Keysight 34401A - Page 1
Forum section: electronics / general

author: cemelec
author_rank: Contributor
timestamp: May 22, 2021, 03:57:37 pm
content: Just acquired a new old stock Keysight 34401A (6.5 digit) at a good price, never used, original packaging, complete with all accessories and calibration certificate dated January 2016.Its accurate in all functions according to the best standards I have - 0.01% dc low voltage, 0.1% resistance, drift after warm up is negligible. Full self test all pass.A general question - what is the main cause of drift in such an instrument? Is it usage or age? It has some age but no usage.I'm contemplating getting it to a calibration service, but it won't be cheap. I know I don't need that level of precision, just want it. Also a cal certificate might improve its value?ThanksCharles

author: Andreas
author_rank: Super Contributor
timestamp: May 22, 2021, 06:32:14 pm
content: Quote from: cemelec on May 22, 2021, 03:57:37 pmCan be both. (usually effect from usage is larger since the voltage reference is heated).Another effect may be humidity during storage.So I would let the device run at least for a week before comparing it to a standard.With best regardsAndreas

author: HighVoltage
author_rank: Super Contributor
timestamp: May 22, 2021, 09:54:58 pm
content: Quote from: cemelec on May 22, 2021, 03:57:37 pmThese instruments are very stable over time.If you get it calibrated, most likely it will be within specification and no adjustments are needed.The value of the instrument is greatly increased with a new cal certificate, especially if you want to sell it.In addition you have a good reference point with the certificate because the cal certificate should tell you how far off the instrument is in all ranges.Just be careful falling in love with such instrument, soon you have two and you do not know which one to believe!

author: cemelec
author_rank: Contributor
timestamp: May 23, 2021, 08:48:07 am
content: QuoteYou are so right - I now have 5 oscilloscopes, at least 7 AF/RF sig gens, 9 or 10 multimeters, etc.etc. plus a mountain of components enough to stock a small shop.Still, its mostly hobby so I don't need to justify it. The 33401A is so nice I'll be keeping it, I'm now searching for a decent voltage calibrator I can afford. Saw an Analogic AN 3100 DC Voltage Standard recently, supposedly 0.005%, maybe worth getting....Charles

author: Andreas
author_rank: Super Contributor
timestamp: May 23, 2021, 08:58:52 am
content: Hello,I'd rather go for Ians PVDS2 instead of the AN 3100https://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-2-mini-detailwith best regardsAndreas

author: Keith956
author_rank: Regular Contributor
timestamp: May 27, 2021, 04:24:59 pm
content: I wouldn't bother getting it calibrated. They don't drift much - I have 3 of them, of varying ages, and the newest was last calibrated in 2013.A quick check shows they all agree within better than 0.01% to each other....

author: bdunham7
author_rank: Super Contributor
timestamp: May 27, 2021, 05:41:10 pm
content: If you do get calibration, make sure you get before and after data (and post the results!). I don't think it will increase the 'value' of your DMM by more than the price of the calibration.When Fluke started including cal certificates with some meters, they published a note essentially saying that you could ignore the cal date on new meters that had been sitting on the shelf for a while (months, typically, not years) and presume that the meter would be accurate for one full cal cycle from the date it was put in service. They claimed this was based on evidence and testing. I don't know HPAK's position on this, but I would guess that if your meter was stored properly (not at extreme temperatures) in its sealed bag with a desiccant, it will likely still be spot-on.I have one that is 25+ years old and has never had the cal adjusted and it is spot-on on the DC rangesafter a warmup, but it does have some short term variation with temperature. I suspect the long-term drift issues are minimal and mostly related to power-on time and power-on cycles ageing the LM399 reference.

