HP 34401A acccuracy test, advice? - Page 1
Forum section: electronics / testgear

author: rosaage
author_rank: Newbie
timestamp: March 14, 2020, 10:24:17 pm
content: I recently bought a used HP 34401A from ebay, marked as "For parts or not working" but the seller wrote he did a self test at the request of a bidder and the unit shows "PASS". It's missing the handle, but this can be found from china for 18usd shipped.Looking at the website of the local calibration shop they want more than I payed for the instrument to calibrate it, so instead of doing this I tried testing it myself against a probably in spec instrument at work (Fluke 1586A).This instrument was bought new in 2018 or 2019, and has a sticker from fluke for the factory calibration marked 21-mar-2017.All voltage tests were done using a Fluke 724 as the source (I have the calibration data for this, but it's a bit old and the specified accuracy is lower than both the 34401 and 1586A)Resistance tests were done using resistors from Elfa (marked in table) using a set of kelvin clips bought from china:https://www.aliexpress.com/item/32954642011.htmlboth instruments were set to 4-wire mode.Current tests were done using a Fluke 707 as the source with a single loop through both instruments at the same time.The fields "HP range %" and "HP reading %" shows the accuracy of the HP in % (from the manual/datasheet) and the "+/- Accuracy" field is calulated "=([reading%]/100)*[HP reading]+([range%]/100)*[HP range]". The "Outside spec" field compares the HP-Fluke difference to the HP accuracy. A positive value here means the HP shows higher than the fluke and negative means lower. Green "in spec" is TRUE, and Red is FALSE. Is this the correct way to calulate accuracy?So I see it is "outside spec" in most voltage tests, but is this super bad values? I know it is way overkill for me, but how is this compared to other older HP multimeters? I have no idea about when it is made, but it is HP branded on the front so I assume not the newest. The only sticker on the back reads "3146A07374" is this the serial number? can it be used to find a manufacture date?Is there anything else I should know about this instrument? I have not opened it.

author: ceut
author_rank: Regular Contributor
timestamp: March 15, 2020, 12:22:13 am
content: Hello,I have a similary problem as yours: I have since some time an old, but in good condition and good working HP34401A (from HP too).But it has little drift too(when compared to a brand new Keysight U1272a with an independant calibration certificate not from Keysight).I only use the VDC on it.I have downloaded the Service Manual, and bought the "4 terminal short" .But as I haven't any more precise meter, I can't go further, so I'm stuckAlso I have tried to find local store to recalibrate it, but it is really really too much expensive to calibrate it (between 400 and 700€), so for instance I don't use it. I'm disappointed as it is a really good meter.If anyone has an idea for us ? Thanks

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: March 15, 2020, 02:13:45 am
content: Accuracy aside, problem is all your test gear are built for other uses are not meant for verifying a 6.5 digit DMM. The Fluke 1586A has too narrow a range to be used as a full range transfer standard for the 34401A as it can only go up to 50V and 100mA. Your Fluke 707 can only source up to 20mA with 0.015%, and the 724 is only good for 10V at 0.02%. They are all below the accuracy level of the 34401A by a large margin. The gain verification on the 34401A has verification points at 100V and 1000V, as well as 100mA, 1A and 2A, which you don't have a way to test. So any adjustment is definitely out-of-the question.For the ranges you had checked, they are out of spec but not terribly off. So it all depends on your use case on whether it should be adjusted.I own two 34401As, but they are both late Agilent builds before they changed to Keysight, so I cannot comment on the drift characteristics of the older HP meters. For my meters, they do drift a little early on, but then they become very stable. In your case, an older meter without any cal history (or who cal it), my guess is it would drift very little now, but could be out-of-spec as your test shows. If you use case dictate accurate 6.5 digit reading, it is a meter that is well worth a professional calibration as it could give you many more years of reliable service (assuming it is in good condition and properly maintained), and delivering an accuracy level none of your current equipment provides when they are used as a DMM.The Cal Count in the meter should also give you an idea of how many time it has been calibrated in the past (thought not when unless they set a message).For the early HP meters, if it has not been modified, all the inputs are red, the later ones are red and black before they move to Agilent. Also the early units were built in the US before they moved production to Malaysia (and even then, the components were made in the US but assembled in Malaysia early on, before they moved component production over there). If it is a valid serial number, my guess is the 31 prefix would make it one of the early production with '91 design (the meter was announced in '92), there is also a possibility it was built the 46 week of '91 in the US, depending on how serial number is applied to this unit. If it is the case, the inputs would be all red. If you open the meter, you may also find more date information on the PCB or chip as well. In any event, it is very likely it is 27+ years old.If you look at the service manual, you will realize that it is very difficult to do any cal on the 34401A unless you have the requisite calibrator. There are just too many verification points that requires high level of accuracy. It is not something that you can do reliably without the proper equipment. For the data you provided, I would either leave it alone, or have it done professionally, but not to try any adjustment with the equipment you have.I often take the abundance and the low calibration cost in the US for granted, as there are shops here that would do a NIST traceable cal for less than $100. I send my meters back to Keysight for cal yearly, it is $240. You may want to check with your local Keysight office to see what they quote you, but I doubt they will charge you 400-700 euro.I still use my 34401As regularly, they are accurate and reliable, and great 6.5 digit meters.

