Joe wrote:
The way I read this is that if I send them a DMM that is within spec, they won't adjust it or provide pre/post data. Is this the case? If I spend over $200 sending a DMM to them, I want it adjusted to the best possible specs and I want the data. I do not want someone just saying that it is good enough and send it back to me. I can get that for $50 in El Paso.
The big difference is not between adjusting and not adjusting -- it is between getting a calibration "with full data" and getting one without data. /The true value of calibration is not the adjustment -- it is the data./
Agilent doesn't just say it is good enough -- they tell you specifically how far off it is and quantify the statistical uncertainty of their measurement. That is everything you need (i) to correct readings you make with the instrument and (ii) to be confident of the potential uncertainty of those measurements.
Let's say your meter has an uncertainty spec of +/- 15 uV (1.5 ppm) total at 10 V. If your calibration certificate says the meter reads dead on at 10.000000 V, the reading shown on the display is your measurement result (with a certainty of 1.5 ppm, or +/- 15 counts from the reading) when you measure a 10 V source. But the cal certificate could just as well say that the meter reads 10.000008 V when measuring a 10.000000 V source. In that case, you know to subtract 0.000008 V from whatever the meter reads when you measure a 10 V source to get your measurement result (again, with a certainty of 1.5 ppm, or +/- 15 counts from the corrected reading). Of course, in the real world a voltage standard will have its own calibration offset, so you will make two corrections when you measure your house "10 V" standard to verify that your meter is still in calibration.
So, why wouldn't they adjust every instrument to be "spot on"? Because metrologists have determined that, as a general matter, not messing with the adjustments results in overall better stability of instruments.
Adjusting instruments inevitably causes a new drift and settling cycle, so if you adjust everything as close to perfect as possible every time you calibrate, you will always be on the steepest portion of the settling curve. On the other hand, you can benefit from the long, ever-decreasing tail of the settling cycle by not adjusting as long as the instrument is within the manufacturer's specifications. Further, seeing the change from one calibration interval to the next, and the next, etc., increases the confidence you have in readings you make when the last calibration is not so fresh any more.
Best regards, Charles _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
