What type of calibration to get when buying used equipment - Page 1
Forum section: electronics / general

author: oscilloscope_guy
author_rank: Newbie
timestamp: June 05, 2025, 06:21:23 pm
content: Hi new member here, I did a search on the forum for this topic but didn't find anything so I'm asking myself. I'm currently doing some research on bench multimeters to buy one from the US and ship it to my country. I'm going towards used ones because of price but I need the seller to do the calibration for me as I won't be able to do that locally. I'm confused because some sites have different terms for calibration, for example Valuetronics has four options:1)No calibration2)NIST Traceable3)NIST Traceable with full data4)ISO IEC 17025 accreditedDepending on the meter ISO calibration can be around 400 USD whereas NIST Traceable is only 100 or less, so should I spend that much money on calibration or is the most basic option enough?I will use the meter for designing and testing my own circuits as well as doing repairs so I'm not looking for some crazy expensive scientific instrument.There are some other used equipment sites that don't state anything about calibration on the product descriptions. I don't want to spend 300+ USD to get a meter that I can't trust because it hasn't been calibrated since it left the factory, could you recommend a reputable buying site for used and calibrated meters?Any input would be appreciated

author: tggzzz
author_rank: Super Contributor
timestamp: June 05, 2025, 07:36:30 pm
content: You don't give the key information...What is the penalty for a measurement being wrong?Alternatively, what is the penalty for not being able to prove a measurement is right?If no penalty, then calibration is irrelevant.

author: andrewtaylor
author_rank: Contributor
timestamp: June 06, 2025, 04:57:17 am
content: As tgzzz already stated correct: IT depends ON your final use.If it is for your Home Lab and intense Hobby: an AliExpress for 39.99USD will doch the Job to your full satisfaction.

author: Psi
author_rank: Super Contributor
timestamp: June 06, 2025, 05:23:32 am
content: Remember that calibration is more about tracking the history and accuracy trendline so you can statistically calculate the likelihood it will stay in-spec into the future, and also knowing when the trendline says it will have drifted out of spec.It's less about tweaking pots to get things back into spec.If something drifted out of spec do you really trust it any more, even if you do adjust it back into spec?The question to ask is,How much confidence do you need that the piece of test equipment will still be in spec when you send it out at it's next calibration interval?If it was found out of spec at that calibration would you have to recall all product made with that piece of out-of-spec test equipment?As said above, it really depends on what exactly you are measuring and why.

author: DH7DN
author_rank: Regular Contributor
timestamp: June 06, 2025, 11:11:41 am
content: QuoteReally depends on your application. I'd recommend a basic functionality check of all measuring functions before even considering a commercial calibration. Maybe you can borrow a trusty DMM from a friend or from the department and do a comparison measurement?If its for some kind of sophisticated hobby project: ISO 9001 compliant calibration ("NIST Traceable", often called just "ISO certificates") will do it. However, you can't be always sure whether it's a real traceable calibration. ISO 9001 compliant calibration certificates might not be accepted by your customers or authorities so be careful when buying a calibration service with a resulting ISO cal certificate.If it's intended to be used for a commercial or quality assurance relevant application, you will need a traceable calibration from an ISO 17025 accredited laboratory. The traceability to national standards is guaranteed and those cal certificates are accepted internationally. Also they are a bit more expensive than the ISO certificates.If you're using it in a cal lab environment, you have to keep records of your equipment and track/analyze the calibration results. You'll have to re-calibrate ISO 17025 calibration initially at shorter calibration intervals. That's where the most money flows...If you find your new DMM in spec, consider a ISO 17025 calibration. If it's out of spec, consider a calibration/adjustment/re-calibration procedure by the manufacturer (or any competent lab).So many possibilities to spend money...

author: oscilloscope_guy
author_rank: Newbie
timestamp: June 06, 2025, 10:45:47 pm
content: Thanks for the help, I'm planning to build a small home lab and this is my first time buying "serious" measurement equipment and dealing with calibration. I did some reading about ISO 17025 and a bit of digging on valuetronics.com and it appears to me that options 2 and 3 (NIST calibration) are basically their own in-house calibration done at their internal lab and option 4 (ISO 17025) means most likely they'll send the device to some external lab (the original manufacturer's maybe) that's ISO 17025 certified.

author: tggzzz
author_rank: Super Contributor
timestamp: June 06, 2025, 11:11:11 pm
content: Quote from: oscilloscope_guy on June 06, 2025, 10:45:47 pmDon't forget the difference between calibration and adjustment.Don't forget that many calibrations only define that the reading is somewhere within the instrument's specified tolerance band, i.e. a go/no-go statement.Do realise that frequentlystabilityis the most important attribute, i.e. if the reading next week has changed it is because the input has changed.It is relatively easy to get (i.e. adjust) two of your instruments to give the same reading. It is much more expensive and difficult to get your instruments to have the same reading as someone else's instrument.