author: VK5RC
author_rank: Unknown
timestamp: March 15, 2020, 05:50:12 am
content: A general comment I would make about calibration is that it is a process that needs to be repeated over time, all instruments are 'incorrect' to some degree and will drift further with timeWhat you need to know is how far it was 'off' before, how fast it has been changing over the last few months /years, and what is the likely trend and if needed, to be able to interpolate the errors onto your measurements. Also because of an inherent 'randomness' of measurement you also need to get into error bars/confidence intervals.Down into the Volt-nut rabbit hole!PS sounds like a great buy!Rob

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: March 15, 2020, 07:25:43 am
content: Great points, plus you also need to take ambient temperature into consideration if your lab is not at a constant temperature. The meter also needs to be on for 1+ hours before any measurement is done so it is properly warmed up.Quote from: VK5RC on March 15, 2020, 05:50:12 am

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2020, 11:13:49 am
content: The Fluke 1586 is not much higher accuracy than the 34401 - more like comparable accuracy. So the difference can be due to errors of both instrument. With that in mind the 34401 may still be in specs - the test is just not very stringent.Depending on the use for the 34401 one could see this as a crude accuracy test passed.There may be some additional offset from the cabling. Just the normal banana cables may add some offsets, that can effect the low voltages.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 15, 2020, 04:25:31 pm
content: It will depend of your needs and the types of measurements you will make.I recapped 2 of my 34401a (electrolytics and tantalums) before 1 was calibrated and the cal shop said i didn't drift too much in many years, 5 years have passed, i was able to get the previous cal report ...So in the end i'm happy, even if i shelled 150$ CAD for a calibration.

author: rosaage
author_rank: Newbie
timestamp: March 17, 2020, 09:33:08 pm
content: Quote from: NoisyBoy on March 15, 2020, 02:13:45 amThank you for good information,I was never planning on adjusting anything, it was more trying to use an instrument with similar specs to compare. If the meter seems "good enough" that is fine for my use. I had no idea how far instruments like this might drift, and this was more a test to confirm if it was way out. I almost never go above 50v right now so currently at least this is no problem. If this is almost in spec that is more than enough for me.I did have the meter connected for around an hour before I started taking measurements, not too sure about the temp but it is 20-something.The current cal count is set to 49 without a message. (is this a lot?), it does say "made in the usa" on the back and all inputs are red.I might check with the keysight reseller here in norway what they charge for a calibration, but the local calibaration lab wanted 285usd (including tax) for calibration, but as I only payed 215GBP for the meter (310GBP total with shipping and vat) I will not do this now at least, but maybe in the future if my needs change

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: March 17, 2020, 11:43:31 pm
content: That's a very low cal count, it means the meter has been recalibrated no more than once since it was put in use, and it was not even a full calibration, probably done by the user themselves on some ranges only (a full cal is an increase of 35 in cal count, every range you touch is a separate count). How little it has drifted in over a quarter century is a testament to the legendary reliability and stability this meter is famous for.If you don't go over 50V, and if you don't ultra accuracy, it is probably good enough for most use. The only reason I brought up calibration is because the meter is capable of a much higher accuracy.BTW, for calibration, contact the Keysight office and not a reseller. Reseller may quote you the price they charge rather than a Keysight price. It is also a good idea to shop around when you are ready to do a cal.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 19, 2020, 05:22:17 pm
content: Those kelvin clips are not wired correctly by the looks of them. The two jaws of the clip should be insulated from each other and the two wires shouldn't be wired to the same jaw of the clip, there should be one connected to each jaw.