Am Wed, 20 Jun 2018 16:15:38 -0400 schrieb Bob Bownes <[email protected]>:
> Florian, > > As one would suspect this is far from a simple problem and it depends > on what your needs are. > This is exactly what I expected. It seems simple, but isn't necessarily. There's probably a reason Calibration labs are not popping up everywhere... > The complexity of calibrating a particular piece of equipment can > range from simple 'hook it up to a known source and if it is withing > cal, stick a sticker on it' to a many hour/day exercise in which > things like drift with time and temperature are measured and > corrected. > As mentioned already in my other response, temperature isn't that much of a concern for me. Lab temperature is set at 23°C, and varies by no more than half a degree over the day. Humidity is to my knowledge uncontrolled in the labs so it does vary, but I have no idea by how much, or if that could be a problem. > It all depends on your needs. Lets take the relatively simple HP > 3468A 5.5 digit multimeter on my bench here. The manual, available > online has a 20 page section on performance test and calibration. > This is for a meter that measures AC volts, DC volts, resistance, AC > current, DC current and not much more. I keep it calibrated by > comparing it to my 3456, (which is calibrated by a commercial > service) because it is my main workhorse for doing electronics > construction and debuging (and the 3456 is more accurate). The 3.5 > digit multimeter I keep in the garage has never been calibrated, but > it's used for seeing if I have 13.8VDC and continuity in circuits in > the cars and not much more. And I'm happy with both. But I answer > only to me. > > So the question is, what are your needs and requirements? Your > industry may require NIST (or other defacto standard) traceable > calibration for some things. Your factory may be using 3.5digit DMMs > to see if there is mains voltage on lines and not much more. Or you > might be in a semiconductor plant where you have a need for agreement > in the last digit of several 3758 8.5 digit meters. All about what > the requirement is. > As a matter of fact, it IS a semiconductor plant I'm talking about. And I don't care about equipment that's used to check if the mains fuse might been blown. I'm looking for a solution for the equipment that needs to be calibrated because we do serious measurements with it. I'm also not talking the stuff we have in place to extract PCM data. This will remain to be commercially calibrated. But defining what we NEED is a bit difficult for me. I have the impression that buying decision for a specific instrument over all the others is made because one thinks that at some point it might be useful to have this or that feature, or because the engineer thinks this would be neat to play with. Or possibly just because everyone else in the industry uses that. With proper setup, even three digits can potentially give better insight than 8.5 digits ever could when they're applied without thought. Or both can be off by orders of magnitude if the setup isn't up to the task. Taking the actual accuracy specs into account, even 6.5 digits seem excessive, indicating a precision that just isn't there. The semiconductor parameter analyzers and SMU mainframes we use all feature accuracy specs of aapproximately 50 ppm for voltage measurements and maybe 100ppm for current measurements. More problematic might be the current range of these instruments: They regularly have as a smallest range 10 pA full range with femtoamp or even sub-femtoamp resolution. Granted, accuracy drops to the order of 1% at that range, but still it seems difficult to do calibration at these current levels. Thanks for all the input in any case! 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.