author: Simmed
author_rank: Frequent Contributor
timestamp: June 06, 2025, 11:42:09 pm
content: Quote from: oscilloscope_guy on June 05, 2025, 06:21:23 pmfor hobby / self checking to mVyou can consider thesehttps://voltagestandard.com/(im not related to this product)Quote

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 07, 2025, 12:09:11 am
content: Even hobby use requires thoughts about purpose. If you are merely checking if power supplies are within limits of the components used very low accuracy is required, and used high spec equipment is usually adequate even if it is drastically out of calibration. Other hobby uses might need high precision, but no need for absolute accuracy. Still other hobbyists basically compete for how many digits of accuracy they can obtain, some approaching the accuracy of national labs.Only you can tell what you want to do now and in the future, and how much you are willing to spend to future proof yourself against future interests.

author: CalibrationGuy
author_rank: Unknown
timestamp: June 07, 2025, 04:59:04 am
content: I own a calibration outfit, so I'm definitely biased. However, there was a time when I assumed that if you bought a new meter, it's readings could be trusted. I was wrong. I say this because I bought a meter once that was new and very inaccurate. It caused me a lot of pain and frustration. Having said that, I've also purchased tons of used equipment, some 20+ years old well within spec, and some only 2 years old completely out of spec. My point is that without testing, you have no idea what you have. You need to start with readings that can be trusted. That can only be done with confidence when you are using a calibrated instrument. The calibration should be done by a reputable company. If the company is ISO17025 accredited, it is more likely to be a company you can trust. For basic electronics and hobby uses, a reputable company can calibrate your instrument. I would never get a calibration without data, ever. Anyway hope that helps. Good luck!TomG.

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2025, 11:38:36 am
content: Quote from: oscilloscope_guy on June 05, 2025, 06:21:23 pmThe difference between calibration options typically boils down to how much information you get. The most basic options just guarantee that the instrument is within specifications (typically the one-year specs for DMMs) and only provides as-found data when it is found out of tolerance (OOT). Options with data tell you what the instrument measured when it was received (as-found) and after adjustment if performed (as left) but they don't specify the uncertainties of the calibration lab equipment. The typical top-of-the-line service will provide you as-found and as-left data and will also list the expanded uncertainty of the calibration lab equipment used at every measurement point. The cheapest option only guarantees you that the instrument is within specs, the best may give you information and perhaps adjustment that can be 10X better than the listed specs.The difference between calibration labs boils down to their scope of capabilities and procedures, meaning how accurately they can measure whatever they are calibrating. A good ISO-17025 accredited lab may offer all of the options and the quality will be excellent no matter which option you select and only the information provided varies (e.g. OEM primary standards labs like Fluke or Keysight), a lab that only offers lower-level calibration options may be less capable and some of them can be of questionable quality when it comes to precision instruments.When determining what you need, don't just assume you need the very best as a top-of-the-line calibration can cost as much or even more than a pretty good used 6.5-digit DMM. Buying an inferior meter so that you an afford calibration may not be your best option. It appears that you don't necessarily need anything ultra-precise so a 6.5-digit meter with the lowest level calibration service from a reputable provider might be plenty good even though that lowest option only guarantees you that it is somewhere within the manufacturer's specifications.Note that buying a brand-new meter from a less-reputable manufacturer may not give you much of a guarantee on accuracy, especially over the long term.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 07, 2025, 01:06:01 pm
content: Quote from: CalibrationGuy on June 07, 2025, 04:59:04 amI totally agree that some testing is necessary. But for some hobby applications a fresh alkaline battery is sufficient.Regardless of the quality of the calibration thought is required to understand the utility and consistency of your measurements.

author: CalibrationGuy
author_rank: Unknown
timestamp: June 07, 2025, 06:09:02 pm
content: Quote from: bdunham7 on June 07, 2025, 11:38:36 amTHIS! If we calibrate an instrument for a client, mostly the ISO17025 certificates are needed for regulatory/quality compliance. If you compare our NIST certificate (we don't offer certificates without data as we don't charge extra to share that data) with our ISO17025 certificate of the same instrument, you will see the same values. The only difference will be a pass/fail qualification statement, some nice ISO17025 badges, and our uncertainties for each measurement.TomG.

