Going to sell Racal-Dana 1992, should I calibrate first? - Page 1
Forum section: electronics / general

author: staze
author_rank: Frequent Contributor
timestamp: July 01, 2014, 05:11:58 am
content: Figure this is the best forum for this question, but it might be a good general purpose question for those reselling gear.So, purchased a non-working Racal-Dana 1992 1.3Ghz Frequency Counter off eBay a couple weeks back. Got it, and repair took 30 seconds (display multiplexer IC had come un-socketed). Going to turn around and sell it on eBay, and I'm curious if I should calibrate it first. I have a GPSDO that's good to 5x10^-11 or better, but the TCXO that this unit has is only good to about 3x10-7 or so (not great). Right now, it's reading my GPSDO 10Mhz signal out at 9.99999755Mhz. Which means it's off by about 2.5Hz. Last official calibration was in 2008 I believe, so I'd be breaking that sticker to calibrate off my own non-traceable standard.Anyone have any opinions on whether a buyer would(n't) prefer a calibration this way? Should I offer it as an option on the sale (free, it would just add a couple days to the turn around since I'd have to account for warmup). So the general question from this is:Does one leave a knownbad, but potentially traceable calibration when selling something, or does one calibrate to a knowngood, but untraceable standard?p.s. Main reason I'm selling it is I already have a 1992 with an OCXO. Don't really need a second counter.Thanks!

author: Paul Moir
author_rank: Frequent Contributor
timestamp: July 01, 2014, 05:53:14 am
content: The way I see it there are four classes of purchasers:1 - needs a traceable cal, which you cannot really provide. There are other things with a traceable cal like proper documentation and recall.2 - doesn't need a traceable cal, but would like it to read "correct"3 - doesn't want to pay for a cal4 - wants the old cal or wants to cal him or herselfI'm guessing like you that 2 is the big category. I'm also guessing that 1 & 4 aren't particularly price sensitive. And I'll suggest you really don't want to sell to 3. A way of hitting 1 2 and 4 is to offer the instrument with a "FREE!!!" non-traceable cal at the buyer's option. I think 4 will appreciate your checked reading as a sign of honesty. 1 will appreciate that you haven't "monkeyed" with it (no offence, but that's what I'd be worried about if I needed a traceable instrument.) You could hit three too if you offered the adjustment as separate charge, but I'm guessing that's impossible.

author: staze
author_rank: Frequent Contributor
timestamp: July 01, 2014, 05:58:59 am
content: Quote from: Paul Moir on July 01, 2014, 05:53:14 amPretty much what I figured, but awesome to have it spelled out that way. Think this is what I'll do. Thanks very much for the input. Anyone else wants to chime in, please! We could probably work up something that would be wiki or sticky worthy. =)

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 01, 2014, 08:10:38 am
content: Ok, first of all that would be an interesting question in the time-nuts forum, where you probably would get a competent answer: "Does a GPSDO, and under what prerequisite, provide a traceable calibration, and to what level?"But first, these arguments:1. Your 2008 cal sticker, or your "traceable calibration" is worthless, because it is long time expired.2. The GPSDO may in fact be a traceable standard, because the GPS timebase is traced backwards to NIST clocks, and even is part of the worldwide set of clocks, which defines the SI Second.Professional calibration labs will also use GPSDO for calibration, because Cs clocks would be too expensive for most of them. (Maybe they use Rb clocks as working standards, but either disciplined or calibrated regularily by GPS.)NIST and other metrology institutes also use GPS as a timebase for synchronization / metrology purposes.The question is only, how to make sure that your GPSDO delivers correct time and timing during calibration.3. Technically, your calibration uncertainty by the GPSDO is probably to the 1e-11 level .4. You have a legal limitation only. That means, you are not entitled to give a certification about your calibration, and you may not put a certified cal sticker on the counter.That's only due to the fact that your lab is not certified, although your equipment and your lab location may be in theory capable of being certified.A good uncertified calibration is in fact much better than an expired certified un-calibration.If the next owner really needs a certified calibration, he has to pay for that additionally in each case.Therefore, go ahead and calibrate the instrument, as the potential amateur buyers will appreciate this.FrankPS: ko4bb hosts an interesting NIST paper about these questions:http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/3)_Manuals_to_be_sorted/Traceability_and_Legal_Metrology_NIST_2002.pptPPS: Here's another, older NIST paper:http://tf.nist.gov/general/pdf/1305.pdf,and I cite this:"As a service to end users, manyNMIs distribute signals referenced totheir UTC time scale. Theseservices allow the end user to directlylink to a NMI andcomplete thetraceability chain."These signals may be LF signals, as DCF77, WWVB, or GPS.Therefore, GPSDO is a traceable time source, perhaps you may access the local correction tables which are also available for free, afaik

