Calibrating 3458A? - Page 1
Forum section: electronics / general

author: Keith956
author_rank: Regular Contributor
timestamp: February 15, 2022, 03:21:58 pm
content: I have been very lucky in finding a 3458A recently. It's in impeccable condition with all the original manuals etc. and was bought by the seller's father's business in the mid 90's and used to verify calibration standard resistors the business made. So far I've just confirmed it passes self test and gives sensible readings (within 0.5mV of a 34401A in cal).What I'm wondering is whether to get it calibrated - I have no idea how much Keysight charge for a standards lab level cal here in the UK but I expect it to be expensive. So what do others here do? Intended use is for lab reference and to calibrate the other 34401A's I have.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 15, 2022, 05:38:11 pm
content: My 3458A was purchased used several years ago. Like yours, it was pristine with all the manuals and accessories and in its original Agilent shipping box. Presumably, it had not be used for 9 years or so. At first, it seemed a bit drifty. After a lot of use it now seems to be settling down. If yours is similar, you might want to give it a lot of use before having it calibrated.

author: tszaboo
author_rank: Super Contributor
timestamp: February 15, 2022, 05:44:00 pm
content: Quote from: Keith956 on February 15, 2022, 03:21:58 pmAs I recall, you only need a 10V and 10KOhm to calibrate it. And yes, sending it to a cal lab is expensive, they need a Fluke 734 to make the calibration.

author: IanJ
author_rank: Unknown
timestamp: February 15, 2022, 05:58:52 pm
content: Hi,Last time I was charged about £700+VAT from Keysight over a year ago. I'm about to call them again for costs as I'm due in March.There are different levels (and price), 3 if I remember. Mine was to UKAS standard.Remember, they don't necessarily make any adjustments. If it's within spec they just detail to you the numbers. Up to you after that.Note: A few years ago they identified my front/rear switch was giving inconsistent results in one of the modes if I remember. Luckily I didn't need that mode and I was fine without repair until the I replaced it prior to the following years re-cal.Ian.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2022, 06:01:04 pm
content: The adjustment only needs a 10 V and 10 K reference, but a proper calibration (legal certificate),still needs the full high grade calibrator to generate all the test voltage to do a full functional test of all the ranges.=.5 mV difference to a 34401 is quite a large difference, unless tested in the 1000 V or 100 V range.For calibration of a 34401 one would still need a stable source to generate the test signals the 34401 wants. The 3458 alone is not enough.Keysight has some problems doing business with private customers at least in Europe. So one may have to look for a different lab (e.g Fluke) in the UK.It may be a good idea to frist use the DMM for some time, so it was on much of the last 4-6 weeks before sending it in for calibration. Some units may show some drift if not used for a longer time. Another point is doing functional testing before: e.g. look at input bias, ADC drift (SN18) and check the date on the battery backuped RAMs. A drifting ADC is bad - it gets even worse directly after calibration.

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 15, 2022, 06:15:31 pm
content: The meter is possibly still well within specifications but you cant be sure unless you get it checked. I Personally prefer meters with documentmented history.Plenty of labs in the uk that can calibrate it, not so many can adjust it but there will be some about. I would suggest finding one with 17025 accreditation (UKAS) and local to you so you can drop it off as the ppms dont like the delivery firms.Not sure how much luck you will have with keysight but if you run into trouble I can see if I can arrange it to be a subcon job through the lab I work at. I don't have a good enough uncertainty to be looking at your meter but as an UK company I can send it to them.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 15, 2022, 06:21:08 pm
content: Your first step is to follow the process in this service note:https://xdevs.com/doc/HP_Agilent_Keysight/3458A/service/3458A-18A.pdfGiven the age of the unit and potential downtime, you should run the test over a couple of weeks of power on, longer if you see drift. There are two objectives:- getting the reference settled. Per KS, long power downs can cause a reversible shift in the reference value. They recommend a 6 week minimum "warmup".- Seeing if you have a drifty ADC.If you have a drifty ADC, the fix is a new board - if you can get one. However, for your calibration processes, the long term stability may not be an issue.Finally, DC calibration is a relatively quick process, and requires just 10V, 10k, a good short, and some care around thermals, warmup and so on. As Kleinstein notes, verification is a much bigger deal.So if you can find someone with good hobby 10V/10k willing to help you, you can get well along the path. Or at least, the start of it...