author: CalibrationGuy
author_rank: Unknown
timestamp: June 07, 2025, 06:18:24 pm
content: I should qualify that last post just a bit. To some companies, especially other labs, aerospace mfg companies, etc., our uncertainty values matter a LOT! Some customers really want to know how good our capabilities are. Most customers just want to know that their equipment is working within specifications and need the certificates to back that up.TomG.

author: J-R
author_rank: Super Contributor
timestamp: June 08, 2025, 12:47:00 am
content: I think "home lab" just adds to the ambiguity we are experiencing in this thread. Are you repairing test equipment that needs to be calibrated afterwards? Or just doing general repairs and DIY tinkering?A typical hobbyist probably just does not need more than a 50,000 count DMM. If you watch the videos from our favorite content creators, you will notice that even a 6,000 count DMM would allow them to locate faults and work on projects just fine.For calibration work, a single calibrated device/DMM is not even remotely adequate. I recently dealt with a company that sent me a reference and based on what I saw on my equipment I just knew right away it was off. I even sent it back for a second calibration and got the same result. After almost a year later it was determined that their equipment had drifted quite a bit shortly after its most recent calibration. They put too much trust in a single, very expensive piece of gear. If I didn't have quite a bit of additional calibrated equipment, I would have ignorantly assumed my single DMM was out of calibration, for example.For sanity checks of your calibrated DMM(s), I would suggest getting thehttps://dmmcheckplus.comas well. Having all those passives is really a good idea and a small device is cheaper to ship.I also 100% agree with the sentiment that you need the calibration data in all cases. The primary reason for this is that a lot of equipment can have somewhat lax specifications, so even if it passes calibration, the last couple digits on the display could be garbage. So you need the data to be able to determine what is going on. Tracking drift between calibrations is also a good idea if you're really going down the metrology path.Which DMM to get? For the metrology path on a budget, you're probably looking at older, clunkier DMMs with tiny displays but well-aged and stable. For general bench work such as tinkering and repairs, you'll probably want a nice, big display and all the modern features.

author: bdunham7
author_rank: Super Contributor
timestamp: June 08, 2025, 11:17:20 am
content: Quote from: oscilloscope_guy on June 06, 2025, 10:45:47 pmI should have asked--which meter(s) are you looking at and what do you anticipate trying to accomplish with your home lab? And what is your overall budget for this? If you're looking for 100ppm basic DC accuracy, that's easily achievable even without any official calibrations--I can do it, for example, and an appropriate meter like the HP 3478A can be had for less than $200. If you want a guaranteed 10ppm, then that's nearly impossible without spending a lot more money.

author: J-R
author_rank: Super Contributor
timestamp: June 10, 2025, 02:48:30 am
content: Actually, the HP 3478A is currently trending closer to $100 right now. I suppose an aged LM399 reference is probably all it has going for it though. It has a quite lengthy list of negatives otherwise.The HP 3457A has a similar set of negatives (battery-backed calibration, poor LCD display, limited ranges and very limited functions).I considered both of these at length back in the day but decided to just spend a bit more money for a calibrated 34401A.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2025, 03:21:28 am
content: Quote from: oscilloscope_guy on June 05, 2025, 06:21:23 pmIf you have to ask the question, then the cheap NIST option is more than enough.In most cases, the old big name bench meters do not generally drift, they either work and are well within spec, or they are broken. So it's no much risk to buy one without calibration.You can always buy a cheap voltage calibrator on ebay to spot check it.

author: CalibrationGuy
author_rank: Unknown
timestamp: June 10, 2025, 04:32:10 am
content: Quote from: EEVblog on June 10, 2025, 03:21:28 amNIST is fine, but never without data!TomG.

author: J-R
author_rank: Super Contributor
timestamp: June 10, 2025, 05:42:20 am
content: An example of why the calibration data can be good to have and also a 1 year accuracy comparison of old vs a modern, but budget option:3V DC range on the HP 3478A: 0.006% of reading + 2 counts. So 3V could read between 2.99980V and 3.00020V.2V DC range on the just-released Siglent SDM4055A: 0.015% of reading + 0.003% of range. So 2V could read between 1.99964V and 2.00036V.1V on both:HP3478A: 0.99992V - 1.00008VSDM4055A: 0.99979V - 1.00021VSo without the calibration data you can be left guessing a bit.

author: bdunham7
author_rank: Super Contributor
timestamp: June 10, 2025, 06:56:22 am
content: Quote from: EEVblog on June 10, 2025, 03:21:28 amI would caution that while this is true for some models, it isn't true for all of them. Completely closed case calibration models are typically OK if they haven't lost the cal data. However when you get to 6.5-digit levels, while the DCV may be within spec auxiliary ranges can be out and those are harder to detect by typical users. My mint-condition never-adjusted 34401A was slightly out of tolerance on AC volts and even my Fluke 8846A was out of tolerance on the lowest capacitance range (but not in a way that would ever give a wrong reading--long story).Older models with trimpot calibration, well YMMV. Once you get them sorted they may be good until they break again.