author: grumpydoc
author_rank: Super Contributor
timestamp: July 01, 2014, 09:35:42 am
content: QuoteIt would probably be worthless if done yesterday if not accompanied with paperwork from that cal and ideally history.I tend to readjust stuff I sell as usually there's no history and, in fact, typically no cal at all when I get it.

author: Macbeth
author_rank: Super Contributor
timestamp: July 01, 2014, 04:54:52 pm
content: As a hobbyist I would trust any other hobbyist who has superior gear and is willing to calibrate an instrument for free over any long expired cal cert.Also, as I understand it, sending something for calibration is only to confirm it is within specification and no adjustment is actually performed!I would much rather have my equipment corrected than have a PASS or FAIL certificate that costs a pretty penny.

author: staze
author_rank: Frequent Contributor
timestamp: July 01, 2014, 05:32:47 pm
content: These are great replies. I'm still leaning toward giving them the option, in case they for some reason think an expired cal is better than some unknown doing it at his house.One of the things I haven't thought about is the fact that it's a TCXO, and isn't all that stable (relatively) anyway. So any call is going to be, at best, a "best attempt".The fact a GPSDO is traceable I hadn't thought of. But I think we get into the notion of traceable vs certified relatively quickly since my GPSDO is home built (jrmiller design). So,Itrust the signal it produces, but...

author: Shock
author_rank: Super Contributor
timestamp: July 01, 2014, 06:19:39 pm
content: Quote from: staze on July 01, 2014, 05:11:58 amDon't remove cal stickers unless you are planning to get it properly calibrated to a traceable standard. It also shows that at that time the equipment was adjusted or checked to be within spec. Calibrations are all about confidence of measurement accuracy. Without it there is none, with it you can see when the equipment was last good, then it's up to you to tell the rest of the story.Quote from: stazeYou have a few options, you can advise where it currently reads or has drifted so they know it's inaccuracy. You can offer a home cal adjustment so it's in spec. The last would be to get it calibrated properly which is not a good idea as unless you get a cheap rate it will be worth more than the equipment.Quote from: stazeYou either say:It's uncalibratedIt's been home calibrated (and give details and results)It's been home calibrated comparatively via a traceable standard (and give details and results and show your standard)It is stickered and in cal to a tracable standard.

author: Shock
author_rank: Super Contributor
timestamp: July 01, 2014, 06:35:43 pm
content: Quote from: staze on July 01, 2014, 05:32:47 pmCertified and traceable is one in the same.It means they have documentation of their lab standard where and when it was calibrated (and that it was in cal at the time of testing) and what the measurements and inaccuracies were. It all must be documented and performed to the standard outlined by their lab and certification type. They must be able to provide their testing methodology.Calibrated is then adjusting to within manufacturers or a specified tolerance of measurement accuracy.Another point, is that part of having confidence is following the drift of the instrument over time. That is why it's important to see what the test results and calibration adjustments were, as you can tell when the instrument is unreliable. The sticker only shows part of the story.