author: Bill158
author_rank: Regular Contributor
timestamp: February 15, 2022, 07:43:03 pm
content: Congratulations on getting a top of the line DVM!Don't forget about the age of the NVRAMS. If they have never been changed then before anything else new ones must be installed. BUT just installing new NVRAMS without copying the old ones and then programming the data into the new ones will loose all of the previous calibration data. If your NVRAMS are original in your 3458A then you are running on borrowed time already. The batteries in the NVRAMS are rated for 10 year life. Yours could be as much as 25 years old. Open the instrument and look at the date codes on the installed NVRAMS. If you send your 3458A into KS then they will take care of this. I am not sure if your "corner" cal lab will change them. But the NVRAMS in your 3458A are probably soldered in so you need to unsolder them from the PCB and take them out, install IC sockets, and then plug in the new ones. Search EEVBLOG for information on how to read the original NVRAMS before you remove them. There are programs out there that can read the NVRAMS through the IEEE 488.Bill

author: Keith956
author_rank: Regular Contributor
timestamp: February 15, 2022, 08:10:15 pm
content: Yep the NVRAMs are 94/95 vintage. I'll take a look for the info on how to read/change them.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 16, 2022, 01:20:36 am
content: Hello,Maybe you also want to become a volt-nuts soon?You might make or buy your own Volt and Ohm references, and compare these with references from other volt-nuts, in your country, maybe at low uncertainty.The 3458A is no standard, by no means.Therefore, for amateur use, you might use such references to check or to unofficially calibrate your 3458A. If you like, you could also adjust it to these references.Here in Germany, there's user ap, from ab-precision, who might offer such services.Frank

author: Keith956
author_rank: Regular Contributor
timestamp: February 16, 2022, 08:06:24 am
content: Quote from: Dr. Frank on February 16, 2022, 01:20:36 amHaha Dr Frank, you may be right! A 10V reference is next on the list, but so many choices!Keith

author: IanJ
author_rank: Unknown
timestamp: February 16, 2022, 10:25:47 am
content: FYI,Keysight UK just got back to me with cal cost - £783.Ian.

author: Keith956
author_rank: Regular Contributor
timestamp: February 16, 2022, 10:53:10 am
content: Thanks Ian. I'll follow the advice here and let it run for a few weeks before getting it into cal.Which level is that for? The basic 'Keysight Calibration' or all the way up to 'Keysight Standards Lab Calibration'?

author: IanJ
author_rank: Unknown
timestamp: February 16, 2022, 11:48:57 am
content: Its for UKAS Accredited Calibration, the standards lab level.From memory, the basic Keysight level cal was a couple hundred quid cheaper. But, since I use mine for calibrating my products and producing my own cal certs i need the UKAS level.Ian.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 16, 2022, 01:28:52 pm
content: Hello Ian,that sounds interesting.Have you found a way to be able to emit your own, official calibration certificates?Do you have an accreditation, or sort of?Frank

author: IanJ
author_rank: Unknown
timestamp: February 16, 2022, 02:12:23 pm
content: Quote from: Dr. Frank on February 16, 2022, 01:28:52 pmKeysights website is a little confusing (as I was!)......so I called Keysight UK just now, here's the 3 levels. No prices as their pricing system was down.1. Keysight Cal.2. UKAS3. UKAS Standards level, tighter tolerances/spec.1st one is the cheapest.I normally get 2nd one......per the price I gave earlier.The guy told me that not many people go for the 3rd one.Other than that, i just produce my own cal certs when i cal my products. Actually, its all automated via my GPIB app (control loop and spits out a Word doc), but i dont issue anything UKAS formally/legally in any capacity if that what you mean.Ian.

author: Keith956
author_rank: Regular Contributor
timestamp: February 18, 2022, 10:48:07 am
content: I've managed to read the NVRAMs using Ian's WinGPIB. Now the question is: what to replace them with? DS1220Y-150 (obsolete now) or DS1220AD-150+ (Farnell have them in stock)?And can anyone recommend a cheap programmer that will can program them using the saved data?

