Adjusting a 3456a to a 34401a - Page 1
Forum section: electronics / general

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 08, 2013, 08:35:47 am
content: Hello there,I would like to calibrate my 3456a. However the price of calibration of this instrument is about the same as a calibrated and adjusted second hand 34401a on eBay.They are both 6 1/2 digits. Is it a good idea to attempt to buy a fresh cal 34401a and adjust the 3456a to match, or am I just going to make my situation worse?Thanks.M

author: saturation
author_rank: Super Contributor
timestamp: January 08, 2013, 03:27:19 pm
content: If you can buy a calibrated 34401a for the same price as just the calibration cost of the 3456a, its worth it if you willing to endure time required to adjust it because those 2 machines are about par spec wise, the TUR ratio is ~1:1, as a meter the -01a has an edge in that its newer, more compact and more functions. However, you can keep tweaking the -56a until it reads identical to the -01a, since you have both units at your lab.You will also need stable references to make the comparisons, i.e. volts and ohms. AC has been difficult for me. Its not easy to find stable VAC sources up to 1kV with the multiple frequencies required, that you can use in lieu of an AC calibrator.

author: robrenz
author_rank: Super Contributor
timestamp: January 08, 2013, 03:38:03 pm
content: If possible make sure the calibration is fresh and with actual values for all ranges and functions. Preferably with pre and post cal values so stability might be implied. If you can get a fresh cal you could use 90 day specs and do your 3456 cal within the 90 days.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 08, 2013, 03:57:11 pm
content: Thanks guys!M

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 08, 2013, 04:03:11 pm
content: And another question, this probably sound stupid as hell: By calibration, is it given that the cal lab adjust the instrument to to correct value, or do I have the specify this in addition to calibration?Thanks.M

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2013, 04:18:57 pm
content: Depends what you ask for. If you just require verification then they do nothing ( important if you are tracking long term drift with the meter, where adjusting it will destroy your drift data) and if you request adjustment then they will do it if needed. Most often with good units the only thing done is the verification so long as it reads within the error budget.

author: robrenz
author_rank: Super Contributor
timestamp: January 08, 2013, 04:26:58 pm
content: I am no expert but this is what I am aware of.No data other than the unit meets all the specificationsData of actual values as recieved (no adjustments performed)Data of actual values after calibration (adjustments are performed if out of spec) IMO worthless ncompared to below.Data of actual values as recieved plus values after calibration (adjustments are performed if out of spec) Best because you can build a history of stability. Without the as recieved data stability will be an unknown.Even though the cal with pre and post data is the most expensive. For hobby/personal If you have a stable instrument it will allow the cal interval to be extended so may be cheaper in the long run.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 08, 2013, 05:55:14 pm
content: Thanks! I've requested pre and post cal with adjustment as necessary. Hope I get a good dealKind regards from Norway

author: JuiceKing
author_rank: Regular Contributor
timestamp: January 08, 2013, 07:02:32 pm
content: Does that meter have to be manually tested and adjusted? If, like the 3457a, a calibration check and adjustment are automated procedures, then I say shop around for a better price. I got my 3457a calibrated with data for $87, but hp and other national cal labs would have charged much more. They just hook it up to a Fluke calibrator and push a few buttons...not a big deal.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 08, 2013, 07:19:42 pm
content: Best price I found for cal in my country was about US700. Most places charged over $1,000. Perhaps Sweden is better value, their price of labour isnt as high these days.M

author: robrenz
author_rank: Super Contributor
timestamp: January 08, 2013, 07:42:09 pm
content: Quote from: king.oslo on January 08, 2013, 07:19:42 pmThat is outrageous!!! %-B Double that price in the US and you can get abrand newFluke 8846A with its full calibration report included.

author: Unknown
author_rank: Unknown
timestamp: January 08, 2013, 09:10:57 pm
content: The 3456a has to be adjusted by tweaking trim pots by hand. This is much more labor intensive than closed case calibration that all decent newer meters support. Cal labs hate this because it requires them to do actual work beyond just hooking it up.The issue is going to be finding stable sources, as saturation already mentioned. The low TUR (I think the 3456a is actually better on some ranges, especially after many years of aging) will mean that you won't be able to verify that it's within factory specs. I would avoid adjustment unless the 3456a is clearly out of spec, no reason to assume the 34401a is spot on.The upside of getting a calibrated 34401a is of course that you get another excellent meter. Make sure the calibration is traceable and to factory specs, anyone can connect meter to a voltage source and tweak the meter to read 10.00000 V.