author: staze
author_rank: Frequent Contributor
timestamp: July 01, 2014, 06:38:23 pm
content: Quote from: Shock on July 01, 2014, 06:35:43 pmThat's just it though, as stated above, a GPSDO is traceable since the GPS timeclocks are considered traceable by NIST. But my home lab isn't certified in any way.This seems like a time-nuts discussion... =P

author: Shock
author_rank: Super Contributor
timestamp: July 01, 2014, 07:08:29 pm
content: Quote from: staze on July 01, 2014, 06:38:23 pmIt's not traceable unless it's been tested and calibrated (if required) to a traceable standard in a lab.If you cannot prove the accuracy you cannot have confidence in the equipment, full stop.Remember this is a business and science requirement this has nothing to do with the home user and how confidant they feel.I'm not interested in a traceable/certified calibration for anything I own. I would buy the equipment find someone who has similar equipment either new or better with factory cal then leech a comparative calibration off them. If your lucky enough to know someone who gets proper calibrations done or can test your gear at work even better. This is about as good as it gets at home without spending money.Many years ago I used to work in a certified lab doing calibrations in case your wondering. It's like this on the forums you need to trace knowledge to the source hheheh. I'm not an amazing time buff or anything but if you brought a GPSDO to me and said it's accurate, unless you can prove how accurate it is with paperwork, it's not.

author: staze
author_rank: Frequent Contributor
timestamp: July 01, 2014, 07:28:40 pm
content: Quote from: Shock on July 01, 2014, 07:08:29 pmThat makes perfect sense, and was my original point in the whole thing. Anyway, this all pretty much says I'll just give the option for the auction. If they want me to cal, great. If they don't, even better (saves me the work). =)

author: Shock
author_rank: Super Contributor
timestamp: July 01, 2014, 07:39:25 pm
content: Quote from: staze on July 01, 2014, 07:28:40 pmI think that is the best scenario, I wouldn't charge for it unless it was adversely time consuming.If you say you will do it for free it's a good selling point (for those that don't have a reference at least).

author: grumpydoc
author_rank: Super Contributor
timestamp: July 01, 2014, 08:09:52 pm
content: QuoteAgreed but for hobby use (and tweaking stuff to go on ebay) if my GPSDO says it can see satellites and is in "fine" discipline mode I'm inclined to believe that it is within its specified 2 parts in 1010simply because it's difficult for a GPSDO to "work a bit". The fact that it agrees with my FE-5680A helps build confidence in both.Yes, it's worthless if you want a traceable calibration with paperwork but if you do you will, presumably, know why you need it and be prepared to pay for the calibration.I'll get my Keithley 2015 calibrated by a UKAS certified lab though because I just don't see the point of owning such a beast without having it calibrated and have nothing as, or more accurate than it is to give me confidence that it is not just giving me a very precise but wrong answer.Well, I have a K2000 and an HP 3478 and they all agree to within their expected error which is nice, but not proof.

author: nctnico
author_rank: Super Contributor
timestamp: July 01, 2014, 09:10:23 pm
content: I prefer equipment with it's original calibration. There may be more to calibrating an instrument properly than meets the eye.

author: staze
author_rank: Frequent Contributor
timestamp: July 01, 2014, 09:15:03 pm
content: Quote from: Shock on July 01, 2014, 07:39:25 pmYup. I'll just give the option. If they want it now, or have a standard, or want to pay for a traceable cal, then go for it. If they want what I got, I'll do it for free. =)

author: grumpydoc
author_rank: Super Contributor
timestamp: July 01, 2014, 09:27:22 pm
content: QuoteTrue on both counts.But equipment which reaches the 2nd hand market more often than not gets separated from the all-important calibration history.An expired calibration sticker on some equipment tells me very little - just that itwascalibrated at some point and it (presumably) met its spec at that point. There's no information about where the item is now.Also chances are that if I'm selling something then it needed repair and that pretty much invalidates the previous calibration history anyway and almost certainly means adjustment is necessary.If I ever do find anything with a documented calibration history that works you can be pretty certain I will leave it alone unless I have a very good reason not to.