author: bdunham7
author_rank: Super Contributor
timestamp: June 10, 2025, 07:04:36 am
content: Quote from: CalibrationGuy on June 10, 2025, 04:32:10 amI would argue that data without specified uncertainties is, well, sort ofuncertain!Suppose I send a DMM with a specified tolerance of 18ppm (on the 10VDC range) to a budget/volume lab that has a scope that lists their ability to measure 10VDC as 8ppm expanded k=2 uncertainty (because they use a 5500-series calibrator and a normal 3458A on a two-year calibration cycle) and they give me "data" that shows my 10VDC measurement to be 9ppm high, is that helpful?

author: tggzzz
author_rank: Super Contributor
timestamp: June 10, 2025, 08:55:30 am
content: Quote from: CalibrationGuy on June 07, 2025, 06:18:24 pmThat sounds sane behaviour from all parties.I come back to the question I don't think the OP has answered: what is thepenaltyif the OP's equipment gives subtly faulty readings?If no penalty, then there is arguably no benefit to calibrating.If $X, then you can justify spending $X.If loss of personal or corporate reputation, then how much would it cost (financially or otherwise) to regain the reputation? I've found asking for a statement of the uncertainty is a good way of finding out what a calibrationlabshop is aiming to do and not do.

author: CalibrationGuy
author_rank: Unknown
timestamp: June 11, 2025, 02:46:16 am
content: As some of you here know, I have a real obsession about lowering our 10Vdc sourcing and measuring uncertainties. In the time since I characterized the USA Cal Lab's FX, I've added 2 more Fluke 732s. Which reminds me that I still have TiN's reference hooked up to our dc voltage reference array.So yes, uncertainties matter. A lot.We have drifted a little off topic, however, so in closing I'll say this: It really depends on your use of the instrument, your goals, your budget, your tolerance for the unknown, and most importantly, how good your friends' equipment is. If you need help, there's a lot of people here who have been very generous with their time and expertise.TomG.

author: martinr33
author_rank: Frequent Contributor
timestamp: June 17, 2025, 04:39:08 am
content: If you are buying a used instrument, a cal cert will (may) wring out the ranges for you. Some cal labs are definitely a bit suspect, so data is important.A good example of a tricky range is the Keysight 3458A 2W Ohms zero. An iffy switch (common) will show up on that particular test. 2W resistance is the most sensitive range for bad contact at the terminals, the switch, or any of the interboard connectors.

author: oscilloscope_guy
author_rank: Newbie
timestamp: June 17, 2025, 08:03:02 am
content: Thank you all a lot for your responses, I was not expecting so many replies!I learned many new things to consider when buying.I asked Valuetronics' support people about what their calibration options mean and they said this: "NIST is simple cal cert, NIST with data includes data, ISO 17025 is accredited cal with before and after". So yeah that agrees with what you guys said.As for me, I'm fine with a 5.5 or even 4.5 digit DMM, I'm looking to get a similar setup to what was available at my university's lab where they have Agilent 34401 DMMs, although those are quite a bit over my budget.One very important use case for me is to check 12 bit ADC/DACs output accuracy as well as their reference voltage.Quote from: tggzzz on June 10, 2025, 08:55:30 amIn my case I like designing analog circuits so I need a measurement instrument that I can trust to give me the measurements I need to make my circuits work properly and verify their performance, basically my penalty is lots of headache and time spent on figuring out why my amplifier's DC OP is so off and not getting the estimated gain and things like that, so I think a good old quality 5.5 or 4.5 digit DMM will do fine for now, if I find myself needing higher accuracy or features in the future I'll go for a more expensive 6.5 digit one.

author: tggzzz
author_rank: Super Contributor
timestamp: June 17, 2025, 08:40:18 am
content: Quote from: oscilloscope_guy on June 17, 2025, 08:03:02 amIf you wanted to push linearity measurements to the ppm level, consider using a KVD (or Harmon divider) with any old voltmeter. That technique has been used since the mid 19th century. We were taught a variant during school physics lessons, to make 0.1% measurements with a 2% meter.FFIhttps://en.wikipedia.org/wiki/Kelvin-Varley_dividerand specifically its reference to Jim William's AN86 about 20bit DACsQuoteOK; that's a useful definition and a sane conclusion