author: quarks
author_rank: Frequent Contributor
timestamp: January 08, 2013, 09:26:28 pm
content: Almost 1000$ is way to high for 6.5 digit DMM calibration.For this much money I would rather buy a new Fluke 8846A (or a identical but cheaper Tek DMM 4050) or spend the money for your own calibration gear.Another way I think of, is looking for a eevblog forum guy close to your location who has the gear you do not have yourself for calibration.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 08, 2013, 10:02:20 pm
content: What reasonable dc ac calibrator would I buy which would be good enough reference for my 3456a?Thanks.M

author: quarks
author_rank: Frequent Contributor
timestamp: January 08, 2013, 10:20:48 pm
content: I am sure there are quite a few Fluke "calibrators" on ebay.I myself like Valhalla very much. Also l think the (German) brands Knick and Burster have really good DC sources.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 09, 2013, 03:13:27 am
content: 34401a datasheet says its 24h accuracy spec is .0015% + .0005 of the range. Is it correct that on the 10V range, the cal lab would do nothing if the instrument was showing 10.00300V?Wouldn't it be better to have the pre cal values, and adjusted all these "in spec"-deviations to 10.00000V? And even if the instrument was showing 10.00006V, wouldn't it be better if they adjusted the instrument to 10.00000V? Is this what I should be asking the cal lab to do?Thank you for your time.M

author: quarks
author_rank: Frequent Contributor
timestamp: January 09, 2013, 09:51:24 am
content: Quote from: king.oslo on January 09, 2013, 03:13:27 amIn all the calibration documents I have, it looks like only the documentation of the values and uncertainties are reported and no adjustments were done. So I guess if it is in spec they will not adjust it.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 09, 2013, 10:57:35 am
content: I have a question. Lets take the 34401a. If it is showing 10.00024 at 10V input, it is in spec on its 90 days accuracy. But it seems to me that the last three digits are uncertain, leading me to think: Why do you even need uncertain digits? I can only rely on the first three and a half digits. Why do they call this a 6 1/2 meter when it is very uncertain beyond 10.000V? Perhaps one could stretch to calling it a 5 digit meter as the least significant digit in 10.0002 can only be as poor as 2.Am I missing some information, or have I understood correctly? To me, it seems pretty useless to talk about its digits which do not measure anything but error. (?)Thank you for your time.Kind regards,Marius

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2013, 01:05:12 pm
content: You have just described the difference between traceable accuracy and resolution. High resolution combined with high repeatability is very useful for many things like measuring drift precisely or matching resistors. Traceable accuracy is not that important in these cases.On adjustments at cal to tweak to be more exact:Long term stability trumps everything so you don't want to do anything that might compromise that.If the cal involves making a mechanical adjustment like a pot you don't want to touch it if it is within the spec. Trim pots are a weak point in long term stability to begin with. Moving one will corrupt the long term stability record.If the cal involves only a computational value adjustment and all the electronics are completely undisturbed including digital pots etc. then the long term stability would be unchanged.Having a cal for a +/-24ppm meter that shows +/-5ppm maximum errors does not make it that accurate. The linearity error of the ADC will allow it to be off in mid range compared to the min and max of the range that was calibrated. The stated accuracy of a meter is a mix of many possible errors including the standards used combined with statistics to allow a % confidence level that the measurements will be within the stated traceable accuracy.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 09, 2013, 01:14:15 pm
content: Quote from: robrenz on January 09, 2013, 01:05:12 pmThanks, that was a very useful post!M

