poor man's calibration? (calibrate 5.5 digit with 6.5digit dmm) - Page 1
Forum section: electronics / general

author: cnt
author_rank: Contributor
timestamp: November 25, 2021, 12:33:57 am
content: So I have a keithley 2461 SMU and a 6.5 digit DMM at work. Both new in good condition. I want to calibrate an old keithley 197 (5.5digit). I'm not going for NIST traceable, perfect calibration, I just want to do the best I can for hobby stuff at home.Is there anything wrong with using the SMU as a voltage/current source for calibration and tweaking the output until I get the correct value on the 6.5 digit DMM. The SMU has 6ish digits of precision but only 3-4 worth of accuracy. The 6.5 digit DMM is much more accurate.For ohms I'm thinking I'll make a resistance box with a high resistance pot in parallel to fine tune. I should be able to dial it into the resistance I need (1.9eN) and assume its stable for the 1min I need it to be stable.Any tips, pitfalls, or caveats I should be aware of, or can I just do the obvious thing? All the equipment is Keithley so I shouldn't have to worry about any off brand electrons(I know won't be able to calibrate the 200V and 1000V ranges this way, or AC, which I don't care about)

author: bdunham7
author_rank: Super Contributor
timestamp: November 25, 2021, 12:49:37 am
content: Yes, that will work well enough if your 6.5 digit DMM is good. You don't have to use the exact listed voltage on the calibration chart, and I think you can even use a much lower voltage for the higher voltage ranges, with some loss of precision. I've done this sometimes for a 'rough' initial setting when I don't have a calibrator on and ready, and it usually turns out to be pretty close.

author: cnt
author_rank: Contributor
timestamp: November 25, 2021, 12:59:37 am
content: Quote from: bdunham7 on November 25, 2021, 12:49:37 amAh thanks for the tip! below is the manual for DC voltage calibration. It doesn't explicitly say I can use other voltages but am I really just "software trimming" it?Could I skip a step and output "1.9v" on the SMU, and then just enter in whatever the 6.5 digit DMM says, rather than trying to fine adjust the SMU to 1.90000v?Same goes for ohms? maybe i don't have to manually trim just make the two displays agree on a number?

author: bdunham7
author_rank: Super Contributor
timestamp: November 25, 2021, 01:11:53 am
content: Quote from: cnt on November 25, 2021, 12:59:37 amYes, most of the better closed-case-calibration instruments give you the option of calibrating to at least a slightly non-standard value. In the case of this one, I think it is just like a digital trimpot and you can use any value, but of course your results may not be as good. For resistance, all you need is stability and a good way of accurately measuring.

author: alm
author_rank: Super Contributor
timestamp: November 25, 2021, 02:03:25 am
content: The value should be somewhere in the ballpark (so not 2.0V or 1.8V). Apart from that, it's the noise and stability of the source (over the time span of the calibration), and the accuracy of the reference DMM, that will limit the accuracy of the calibration. For example, according to theKeithley 2461 datasheet, it has 2 uV noise in the 2V range, or about 1ppm of a 1.9V signal. I don't see a spec for short-term stability, but sourcing accuracy (which will include stability and temperature coefficient over the specified temperature range of +/- 5 K) is 0.015% + 300 uV, or 300 ppm of a 1.9 V signal. So the upper limit for uncertainty would be 300 ppm. Short term stability is likely to be much better, but it would be up to you to characterize how much.

author: cnt
author_rank: Contributor
timestamp: November 25, 2021, 07:38:19 am
content: Quote from: alm on November 25, 2021, 02:03:25 amare you saying in general or for the 197 specifically? for the 200v range the best I can do is 105v. Are you saying I shouldn't calibrate there (ie not calibrate at all for that range) or that setting close to specified and dialing in agreement with the more accurate meter is optimal.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 25, 2021, 08:31:09 am
content: In most cases a calibration is just checking of the readings are inside the range they are suppposed to be and no adjustment at the instrument. It would be only of the meter is off too much than an adjustment is acutally done. There is no problem to do the check with whatever voltages are available.For the adjustment and thus the CAL menue / functions of the DMM it depends on the meter if it accepts a calibration at a much different voltage. Some meters do and some don't. I would consider 2.0 V (e.g. in this case more 1.999 V) or 1.7 V still close enough to the 1.9 V suggested voltage.105 V instead of 190 V may also be still acceptable, as the 200 V range is usually still reasonable linear. The specs of the 200 V range are often not that great anyway and the 200 V range my be one that may have drifted quite a bit.The long term stability specs for the SMU do no really matter and are more like confusing. One could upfront check if the ouput is stable enough - I would expect so.

author: mzzj
author_rank: Super Contributor
timestamp: November 25, 2021, 11:28:13 am
content: Quote from: cnt on November 25, 2021, 07:38:19 amAs Kleinstein already mentioned you should first only verify the accuracy and adjust only if needed.And the official definition for calibration is to only compare the readings between the reference and instrument under test.("common abuse" or historic use of word calibration often implies also adjustment )

author: DH7DN
author_rank: Regular Contributor
timestamp: November 26, 2021, 03:53:03 am
content: Quote from: mzzj on November 25, 2021, 11:28:13 amCalibration is actually a two-step procedure (shamelessly stolen from BIPM VIM)1) establishing a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties2) using this information to establish a measurement resultWhat Kleinstein mentioned ("checking of the readings") is not a calibration, it's a verification.For hobby purposes: Verification of test equipment is easily possible with accessible gadgets. You just have to "steal" or "borrow" the measurement results from a higher quality standard (e.g. 6.5 digit DMM for a 5.5 digit DMM) and somehow transfer it to your test equipment. Your SMU/Source has to be short-time stable, just enough to steal those precious ppmsMy tips:- experimental and environmental conditions should be taken into consideration (temperature, humidity, contacts and cables, shielding, warm-up time) in order to have comparable results- try not to use cheapest low-quality chinesium gear (cables) although there are very cheap but effective methods to get good enough results- if you have to adjust your device under test, take measurements before and after the adjustment- always document every number, experimental conditions and settings- Don't go down the metrology rabbit hole

author: lugaw
author_rank: Regular Contributor
timestamp: November 26, 2021, 12:45:02 pm
content: Quote from: DH7DN on November 26, 2021, 03:53:03 am

