Am Wed, 20 Jun 2018 21:55:52 +0000 schrieb "Poul-Henning Kamp" <[email protected]>:
> -------- > In message <[email protected]>, > Florian Teply writes: > > >Now, as far as I understand, calibration at first sight is merely a > >comparison between what the meter actually reads and what it is > >supposed to read. As long as the difference between the two is > >smaller than what the manufacturer specifies as maximum error, > >everything is fine, put a new sticker to the instrument and send it > >back to the owner. > > What the sticker really says is that you have credible statistical > reasons to think the meter will be inside spec until the date on > the sticker. > > This is why you can go longer between calibrations if you have > the calibration history for the instrument. > > If for instance you instrument over the last five yearly calibrations > have been found to show 0.10, 0.15, 0.20, 0.25 and 0.30, then there > is every statistical reason to expect it to show 0.35, 0.40, 0.45 > and 0.50 at the next four yearly calibrations, barring any unforseen > defects or mishaps, and the date for next calibration can be chosen > accordingly. > > If on the other side its calibration history contains something > like ... +0.25, -0.35 ... you know it can change 0.6 in one year > and you may have to pull up the date on the sticker accordingly. > > If the instrument has no history and reads 0.35, you will have to > consult the manufacturers drift specs and project forward and see > what the earliest date the instrument can become out of spec, and > write a date conservative to that estimate on the sticker. > 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. 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. But would the evaluation of drift rate still be possible if adjustments have taken place? I guess I couldn't count on actually knowing what has been changed unless I did the adjustment myself, and what effect that change has on future drift rate might be pretty difficult to predict even for insiders. > >Background of my questions is me wondering if it would be feasible to > >do the calibration in house instead of sending equipment out for > >calibration. > > 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? > The second biggest advantage is that you can perform the calibrations > in the target environment, rather than at some artificial enviromental > conditions, which don't apply in real life. > > The third biggest advantage is that the calibration doesn't take > the instruments out of commission for several days due to transport > and scheduling, and they don't get damaged and lost in transit. > > The biggest disadvantage is that you need to maintain suitable > cal-standards in-house. > > If it is just DC and AC voltage/current/resitance in the audio > range, a HP3458A will handsomely pay itself back. > 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... _______________________________________________ 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.