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 18, 2022, 12:35:25 pm
content: The difference between the different types was the voltage compliance, and speed, afair.The circuit is fine with any of these as long as speed is 150ns or faster. So probably the AB is also ok, of course the -100 types.Maybe you check if your 5V digital supply is really within +/- 5%.Get what is the cheapest, i.e. the DS1220AD-100+, and don't forget to order two 32kx8 as well, i.e. DS1230xy which are about double the price of the 1220.Please order precision DIL sockets for these ICs.Maybe you want to add 128kB Ram, i.e. 4x 32kx8, 62256 SRAMs, available cheap for <2€/EA.Then I recommend to update the firmware, to 8 or 9, as older versions still contain a lot of bugs.6 or 1 chip EPROM images can be found elsewhere. If you have already the single 27C4002 version, get a new EPROM, plus DIL socket.The older EPROMs, 6 chip version, might not be available any more and they then need to be erased first.All programming can be done with the cheap TL866 II from XGecu.I recommend ordering from the original distributor, and to add the complete adapter set.Frank

author: Oldtestgear
author_rank: Regular Contributor
timestamp: February 21, 2022, 12:27:53 pm
content: QuoteThey charged me £600 inc VAT in November 2021 for a standard ISO17025 qual. That really is inflation unless you specified a different qual. level.Phil

author: martinr33
author_rank: Frequent Contributor
timestamp: February 21, 2022, 09:26:46 pm
content: Regarding replacing the nvm on the 3458A...Follow Dr. Franks' lead on this. A little more detail on desoldering below.You will have to desolder the existing memories, and the code EPROM(s). This is not so easy if you have not done it before.You will need a solder sucker; some solder; and maybe a bottle of flux (a small brush bottle is fine).- paint the existing solder with flux. This makes the solder more mobile.- Melt it with the iron, to the point where the whole pin is in molten solder. This takes a couple of seconds after you first see it melt.- Pull the solder with the solder sucker. Be quick.- If the pin is not loose in the hole, poke it with the tip of the iron until it breaks away from the side of the hole. If you push it just right, it will be loose in the hole when you are done. This is important.- if a pin does not come clean, add some solder and flux and try again. Adding a little solder helps draw out the residuals. You can also add alittle solder to the pin before you try to desolder it, this approach can help.When you are done and all the pins are loose, the chips should pop out with minimal assistance. Sometimes, I use a flat blade screwdriver to gently twist off the chip - but BE CAREFUL. If they do not, check the pins again. The risk is that you will pull the copper out of the PCB, which is a nuisance. 3458A boards are very robust, but you still need to be careful.Also, after programming there is a chance that the chips will be marginally corrupted and throw an error. If that happens, just reprogram the part.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 21, 2022, 11:42:39 pm
content: @IanJ:I thought that the KS calibration/maintenance multiyear contracts were quite reasonably priced. Might be worth a look.

