-------- In message <[email protected]>, Florian Teply writes:
>Let me see if I understandd that correctly: >Assuming no adjustments have been made to the instrument in between, >with calibration history I could work out the actual drift rate of the >instrument. Of course, the more datapoints I have, the more >accurate that estimation might be. Then I could use that to project >into the future to see when it will likely drift out of spec. Provided it has a uniform low-ish drift rate. That is probably something you will only see on the high-end kit. Low range kit will probably be dominated by all other sources of noise. >And, additionally, given that I worked out the drift, I could even try >and post-process the data taken with that instrument and correct for >the drift we just established, if this extra precision actually has >some value to someone. After all, in the end it's just the removal of >some systematic error I just happen to know after the analysis. If you do that, your uncertainty calculations just got a fair bit more complicated, because now you also have to factor in the uncertainty of the drift rate. But yes, that is basically how all cal-labs without Josephson Junctions estimate their Volt and Resistance. >But would the evaluation of drift rate still be possible if adjustments >have taken place? No. Until you have solid evidence to the contrary, you have to assume that adjustments changed the drift rate. One interesting idea in this space is to maintain per instrument Kalman filters on the calibration results. The predictions+uncertainty you get out will be way better than the formal uncertainty calculation, because the Kalman filter does not factor in risks (ie: things that _could_ happen) until they actually _do_ happen, whereas the manufacturers specs have an allowance for anything they could imagine or have heard about (jumps, thermals, air pressure, etc.) The main trick is that if you ever see your formal uncertainty dip below the Kalman filter, you know something is seriously wrong and in the mean time, the filter *probably* tells you what the situation is much more precisely than the formal numbers. >> The biggest advantage to inhouse calibration, is that you can do it >> much more often, and therefore don't need to do it as precisely >> as the cal-lab, because the sticker only needs date some months >> ahead. >> >One more question to that, as it's not entirely clear to me what >exactly you mean here: >Do I take it correctly that in case I would be willing to re-cal in, >say three months instead of next year, the instruments used for >calibration do not necessarily need to be as precise as if I needed >calibration good for one year? Or did you mean that I could afford >coming closer to the manufacturer spec limit? Or something else >altogether? All of the above, but you need to do the math to show that it is ok. If you stick to the manufacturers instructions, they did the math, and you wont need to. If you invent your own schedule, you need to do the math to find out the consequences for your uncertainty. Linear scaling is a good first approximation for drift, but not for other sources of noise or failure. >I guess a good Calibrator like a Fluke 5730A might do the trick as >well for the mentioned measurement range if low currents don't mattter >too much. And might be easier to get nowadays as even well-known >distributors don't quote a 3458A anymore. Might try to get a quote for >a Fluke 8508 and a Keithley 2002 as well... If you are in EU, I think you need to buy the 3458A directly from Keysight for ROHS reasons. I don't think I'm qualified to recommend specific equipment. I only mentioned the 3458A because it is generally seen as the "gold standard" and it is a damn good instrument in my own personal experience. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [email protected] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://lists.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
