34401A as transfer standard for poor man’s calibration - Page 1
Forum section: electronics / general

author: Jester
author_rank: Frequent Contributor
timestamp: December 24, 2021, 02:42:14 pm
content: I have 3 x 34401A and 2 x 8842A DMM’s that have not been calibrated for many years.I also have a Data Precision 8200 and clarke-Hess 828 calibrators, also not calibrated recently, but do allow generation of stable V & I.I have had the 34401A’s for a long time and they drift very little relative to each other from year to year.The 8842A’s are new to me but obviously old, they both came from the same lab and were likely calibrated with the same source as they agree with each other to 1 or 2 LSD. The Flukes read about 0.03% lower than the HP’s on some ac ranges.I would like to get all of these meters to match and then I can observe over time how much they drift at least relative to each other. I don’t need to have them traceable to NIST etc. as I’m retired so this is a hobby at this stage.So I’m thinking of dropping of one of the 34401A’s for calibration at Pylon in Ottawa, I called this morning and they charge $150 for an actual calibration with printout (but not with official certification).Upon return I would generate the required V&I based on the measurements from the freshly calibrated 34401 and calibrate the other 4 meters to match. I’m not concerned with the resistance calibration at this time.I’m pretty uniformed regarding the nuances of metrology, so other than at least a 24 hour warmup I’m not planning on doing anything other than connect them all up with conventional banana leads and go at it, unless the experts here tell me don’t do that or did you consider this?One potential issue I see is that the calibration procedure for the Fluke meters calls for 1kHz ac waveforms and then later for an even higher frequency calibration Perhaps 100kHz? And the Clarke-Hess only does 50, 60 and 400Hz. I’m not really concerned with ac accuracy at higher frequencies for the work I do so hopefully calibrating at 60Hz or perhaps 400Hz will be acceptable.Comments and suggestions please.

author: alm
author_rank: Super Contributor
timestamp: December 24, 2021, 03:52:52 pm
content: Did you check the 8842A manual to see if you can adjust DCV/ACV without the tricky ranges (like high voltage highish frequency) or Ohms/current? Some DMMs do, and some require you to complete the entire process before they will save.I'm a bit concerned about substituting a lower frequency for AC, because they probably use that point to adjust the entire flatness of the AC function. So all frequencies beyond a few hundred Hz might be off. It's even possible the meter will reject it if it's too far off what it's expecting, although I doubt it for this vintage.Keep in mind the 34401A specs are very close to the 8842A. The 8842A is actually a bit better on DCV. So nothing like a 5:1 or so test uncertainty ratio. So you won't be calibrating the 8842A to its 1 year specifications. But it sounds like you may not care.I believe 0.02% difference on ACV is within 1 year specs for both the 34401A and 8842A, so you may not actually be making the 8842A any more accurate. It's almost equally likely you'll adjust it further away from the ideal value. But if you just want similar numbers, then I think your idea should work. You don't even have to send the 34401A out. Just pick one and adjust the rest to match. Make sure your sources are stable for the couple of minutes it takes to compare and adjust. Or it will be a painful process (speaking from experience here).

author: eplpwr
author_rank: Regular Contributor
timestamp: December 24, 2021, 06:00:50 pm
content: Quote from: Jester on December 24, 2021, 02:42:14 pmPlease note the difference between "calibration" and "adjustment", this question has been discussed in numerous threads. It is possible, even probable, that you get a certificate with "Calibration: Passed", nothing more. That means the cal lab has checked your meter against their traceable standards and the 1 year specs of the meter and all readings were in the allowed +/- ranges. It does not mean they've adjusted the meter to be as precise against their standards as possible, that's called "adjustment". A few cal labs will always start with an adjustment, but that's uncommon.

author: bdunham7
author_rank: Super Contributor
timestamp: December 24, 2021, 06:28:46 pm
content: TBH, it doesn't sound like a good idea. Perhaps getting one of your 34401As calibratedwith datamight be OK, but given how closely all your meters match now, I doubt there will be any surprises. I would definitely not recommend making any calibration adjustments in this way though. You're likely to make things worse.The Fluke 8842As are likely agreeing with each other because they haven't had their calibration data changed since they left the factory. The voltage reference in these is unheated and even less prone to drift than the heated LM399 in the 34401. How closely do all 5 meters agree on DC volts? If it is 1-2 LSD of the 8842A or 5-8 LSD of the 34401A, it isn't realistic to expect any better. And for AC volts, 0.03% is half of the 24-hour spec of the 8842A, so again that's already far better than any reasonable expectation. That small difference could be due to how the meters react to your particular stimulus, especially considering that you don't have a 1kHz AC source. The problem with trying to 'match' them is that thermal variations, measurement uncertainties and noise may be greater than the differences between meters. So one might be higher one day, then if conditions change (usually temperature) the relative readings may change.You don't need to match them to observe drift or variation due to temperature. You can just record the various readings and note the minor, within-specification discrepancies. Then compare those discrepancies over time to see if there is any variation and if that is correlated to temperature or anything else. If you see a consistent trend or a one meter that is consistently a small amount off, then you might consider a calibration adjustment.