author: saturation
author_rank: Super Contributor
timestamp: January 09, 2013, 10:38:53 pm
content: Quote from: king.oslo on January 08, 2013, 10:02:20 pmAfter you buy a calibrator, you still have get it calibrated, so you can be sure of your references. This often adds more cost to the initial purchase, plus having it periodically calibrated. If you buy it from eBay, you have to insure its really stable and precise, so all it needs is tweaks for calibration.Calibrators of reasonable price [$500-1000]each, are close in ppm as the 3456a itself, but it depends on the model. However, it also provides the reference output voltage, current and resistance you need to compare your devices too. These are all manual types, c 1970s or earlier. You'll need at least each for AC, DC, and ohms. Sometimes a later model comes available, usually for $1500+ that is an all in one.http://www.ebay.com/itm/Fluke-5100B-Calibrator-with-03-option-Tested-and-Works-great-/190773912874?pt=LH_DefaultDomain_0&hash=item2c6b02e52aIs it worth it? It depends on how much it costs to calibrate in your country. In the USA, it costs about $100 to calibrate a 3456a, so in lieu of buying a calibrator, I can have the calibration done 15x rather than owning all 3 units, not to mention storing it [ they are often large, being old designs] or repair/recondition it after purchase.For example, for AC only:http://www.ebay.com/itm/Fluke-5200A-Programmable-AC-Calibrator-/110872704284?pt=LH_DefaultDomain_0&hash=item19d087351cWhat I ended up doing is buying a unit already calibrated, and comparing its ranges against my units and other devices.In this video, I used my own calibration procedures on 3456a on the top and bottom against a current Geller SVR board and some very tedious resistance dividers, over 1 year before this video was made. I later purchased a Krohn Hite MV106 that I calibrated against the Geller. The video demonstrates 1.000000 VDC range outputted by MV106. The middle meter was calibrated by a cal lab.http://youtu.be/T1SK2I4Wl2oAt this point I don't know who is spot on accurate, which meter is the most accurate one, or even the reference being measured, the MV106? I'll have to presume its the middle one, as its professionally calibrated. Otherwise all 3 are still in spec but it shows adjustments can be made to track very tightly against a reference using DIY procedures assuming your DVMs are in "factory spec" working order.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 09, 2013, 11:06:31 pm
content: Thanks Saturation. It was fun hearing you and seeing your metersM

author: saturation
author_rank: Super Contributor
timestamp: January 10, 2013, 01:21:36 pm
content: My pleasure. I may have a video of the same test using 10mV many months later, showing the 100nV resolution. I'll post it when I find it.In summary, you can calibrate ranges you can get a stable transfer reference from to use against a calibrated meter even if the TUR is 1:1, but it takes a lot of perseverance, the trick is to make them track identically, to reduce the risk of variations suggested by the TUR concept. It took several days of adjusting for a full week to get the meters sync'd. Thus, if I had done the same against the available professionally calibrated meter, chances are then all 3 would be sync'd.I do not have a source for AC calibration, so I have faith in the DC ranges and ohms, but the AC ranges have not been adjusted.Quote from: king.oslo on January 09, 2013, 11:06:31 pm

author: robrenz
author_rank: Super Contributor
timestamp: January 10, 2013, 01:50:13 pm
content: Thanks for the video saturation. I can now put a voice with the name. BTW I usualy work in the dark also

author: quarks
author_rank: Frequent Contributor
timestamp: January 10, 2013, 05:41:33 pm
content: Hello king.oslo,I just saw your older post "100VDC reference circuit" so you hopefully already have at least one usefull source.Therefore I feel I have to clarify my posts to your subject.I fully agree with saturation statements. For what you want to do "Adjusting a 3456a to a 34401a" you of course do not need a calibrator. Also I would not adjust a 6.5 digit meter unless it needed repair work or is by any other reason out of spec.I only suggested it, because I would not spend 1000$ for a one time calibration and instead spend the money on gear, I can use hopefully a very long time. I also thought you might not have good enough stable sources in the ranges and functions you want to do the transfers at. So a calibrator (even if it is not in its own calibration specs anymore) should still be a very good clean and stable source for the job and for me it is a better investment than the 1000$ calibration. For my own lab I did exactly what I suggested to you and I am happy with my decision to spend the money in calibration gear. I regularly check my meters against the calibration data I already have and try to build up history database. So far I have had no need to do any adjustment.byequarks

author: saturation
author_rank: Super Contributor
timestamp: January 10, 2013, 06:55:07 pm
content: You're welcome. Yes, a voice humanizes the bland name, isn't it? I like the dark as it covers a lot of my lab's dirtbut it also focuses my attention to whatever is lit. The videos are a bit darker than life due to my cellphone camera.I just uploaded this video I just took to show the nV resolution of the 3456a:Quote from: robrenz on January 10, 2013, 01:50:13 pm

author: robrenz
author_rank: Super Contributor
timestamp: January 10, 2013, 09:42:07 pm
content: What portion of that SD is the voltage source and what portion is the 3456?Just for fun I fired up the 8846A and a PD2010 to see what it looks like after 11400 samples. (in the dark of course)We will have to wait about 5 hours for the picture at 100NPLC10nV resolution but SD is 10 times worse with the PD supply as a standard