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 27, 2021, 03:28:39 pm
content: Quote from: DH7DNCalibration,https://jcgm.bipm.org/vim/en/2.39.htmlQuoteVerification,https://jcgm.bipm.org/vim/en/2.44.htmlQuoteI quite like how they say not to confuse them but given the number of people that get confused on the matter, it is clearly not as straightforward as they would like.Getting back to the original subject, I am guessing you just need confidence in your meter is reading roundabout the right numbers so checking a meter against the other is fine. The issues are that you have a very big uncertainty (how sure you are of the readings) and the stability of the readings you have taken. But go for it, its 1 better than nothing. As you head deeper into the bit of despair or rabbit hole you will gradually start chasing ppms and the ppb. a recent clamp down from ILAC means UKAS are steering away from ppm as its meaning isn't clearso now I am using µV/V and eventually X to 10-9

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2021, 04:56:38 pm
content: When you send in a meter (especially a commercial one) for calibration, the main part you get is a verification - that is the statement that the meter is still within specs. In addition there can be some explaination for the specs and reference uncertainties used - this may be important if the standards are not very good or the meter is quite good. There is a little extra paper-work that comes with the calibration, but for hobby use this is of very limited value.It is only if you also ask for measured values that you get more: a few point with actual data.If required that can be an adjustament as an extra step. Normally, if the meter is well within specs there will be no adjustment.Using a 6 digit meter to check a 5 digit meter is not so bad. One will see of the source is stable enough to get usefull readings - in most ranges I would expect the SMU to get stable enough to get a good reading fast and with reading from the display, with no need to use extra measures like recoring of both readings and than check afterwards.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2021, 05:36:00 pm
content: Quote from: Kleinstein on November 27, 2021, 04:56:38 pmIt will depend a bit on which 6.5 and 5.5 digit meters are involved, but yes--with some care, you can do a decent job on a 100ppm meter using a 35ppm meter as a reference, at least on the lower DC ranges. As for the stimulus, a good quality SMU will typically be excellent for this, even a very good PSU may do. My old Power Designs 5020 is more stable and less noisy than typical mid-to-lower end calibrators.

author: DH7DN
author_rank: Regular Contributor
timestamp: November 27, 2021, 06:24:38 pm
content: Well, now we know what "calibration" and "verification" means according to VIMI'm just telling because I spent few hours looking for measurement uncertainties in DMM datasheets and I couldn't find them (only accuracies). They have to be evaluated separately or they are stated in a (accredited) calibration certificate. That's the part with "going down the rabbit hole"Quote from: mendip_discovery on November 27, 2021, 03:28:39 pmInteresting! Is there any publication on this matter? Accredited calibration laboratories are obliged to use legal units for reporting their measurement results. I'm not sure about ppm or µV/V, haven't seen it yet...

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 27, 2021, 08:22:02 pm
content: Uncertainty is a combination of various elements. Spec, imported unc, temp conditions etc etc. I did start a topic on the subject once before, I am willing to pass on my limited knowledge, I learn by making mistakes and learning from it.I will see if I can find any reference to how unc are to be expressed on the scheduls. I get a feeling it's been a rule for a while but UKAS have just made a decisions on it. Some of them prefer ppm, and I got a larf when I suggested that the dimensional should then all meters and none of this microns and stuff like 10 + (8 × L in m).

author: mzzj
author_rank: Super Contributor
timestamp: November 28, 2021, 01:36:21 pm
content: Quote from: bdunham7 on November 27, 2021, 05:36:00 pmHP6632 outputting 10 volts is also stable to 5ppm over a time perioid of few minutes.

author: mzzj
author_rank: Super Contributor
timestamp: November 28, 2021, 01:45:57 pm
content: Quote from: mendip_discovery on November 27, 2021, 08:22:02 pmLatest trend was that accredited ranges can't overlap on accreditation scope.1 to 10 volts @ 6ppm and 10 to 100 volts @12ppm is no good as the customer wouldn't know what uncertainty they get at exactly 10.00000000000 voltsHad to redefine the scope with mess of smaller or equal or larger <>= markings so that some poor bastard would't get confused.I'm also eagerly waiting when they ask us to break down the thermocouple uncertainty for every thermocouple and every temperature range, resulting in something like 12x12 matrix of possibilities.

author: trobbins
author_rank: Frequent Contributor
timestamp: November 28, 2021, 10:24:32 pm
content: cnt, you may find that the biggest uncertainty with the 197 is how well it has aged - mine still has a jitter significant enough to influence the last 2 digits that I couldn't exorcise after identifying a worse jitter source.

author: cnt
author_rank: Contributor
timestamp: December 04, 2021, 09:52:16 pm
content: interesting. I didn't know that was an issue. with my testing I only see the last digit move 1-2 counts over a few seconds in ohms or volts, with a really stable source.What timescale is the jitter you experience on?