.Until you have a standard by which to gauge your adjustments, you can adjust one of your meters to match the other. Which one you choose doesn't really matter if they're not significantly different from one another.Another option is if you have an adjustable power supply that you're confident is accurate and is stable, you can use it as your "standard" and adjust everything to it. Unless you're working on projects that require high accuracy or traceable calibration, the absolute values aren't going to be that critical when all your devices are already pretty close to one another.With regard to buying a bench meter as your in-house standard, note that unless you buy one that is calibrated (or get it calibrated), just because it has 5.5, 6.5, or more digits of resolution doesn't provide any indication of its accuracy. It could be worse than your current DMMs, just with more incorrect digits of inaccuracy.

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2016, 05:38:17 am
content: In practice just using one calibration point is close enough most of the time.Usually you don't trimm any linearity in the ADC on the multimeter and the main source of error tends to be the internal reference and the resistor network that divides down the input signal. So if you short the probes and it reads 0V and then give it spot on 10V and the meter reads 10.01V, you are good. But you do have to calibrate each range separately since they use different resistor dividers inside.You don't necessarily need to spring the cash on a 6 1/2 digit or more bench meter. I bought a brand new Agilent handheld meter(The one with the OELD display and AAA batteries, forgot the model number) and i got with it a full calibration printout of every range. It showed its pretty much within 1 or 2 counts on the DC voltage ranges . I have a 8 1/2 digit Keithley bench meter that i used to verify it with and it is indeed still 1 or 2 counts out on DC volts even after 2 years.True that that handheld meter is only 4 1/2 digits but you can be confident that those digits don't lie at all. Still this meter costs about 300 bucks.

author: Enigma-man
author_rank: Regular Contributor
timestamp: August 19, 2016, 02:48:59 pm
content: Or...Visit this site and buy a precision calibrator for $53.00 bucks US which is calibrated by their Agilent/Keysight 8.5 digit meter which hasa Certificate of Calibration so you know it will be accurate. Other voltages are available as well.Calibration of the 5.000V standard is free for two years, then it's only $5.00 plus postage after that. Not a baddeal seeing you are in the USA. You could see which of the meters was starting to drift with a simple test.It might be worth a look since you do not have super precision meters at the moment.I don't have one of their products, nor do I endorse them.http://www.voltagestandard.com/Home_Page_JO2U.html

author: raptor1956
author_rank: Frequent Contributor
timestamp: August 20, 2016, 01:54:26 am
content: Quote from: bitseeker on August 19, 2016, 05:31:09 amI have a Rigol DP832 bench power supply that appears to be pretty good though I don't know I'd trust the accuracy of the mV settings. A good bench meter and a stable power supply would make a decent combination as you can set the output to whatever voltage then check that the handheld DMM's agree with the bench meter within the tolerance they have. It is true, of course, that without a calibrated bench meter you still don't know what you have and perhaps that's where the voltage references come into play.Of course, all this gets you is greater confidence in the DC voltage but there's others metrics as well -- DC current, AC voltage and current, resistance and capacitance. If you open the door you find a darkened room and a floor with trap doors -- perhaps you just have to 'trust' the meters are OK and leave it at that as the full schmear of testing and calibrations are not for the average Joe and sending all your instruments in every 6 months for re-calibration is not withing mortals budgets.Brian

author: singapol
author_rank: Frequent Contributor
timestamp: August 20, 2016, 04:24:10 am
content: Maybe you should be reading this.https://www.eevblog.com/forum/metrology/mini-metrology-lab-build/I would suggest buying 0.1% resistor and save yourself the null meter matching of 1% resistor soaked in heated oil. Alternatively for the 10V reference "standard" a REF102C (2.5ppm) or an "A" (10ppm or 0.01mV) version will suffice.It saves the trouble of getting the chopper opamp to make 10V from a 5V reference diode but who knows it may be better?

author: Fungus
author_rank: Super Contributor
timestamp: August 20, 2016, 11:32:13 am
content: Quote from: singapol on August 20, 2016, 04:24:10 amWhy settle for 0.1%?http://www.ebay.com/itm/121031115671OK, they're used, pulled from somewhere. I've bought a few of them and they've been fine...

author: bitseeker
author_rank: Super Contributor
timestamp: August 20, 2016, 05:47:38 pm
content: Quote from: raptor1956 on August 20, 2016, 01:54:26 amIndeed!

author: Pack34
author_rank: Frequent Contributor
timestamp: August 20, 2016, 11:41:17 pm
content: If it's equipment you're using to make money you need to have the equipment sent off once a year for calibration.

author: singapol
author_rank: Frequent Contributor
timestamp: August 21, 2016, 04:57:08 am
content: Quote from: Fungus on August 20, 2016, 11:32:13 amNot very practical/expensive (even if used,how about the right resistance values) when you need 11 resistors of different values per decade to make a decade for a KVD (kelvin Varley voltage divider) This DIY/home metrology project kvd needs 6 such decades =66 + a few more to make a KVD.http://conradhoffman.com/mini_metro_lab.html