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: May 27, 2021, 09:04:32 pm
content: Quote from: Keith956 on May 27, 2021, 04:24:59 pmAll diferent readings?Well, you definitely need another (better) one to trust!

author: bdunham7
author_rank: Super Contributor
timestamp: May 27, 2021, 09:19:02 pm
content: Quote from: Keith956 on May 27, 2021, 04:24:59 pmI'm not sure you make the best case for not checking calibration--yours are spread 49ppm apart and no two agree to even 5 digits... Are they fully warmed up?

author: Keith956
author_rank: Regular Contributor
timestamp: May 27, 2021, 09:59:29 pm
content: I suspect it's noise from the power supply - the last 2 digits are constantly changing, and they are triggering at different times. Maybe I'll try a battery and shorter leads...

author: mc172
author_rank: Frequent Contributor
timestamp: May 27, 2021, 10:34:44 pm
content: Quote from: Keith956 on May 27, 2021, 04:24:59 pmNice!

author: guenthert
author_rank: Frequent Contributor
timestamp: May 28, 2021, 01:32:26 am
content: Quote from: bdunham7 on May 27, 2021, 09:19:02 pmThey differ by 11ppm and 37ppm of each other respectively. Quite possibly they are still within spec. Of course, it's impossible to tell by this measurement alone.

author: bdunham7
author_rank: Super Contributor
timestamp: May 28, 2021, 02:11:17 am
content: Quote from: guenthert on May 28, 2021, 01:32:26 amWell, yes, if you take the 1-year uncertainty of 47 counts at 1 volt, you can conjecture that if the input is actually within a certain range, all 3 meters are within 47 counts of it. However, I would expect 3 such meters on the same source, in the same room, all warmed up, to be much closer. The entire uncertainty budget includes the standard distribution (2 sigma), guardbanding and most importantly, +/-5C worth of tempco. His explanation about the PSU being noisy actually is more plausible than those 3 meters disagreeing that much, IMO.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 28, 2021, 03:09:49 pm
content: In my experience they do not drift much at all.I think I took this picture in 2017 and these shown instruments had not been calibrated in years before this picture was taken.

author: mendip_discovery
author_rank: Super Contributor
timestamp: June 19, 2021, 08:44:19 am
content: If I was buying a meter for a lab then history is nice, much like service history for a car.Get a UKAS calibration on it. A good lab will give you before and after readings if they adjust it.Here are the plots for mine to give you an idea of stability.

author: Keith956
author_rank: Regular Contributor
timestamp: July 07, 2021, 10:23:47 am
content: Quote from: bdunham7 on May 27, 2021, 09:19:02 pmHere's a pic of the 4 I have, all connected to a battery. 2 have just come back from calibration, the other two have not been in cal for almost 10 years. Can you tell which two are in cal?

author: DH7DN
author_rank: Regular Contributor
timestamp: July 07, 2021, 11:04:11 am
content: Quote from: Keith956 on July 07, 2021, 10:23:47 amWhich value and uncertainty has your voltage standard?

author: Keith956
author_rank: Regular Contributor
timestamp: July 07, 2021, 11:24:20 am
content: 3041A calibrator. Think that's 25ppm?

author: DH7DN
author_rank: Regular Contributor
timestamp: July 07, 2021, 12:07:45 pm
content: Hmm, those Agilent 34401A measurements are very difficult to interpret without stated uncertainties25ppm = 1 year accuracy of your calibrator, not its uncertainty! If has a traceable calibration, it should also have an uncertainty for certain voltage values!

author: Keith956
author_rank: Regular Contributor
timestamp: July 07, 2021, 01:13:17 pm
content: Quote from: DH7DN on July 07, 2021, 12:07:45 pmThe cal certificates I have state the uncertainties (for DC voltage):2V: 5.6ppm +/- 10uV; 20V: 4.4ppm +/- 73uV; 200V: 5.1ppm +/- 0.7mV; 1kV: 5.7ppm +/- 7mV +/- 1LSD

author: guenthert
author_rank: Frequent Contributor
timestamp: July 07, 2021, 04:45:35 pm
content: Quote from: mendip_discovery on June 19, 2021, 08:44:19 amWell, I do not work in a calibration lab and need help interpreting those graphs. On the abscissa the range (or calibration set point? w/o uncertainty?) is plotted, but what is on the ordinate? The deviation (in ppm)?Am I wrong to have expected some time series instead, if those graphs are meant to demonstrate the instruments stability? Stability over time was meant, wasn't it?