author: Keith956
author_rank: Regular Contributor
timestamp: March 29, 2022, 05:01:07 am
content: Out of interest, does the data in the cal NVRAM change over time? I read the cal ram at the beginning of the month, then I did it again yesterday. Several bytes in groups (presumably float values) have changed.Considering the cal ram is dated 94/95 (like most of the other ICs) so is over 25 years old, it's remarkable that the nvrams are still working. I get the impression this 3458A was not used a lot - the cal count is 1, the seller told me it had never been calibrated since new.Martin - I have a Hakko fr-301 which I've used to desolder 28pin DIL packages before, hopefully it will be straightforward to remove the cal ram and the settings rams and replace with socketed ones. I've got a TL866 programmer so can flash the new ones and verify them.Dr Frank - the EPROM (the board is the one with the single EPROM) is version 9 already.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 29, 2022, 09:03:55 am
content: Quote from: Keith956 on March 29, 2022, 05:01:07 amHello Keith,no, the NVRAM data do not change over time. Only if the internal back-up battery is depleted, it will lose its content, but then the 3458A will display a checksum error.The CAL nvRAM contains the calibration, Autocal and maybe some setup constants, see calibration manual, command CAL?.Therefore, whenever you do an ACAL, its content will change.Yes, the batteries might hold for 15 years or longer, but may fail all of a sudden, so you are safe already, as you have downloaded the content of the CAL nvRAM. Seems that somebody serviced the µP PCB already. Would be nice if you could provide a picture of the whole PCB. From about 1995 onwards, the 40k reference resistor R207 was changed to a VHP101 type, which is much more stable than the initially used one. Maybe you have a look and a picture on that, also. This resistor might be bent backwards a bit, but it's uncritical to align it upright, carefully.I Thank - Frank

author: bsdphk
author_rank: Regular Contributor
timestamp: March 29, 2022, 12:29:10 pm
content: Quote from: Dr. Frank on March 29, 2022, 09:03:55 amThat is not quite correct...The calibration constants only change when calibration procedures are followed, but the NVRAM is used to store a couple of undocumented variables which change outside calibration context.The `CAL?` command has at least the following undocumented uses:# Undocumented CAL? queries# CAL? -1 21249=NUM_SAMPLES# CAL? -2 861=BURST2# CAL? -3 1149=BURST1# CAL? -4 11=NREADINGS# CAL? -5 2010=SMPL_RATE# CAL? -6 1=SSMP_INC# CAL? -7 3.54147092E-05=PERIOD# CAL 2437 16611, DEFEATS valid 2437# CAL 2941 0, DESTRUCTIVE OVERLOADS valid 2941The last two of which are stored in the NOVRAM

author: Keith956
author_rank: Regular Contributor
timestamp: March 29, 2022, 01:05:37 pm
content: Dr Frank, here are some photos. Looks like the Vishay 40k resistor is there (R207). The IC datecodes are nearly all '95.Keith

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 29, 2022, 01:24:29 pm
content: Quote from: bsdphk on March 29, 2022, 12:29:10 pmHello Poul-Henning,thank you again for uncovering these secret parameters.I simply took the question literally "do the data changeover time?",, i.e. an nvRAM is intended to keep its content stable over at least 10 years.The data of course change over a write operation, but that's a different story.Btw.: We still have not tested the ADR1000 inside the 3458A for lower noise prformance, yet.You kindly assisted when I changed the CAL limit constants in the EPROM.Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 29, 2022, 01:35:27 pm
content: Quote from: Keith956 on March 29, 2022, 01:05:37 pmHello Keith,you got FW 8, so that probably was originally installed in 1995 already, at least the EPROM and the solder joints look original. That's fully ok, no need to update to V 9, as all of the severe bugs are fixed with V8 already.The type designator of R207 should be on the back side, but probably there's only 40k000 and 0.1% imprinted, then you'd still have the older resistor version inside. I've got the VHP101, which is rock stable over time (a decade), and has quite a low T.C. , as far as I can tell.Frank

author: bsdphk
author_rank: Regular Contributor
timestamp: March 29, 2022, 01:38:29 pm
content: Quote from: Dr. Frank on March 29, 2022, 01:24:29 pmWhat can I say, the HP3458 firmware is some of the most interesting I have disassembled yet :-)(Disassembling old code is a computer-archaeology hobby of mine, and my HP instruments have made for good research material:https://github.com/bsdphk/PyReveng3)Since the HP3458 is a current product, I can obviously not publish the (tools for) reverse-engineering that.