author: Jester
author_rank: Frequent Contributor
timestamp: December 24, 2021, 07:30:35 pm
content: Quote from: eplpwr on December 24, 2021, 06:00:50 pmI specifically asked if they actually calibrate or just verify the calibration, and the guy on the phone stated actual calibration. If I decide to go this route ( I’m having second thoughts), I will confirm that they actually calibrate, before making the drive. Pure speculation but I would have thought calibration houses would have software for common meters like the 34401A that would calibrate without user interaction. The production calibration rigs we designed did that 30 years ago because too much operator error otherwise not to mention an incredible waste of resources.

author: bdunham7
author_rank: Super Contributor
timestamp: December 24, 2021, 07:40:47 pm
content: Quote from: Jester on December 24, 2021, 07:30:35 pmThe exact question you want answered is whether the service includes before and after data (as-found and as-left) with uncertainties.

author: alm
author_rank: Super Contributor
timestamp: December 24, 2021, 09:54:18 pm
content: Also relevant is if they do adjustments, and under what circumstances. It might be that they only do adjustments if the measurement is out of specification, or close to that. This is often better for the customer, because if you're tracking drift yourself, then any adjustment will destroy that history you built up.They will usually have a multi-function calibrator likethisthat they just hook the meter to, and software that controls both the calibrator and the DMM via GPIB. So calibration is pretty much fully automated unless it's a DMM with a manual function switch, like a handheld DMM. But whether the program is set to make adjustments depends on the specific service you buy. Like I said, many serious customers will prefer no adjustments unless it's really necessary. The "give me a sticker so my compliance officer shuts up" crowd won't care.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 28, 2021, 12:57:45 pm
content: If you want to recalibrate the 34401a as i did with certificatesI professionally (with proper tools) changed all the tantalums and electrolytics in them, paid 160$ each to get them recalibrated with cal reports, one of them had the vfd gone dim, took a chance on aliexpress for cloned vfd'sStill work a threat, and they havent drifted much, before and after, still good meters for their age

author: Arhigos
author_rank: Regular Contributor
timestamp: January 03, 2022, 02:39:59 am
content: What you asking about is not a "transfer standard". It's called characterizing calibrator and you can find a lot information how to do that herehttps://download.flukecal.com/pub/literature/ImprovingTURs2007ncslpaper.pdfShort answer is that you don't have enough equipment listed in OP post to perform a calibration of 8842a dmm

author: Jester
author_rank: Frequent Contributor
timestamp: January 04, 2022, 12:39:26 am
content: Quote from: Arhigos on January 03, 2022, 02:39:59 amThank you for the paper on Characterizing & Disciplining an Electrical Calibrator, I read it and this is exactly what I had in mind, except they described doing it with a 8.5 digit meter.Both of my calibrators go up to 1000V, however only to 400Hz. The 8842A calibration procedure, states that you can calibrate at lower frequencies and this will actually improve accuracy at that frequency at the cost of less accuracy at higher frequencies. So I think I can do a full calibration of the 8842A with that in mind.The 34401A has a similar procedure, however they do not mention the possibly of calibrating at other than the specified frequency. I suppose I could use my HP signal generator, however only over a limited range so I don't think calibrating ac on the 34401A is possible unless I upgrade my ac calibrator.

author: alm
author_rank: Super Contributor
timestamp: January 04, 2022, 02:14:47 am
content: Quote from: Jester on January 04, 2022, 12:39:26 amDid you do the math for what they called the test specification ratio in that paper? What kind of figures did you get comparing the 34401A 1y specs with the 8842A specs? There's a reason why they use an 8.5 digit meter to calibrate a 6.5 digit meter, and it's not just because they want to sell you an expensive multimeter.

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2022, 03:56:27 am
content: Quote from: Jester on January 04, 2022, 12:39:26 amWhat you should keep in mind here is how the calibration constant adjustment process works on these meters regarding AC frequency. The lower (main) AC adjustment will set the calibration constant for that range. It's true that if you are going to be making measurements at a specific frequency, say 400Hz, that you can slightly improve the accuracy of the meter at that frequency by setting the calibration constants using a calibrator at 400Hz. The upper frequency adjustment does not change a calibration constant in the same way, but rather alters the actual frequency response of the input. If you think about it, the DMM doesn't know what frequency or mix of bandwidths you are testing and it needs to be able to do a TRMS reading on whatever it is seeing. The result is that the upper frequency adjustment really has to be done at the specified frequency unless you really put some thought and effort into it. Fluke does discuss optimizing both below 1kHz using the main cal constant and above using the HF AC adjustment, but you should be very careful with this, and you really can't use over 100kHz without causing issues. I always try to make sure, where I can, that the meter rolls off smoothly. If you aren't careful, you can end up with a meter that significantly peaks near the top of its specified bandwidth and then rolls off.Your setup may be fine for checking, but I'd think thrice before changing anything.

author: Jester
author_rank: Frequent Contributor
timestamp: January 04, 2022, 02:02:34 pm
content: Quote from: alm on January 04, 2022, 02:14:47 amWell unless I get a better meter (7.5 - 8.5) the ratio would obviously be unacceptable from a metrology perspective. I’m keeping an eye out for one. Thanks.