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. > > >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. > And especially, I could do the calibration whenever it suits me, and wouldn't be bound by calibration schedules. Yes, exceeding the interval we specified by too much might still cause questions from QM, but then that's the situation I already have, with ISO 9001 audit next week and equipment being overdue for up to four years. > 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. > Luckily, we also maintain the 23°C in our labs that usually are quoted for a calibration environment. Humidity in the labs is somewhat undefined though if I remember correctly. > 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. > This is the one thing my direct supervisor is afraid of. Damage or loss hasn't happpened until now as far as I know, but it's always a certain risk to send equipment out. > The biggest disadvantage is that you need to maintain suitable > cal-standards in-house. > Umm, if I could convince my boss that a few fully loaded Fluke 734As are absolutely essential to get this done, I'd expect to be in the right place at volt-nuts ;-) But I'd consider myself lucky if we would get a single 732B. Of course that always depends on how much time and money that would save us. A set of standard capacitors I've seen somewhere, and I wouldn't be surprised if we also had a set of standard resistors sitting on a shelf. > If it is just DC and AC voltage/current/resitance in the audio > range, a HP3458A will handsomely pay itself back. > > Up to about some hundred MHz you do somethign similar with > a good vector network analyzer. > > In GHz territory it gets nasty. > For now, DC is all I would consider, exactly for the reason that RF tends to become a bit more involved. Even though we also do have plenty of RF equipment up into the 50/67 GHz range, and some even going into three-digit GHz territory. But I didn't want to spend all my time just on calibrating stuff as fascinating as that might be. Coincidentally, we do happen to have an Agilent 3458A in one of our labs. Funnily, it's with the digital guys, where I'd be pretty surprised if they actually cared for the 4th digit. Has neven been calibrated though after it has left the factory. What a waste... Repurposing this one for internal calibration probably would require having it calibrated externally for a few years to come to establish ist own performance. Should still be good to manufacturer drift specs if used shortly after calibration. Best regards, Florian _______________________________________________ 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.