author: Keith956
author_rank: Regular Contributor
timestamp: March 29, 2022, 01:49:29 pm
content: Quote from: Dr. Frank on March 29, 2022, 01:35:27 pmI had thought from a cursory look that it was Rev 9 but you're right, that 9 is really an 8.I'll take a closer look at R207 next time I've got the top off, which will be when the cal ram needs to be replaced.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 29, 2022, 02:56:56 pm
content: Quote from: Keith956 on March 29, 2022, 01:49:29 pmSorry, I have to correct myself. I found the part number of your FW version @ xdevs. 03458-88887 is FW 6 only.That makes much more sense concerning its vintage.So I recommend to upgrade to V 8 or 9, when you change the nvRAMs.Single EPROM binary is available, maybe not on xdevsEither buy a new EPROM, 27C4002-100, I see it for expensive 11€ at Reichelt, Germany.Or erase the old one if you got the equipment.Frank

author: Keith956
author_rank: Regular Contributor
timestamp: March 29, 2022, 03:29:45 pm
content: Is that correct?If I use the REV? command from the front panel it returns : 8,2And the EEPROM has Rev=8 on it.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 29, 2022, 04:30:14 pm
content: Yes, you're correct, of course. And I just got my own original replaced EPROM, V8.It's wrong here:https://xdevs.com/fix/hp3458a/#firmware.If you scroll down a little bit there, TiN has put some information about the version of that R207.

author: martinr33
author_rank: Frequent Contributor
timestamp: March 29, 2022, 05:46:51 pm
content: Here's a version of the V9 software that programs directly into a 16-bit EPROM. The original version I downloaded was byteswapped.The checksum of this version is 03FCB1D6.https://www.dropbox.com/s/7h7pqodpv5ifftm/output_rev9.bin%20-%20CORRECTED%2003FCB1D6.bin?dl=0

author: alm
author_rank: Super Contributor
timestamp: March 29, 2022, 09:50:31 pm
content: Quote from: Keith956 on March 29, 2022, 01:05:37 pmWhere do you see the VHP101 markings?I believe this is a picture of the VHP101 resistor (by TiN fromhere):And this is of the old resistor they used before they switched to the VHP101 (by TiN fromhere):

author: TiN
author_rank: Super Contributor
timestamp: March 30, 2022, 12:30:26 am
content: 301031 is still VHP resistor, just custom order P/N. I have not seen evidence that this older marking resistor is any worse than standard VHP101 found in newer units.QuoteBit info on this. I can say that noise performance so far unchanged with ADR instead of original A9 in my unit. This lead me to think more into A3 modifications, but due to other ppm activities didn't resume those experiments yet.

author: alm
author_rank: Super Contributor
timestamp: March 30, 2022, 10:49:26 pm
content: Quote from: TiN on March 30, 2022, 12:30:26 amThat's interesting. So the change didn't take place around 1995/1996, given the 1990 date code on the picture I got from your website, and I'm pretty sure that I have a fairly early (1990ish) unit that has the same hermetic 301031 part number. So you're saying these 1990-vintage VPG parts that were made well before the VHP101 series came out performed very similarly in your experience, and might have been using the same C/K foil combination? I thought they would have been K-foil parts with a significantly higher tempco than the VHP101 with its combination of C and K foil to balance the tempcos.I had in the back of my mind that if I did a major revision on one of my 3458As that would require recalibration (like lowering the LTZ1000 temperature set point), I might as well replace R207 with an VHP101 part. But sounds like this may not yield much improvement, and may actually perform worse given the soldering heat will partially reset the aging.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 31, 2022, 06:55:09 am
content: TiN, Alm, sorry, but your statements contradict what has been found out in TiNs first 3458A repair report, credits:https://xdevs.com/fix/hp3458a/#ohm_pimp"Most stable resistor in circuit, 40KΩ is currently specified in CLIP’s BOM as ±1.3 ppm/°C. But actual resistor on our board (and any newer 3458A’s as well), is Vishay Precision Group Foil VHP101, in hermetic oil-filled can. This series were released by VPG in 1996, so it’s later change, not reflected in old CLIP. Actual specification for VHP101 shelf-life stability is ± 0.0002% (or 2 ppm) stability in 6 years, and typical TCR ±0.3 ppm/K in +15°C to +45°C window."The difference to the original resistor is the explicit type designator, VHP101, which is not imprinted on the old version, and obviously was not available from Vishay before 1996.From TEMP? vs. external ohm reference measurements, I estimate that my 2001 unit with a VHP101 resistor built in has a T.C. of < 0.3ppm/K, confirming TiN own findings of VHP101 specification, and that again is in contradiction to the 1.3ppm/K in the CLIP and to the 1.1ppm/K in the 3458A specification.Then I also want to hint to the individual serial number on top of each VHP101 type resistors, which is missing in these earlier units.Obviously, hp ordered individually characterized (probably for T.C.?) resistors.FrankThe diagram shows TEMP? vs. the calculated deviation of R207 in ppm, for two different time periods, in between I made a re-calibration.The VHP101s T.C. is specified by a window, < 10ppm in [+15 ..+ 45°C], so that's not the usual "typical" T.C. specification by Vishay.