author: saturation
author_rank: Super Contributor
timestamp: January 11, 2013, 12:24:18 pm
content: Good question, difficult to say, the meter is at/near/over it spec sheet limits, and the voltage source, MV106, is over its spec sheet; however the MV106 has a claimed resolution of 10nV which I've yet to see given the limits of my equipment. However, good designers should report a conservative value in spec sheets for gear. Would be interested in your report. I've also tried different PLC settings on the 3456a, and it does improve on SD values a little bit, suggesting some variation is still from the 3456a.Nevertheless, the video hopefully shows a 3456a still provides excellent capability as a DC DVM despite its being 30+ years old and that acceptable DIY calibration can be done with some perseverance. Depending on what is costs in your country, 3456a is often a low cost DVM that provides stable 1uV resolution easily and can be accurately calibrated by owners, also for low cost. Not to mention general ease of repair, should it happen, making the TCO, fairly affordable at least to USA owners [ because I'm most familiarly with the going prices].Quote from: robrenz on January 10, 2013, 09:42:07 pm

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 11, 2013, 07:17:51 pm
content: How could you calibrate against the geller board? How did you get the 100V?M

author: quarks
author_rank: Frequent Contributor
timestamp: January 11, 2013, 08:00:37 pm
content: Quote from: robrenz on January 10, 2013, 09:42:07 pmThat's interesting. I just try to do the same with my (new/old) Knick JS300. It is quite difficult to hit 10,000mV with the single turn pot. Now I think of pimping it with a 10-turn pot.Attached is the actual data. Is this worth letting it run longer?edit: it does not seem to change much (see second + third picture), I stopped it after that

author: robrenz
author_rank: Super Contributor
timestamp: January 11, 2013, 08:24:01 pm
content: Have you done a long term drift check of your meter with shorted inputs to characterize its behavior? Without that you dont really know what is meter and what is calibrator.

author: quarks
author_rank: Frequent Contributor
timestamp: January 11, 2013, 08:37:08 pm
content: I stopped the other test. Now I have shorted the inputs and started it again. For how long would you let it run?

author: robrenz
author_rank: Super Contributor
timestamp: January 11, 2013, 08:49:54 pm
content: I have done it for a week and I take pictures at 1, 2, 4, 8, hrs and each 24 hr period after that. I use 6.5 digit 100NPLC. Make sure your lab temp is reasonably stable.

author: quarks
author_rank: Frequent Contributor
timestamp: January 11, 2013, 09:03:49 pm
content: Uh that is a long time. I don't think I can do it that long soon, because I need the meter every day. What was your result after all?I will let it run and have a look tommorrow morning. Right now this test already shows SD around 50n.

author: robrenz
author_rank: Super Contributor
timestamp: January 12, 2013, 01:55:32 am
content: Min. Max. span of 1.08µV after 152 hours. You only need to test as long as care about. If you want to check a power supply stability for a week you need to know the meter stability for a week. If you only care about an hour check it for an hour.

author: quarks
author_rank: Frequent Contributor
timestamp: January 12, 2013, 08:59:22 am
content: I now had it running all night. That is long enough for now. The temperature dropped at night about 1.5 °C in my lab room. Min - Max difference is around 0.89µV and SD about 78.9n.

author: saturation
author_rank: Super Contributor
timestamp: January 12, 2013, 05:24:25 pm
content: The Geller SVR provides 10.000 00 VDC, but is divided by a precision resistor divider to check or adjust the lower ranges of the 3456a. You need at least a calibrated ohmeter to calculate the accurate ratios of the divider.Once the lower ranges are calibrated, I used the 3456a ratio mode to take a stable 100.000 0V source [ or are best as you can obtain close to 100V, but best stable to 100uV] and ratio it against the Geller board. The ratio output is then scaled to <= 10V, which uses the just calibrated range of the 3456a. Using this value x 10, you can now adjust or compare the 100V range of the 3456a.I initially used a linear PSU, but found stacking 9V batteries x 10 or 11 provides a stable ~100V output down to 100uV.https://www.eevblog.com/forum/projects/poor-man-calibration-facility/msg97350/#msg97350Quote from: king.oslo on January 11, 2013, 07:17:51 pm