[volt-nuts] Agilent calibration

[Dr. David Kirkby]

On 14 August 2013 06:41, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 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

One of the advantages of modern instruments over older ones is that
measurements are often more convenient to make. This can reduce your
measurement time and so cost. For many companies, a case can be made
to upgrade if a newer instrument will save time and money.

As a rough guess, I would assume 99.999% of instruments sold sold by
Agilent are for commerical non-metrology work. Those 99.999% of users
do not want to remember to subtract 0.000008 V -  they want that
instrument to be as accurate as possible.

Now if you take an instrument like the Agilent 3458A 8.5 digit DVM,
then the intended user base is going to have a lot of metrologists.
Those people might prefer their instruments are not adjusted, but I
think for 99.999% of users of test equipment, they would want the
instruments adjusted. With so much done in software now, arguments
about pots drifting once adjusted dont make any sence.

By its very nature, the readers of volt-nuts will often fall into the
0.001% that might not want their instruments adjusted, but I think it
is fair to say most would.

Agilent must have thought about these arguments, and have come to a
decision not to adjust. I'm a bit surprised myself, but they obviously
have their reasons. Clearly if an adjustment requires someone to go in
with a screwdriver, then it takes time, has some element of risk of
causing accidental damage, and it might well cause things to drift
more in the short term.

Dave