author: TiN
author_rank: Super Contributor
timestamp: April 01, 2022, 03:27:51 am
content: I didn't say there is no difference in resistor, resistors are different at least in available specs and designators indeed. My point is that replacing old 301031 resistor with random new VHP101 will not magically improve performance as I have tested multiple 40k VHP101, VHP102 and VHP202s and they all weren't best and not usually <0.3ppm/K but larger about 0.5-1ppm/K mark. So there is little improvement to be gained replacing old 1.3ppm/K resistor with new 1ppm/K *actual* VHP101 for example.Individual handwritten marking is a order batch number, often applied to resistors even without any special treatment/selection processes. Real PMO'd or matched resistors from VPG usually have special customer partnumber instead of generic VHP*** label on them. For example my custom VHP101 with guaranteed 0.3ppm/K:Here's other resistor from same setNote same part-number 321217 despite different value

author: alm
author_rank: Super Contributor
timestamp: April 01, 2022, 08:46:06 am
content: Quote from: Dr. Frank on March 31, 2022, 06:55:09 amHow did you measure this? Hook up wires to R207 with an external 3458A or something better, and then turn on the 3458A under test and let it warm up? Or did you measure using a more indirect method?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 01, 2022, 09:24:04 am
content: Quote from: TiN on April 01, 2022, 03:27:51 amTiN, pictures from your site are not available in Belaruswrites "409 Conflict". It may be possible to load pictures on the EEVBLOG website?

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 01, 2022, 12:14:17 pm
content: Quote from: alm on April 01, 2022, 08:46:06 amDescription of the diagram is on the bottom of my post, i.e. X-axis is TEMP? , y-axis is calculated deviation of R207 in ppm.Latter is measured as follows:Since 2011, I maintain a group of 10kOhm resistors, 5EA VHP202Z and later, one inside a FLUKE 5450A, which is the most stable.I regularly measure the absolute values of all 6 resistors (once or twice a month) by my 34548A and calculate the mean value of the group, which is then my current reference value.Btw.: These transfer measurements are uncertain to 0.2ppm StD, meanwhile, as specified in the HFL option.This mean value is compared against calibrated references from time to time, e.g. on the Metrology Meetings, or by ab-precision, or by PTB on the Hannover Maker-Faire.From these 'baseline' comparisons, I estimated that the VHP202Zs all show a constant drift of about -1ppm/yr. each, which is used as a prediction parameter. The 10kOhm inside the 5450A seems to be very stable.All measurements by the 3458A can then be corrected. The correction factor, like 0.9999991, is also used to estimate the absolute value of R207, and its drift over time. So that's an indirect measurement.As R207 is quite stable over time, I can nearly neglect its timely drift for the T.C. estimation.The temperature of R207 is indirectly measured, as the sensor for TEMP? measurement is sitting on A1 board as well, not far away from R207, but not directly coupled of course. That generates these big outliers from the linear regression curve.You would have to extract the timely from the temperature drift more precisely, but that's good enough for this purpose, I think.As I would never de-solder R207, and also would not measure either its resistance nor its temperature in situ, this rough estimation method is a nice byproduct of the tracking of my resistor group, nothing more. I also use the 3458A as a ratio instrument / comparator only, neither as a voltage, nor as a resistance reference.Frank

author: alm
author_rank: Super Contributor
timestamp: April 01, 2022, 12:51:17 pm
content: Quote from: Dr. Frank on April 01, 2022, 12:14:17 pmThanks for explaining it! So in brief you run ACAL, and then measure a known resistance. And any change in this measurement is predominantly caused by a change (temperature or long term drift) in R207. And from this and the natural TEMP? variations due to change in ambient temperature you calculate the TC. I'm impressed that you manage this level of accuracy with so many factors involved!