author: DH7DN
author_rank: Regular Contributor
timestamp: July 07, 2021, 07:02:23 pm
content: I made some calculations and actually three of four of your DMM measurements agree within manufacturer's specifications (see diagram).DMM: Keysight 34401A, DC Voltage, 1 year Spec.Range 10 V: +/- (0.0035% of reading + 0.0005% of Range)I'm not sure about the values of your calibrator. Usually, in a calibration certificate, one gets the deviation from a nominal value (e. g. 10.00000 V +/- Deviation) and a stated expanded uncertainty of the standard.Well, just to compare the Keysight 34401A measurements: I assumed you set the calibrator to a nominal value of 7.757000 V (+/- 0.000107 V). If so, then only two of your DMMs agree with the measurement against the voltage standard.Just an estimate, I could bewronguncertain

author: Keith956
author_rank: Regular Contributor
timestamp: July 07, 2021, 07:56:14 pm
content: Quote from: DH7DN on July 07, 2021, 07:02:23 pmI have the docs from the cal lab specifying the expected deviation etc. if you're interested. Only one needed adjustment to meet the cal spec, and that was on the 100V/1kV AC volts ranges only. So as said before, they don't drift much over a 10 year period, and calibration only makes sense if you need to prove it...

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2021, 08:00:15 pm
content: The usual calibration does not include an adjustment. So in essentialyl all cases the DMM will give the same reading after calibraiton as it would heve shown without the calibration (excluding the extra stress from transport). So the meter itself does not get better, one only knows the meter was checked and in some cases one gets actuall readings of the cal soures, so one could numerically correct the deviation to some degree.The 4 meters shown are still close enough togehter, that all of them could be still in spec.The difference between the extremes is still relatively large. So likely at least 1 of the meters seem to have drifte quite a bit.

author: bdunham7
author_rank: Super Contributor
timestamp: July 07, 2021, 11:31:17 pm
content: Quote from: Keith956 on July 07, 2021, 10:23:47 amIf your meters have all been warmed up for at least an hour, I sincerely hope it is the bottom 2.

author: Keith956
author_rank: Regular Contributor
timestamp: July 08, 2021, 06:56:22 am
content: Quote from: bdunham7 on July 07, 2021, 11:31:17 pmYou would think so, but actually the top 2 went for cal. I did send the two that readings furthest from the mean of the 4, and the cal lab tests first and only adjusts if out of tolerance.The cal house report stated (amongst many other things) that on the 10V DC range, the test tolerance was 400uV. (that tolerance is the HP quoted 1 year figure of +/- (0.0035% of reading + 0.0005% of range).1 meter read 10.00021V with 10.00000V applied, the other 10.00013V. So both are 'in spec'.So, with one pair of meters out of cal for around 10 years, the other in cal, both pairs are within the manufacture's spec. Which reinforces my opinion that getting this particular meter calibrated only makes sense if you need to prove your readings are within spec.

author: cemelec
author_rank: Contributor
timestamp: July 14, 2021, 04:26:24 pm
content: Following my original post, I thought I'd follow up with my further testing. Basically I wanted to know how well a NOS 5 year old Keysight 34401A would hold its calibration.I mentioned I might get a voltage calibrator, And Andreas suggested this:https://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-2-mini-detailSo after checking various sources, YouTube, EEVblog and Ian Johnsons website, I went ahead and spent some of my lockdown savings on the PDVS2mini.This enabled me to do some low voltage DC testing, the results are in the attached PDF and Excel files (both the same data)the 34401A is certainly within the 1 year Keysight spec.Obviously the results are totally dependant on the PDVS2mini accuracy, but the PDVS2mini build quality is top class, the manufacturing and test processes top class, and final calibration carried out using very high end test equipment, so I have absolutely no doubts on that.Its not a rigorous calibration process, for which I neither have the equipment or the knowledge, but so far I'm happyIt only covers DC up to 10 volts, but I don't see any reason for ACV, current and resistance to have driftedHope its of interest to someone!