Sorry if i´m being naive, but what´s the difficuty of making a digital
equipment with a memory to store the offset of each scale ans subtract
it before sending to the display, no pot trimming involved?
Why aren´t all of the ones made after, let´s say, 1995, like this?
Daniel
Em 14/08/2013 20:29, Dr. David Kirkby escreveu:
On 14 August 2013 06:41, Charles Steinmetz <csteinm...@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
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