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 01, 2022, 01:06:01 pm
content: Hello alm,yes, that's about the procedure. I assume, that anybody might extract this from his own monitoring data, if it's done on such a regular basis... what any volt-nuts should do, anyhow, when maintaining a bunch of Volt and Ohm references.But I would not speak about any degree of 'accuracy' (you shouldn't use this term anyhow), it's really a very course estimation of value and T.C. of R207 only!Frank

author: alm
author_rank: Super Contributor
timestamp: April 02, 2022, 05:43:49 pm
content: You're right that the word accuracy should not be used in metrology. When looking at the data I have from the past two years of a 1992-vintage 3458A measuring an SR104, I estimate a temperature coefficient which is quite a lot higher: -0.99 ppm/K. But looking at the graph, I'm not quite sure if this is purely the temperature coefficient of R207 or if there's more going on.This 3458A (my #2, #1 has a temperature-dependent ACAL OHMS issue that I need to investigate so it's relegated to measuring DCV) has been measuring the SR104 (not continuously) for slightly under two years, automatically has ACAL ran anytime TEMP2? changes by more than 1 K or every 24h, and all this data gets recorded. I have a Keithley 2000 measuring the thermistor inside the SR104. From the thermistor measurement I calculated the estimated value of the SR104 based on the data in the lid re temperature coefficient of both the thermistor and the actual resistance element. I took 10 points after every ACAL, took their mean, and calculated the PPM deviation from this calculated value of the SR104, and plotted it (with the standard error of the mean, but the error bars are smaller than the points in the plot). Then I calculated linear regression, but looking at the graph and the coefficient of determination is only 0.46, I'm not very convinced that this model represents the data well. It looks like there might be two populations and a whole bunch of outliers in the data. So these would need investigations if they are related to time, changes in the setup like cabling, warmup, or other factors. But this is my best guess for now.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2022, 07:01:56 pm
content: Right after ACAL the resistance measurement in the 10 K range should be mainly effected by R207. ACAL should essentially transfer the R207 defined scale to other parts that need to be stable from the ACAL time to the measurement. When waiting longer after ACAL there are more parts that can dirft: there is the x10 gain to measure in the 1 V range (with 100 µA test current) and the stability of the current source itself (divider for voltage and resistor in the current source). One would see such effects when just measuring the 10 K ref for a longer time.The noise during the resistance measurement should also be only one part of the noise. The ACAL step from R207 to the test current source also includes some noise. So repeated ACAL may give slightly different results. The 10 K range in this respect is still good and other ranges would have more of this ACAL effect.The data indeed look a bit strange, with quite some scattering at the same temperature reading.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 02, 2022, 07:17:20 pm
content: From the International Vocabulary of Metrology:(https://www.nist.gov/system/files/documents/pml/div688/grp40/International-Vocabulary-of-Metrology.pdf)2.13 (3.5)measurement accuracy (1)accuracy of measurement (1)accuracy (1)<classical approach> closeness of agreement between a measured quantity value and atrue quantity value of the measurandNOTES1 — The concept ‘measurement accuracy’ is not given a numerical value, but a measurement is saidto be more accurate when it offers a smaller measurement uncertainty. Measures ofmeasurement accuracy are found in ISO 5725.2 — The term “measurement accuracy” should not be used for ‘measurement trueness' and the term“measurement precision“ should not be used for ‘measurement accuracy’.JCGM/WG 2 Document N31848/1272.14measurement accuracy (2)accuracy of measurement (2)accuracy (2)<uncertainty approach> closeness of agreement between measured quantity values thatare being attributed to the measurand