[Test-Equipment] STD Cells----NBS Monograph 84

[Tom Dawson]

Well, Bob, the bad news is that the national standards organizations are
always going to be providing standard values with uncertainty values based
on the mean values and standard deviations of a large number of measurements
made over time, and usually derived from intercomparison between different
standards.

>From the NIST website:

"The U.S. Representation of the Volt is maintained by monitoring the emfs of
several groups of saturated standard cells in ovens on a monthly basis using
the ac Josephson effect. "

Most likely they take the mean and then assess the uncertainty based on the
standard deviations of the monthly readings.  NIST's uncertainty for voltage
measurements in => 0.2 ppm.

With respect to local groups of standard cells, I don't recall anyone saying
that they were obtaining an absolute value based solely on the average of a
bunch of measurements.  If you read Technical Note 430, the starting point
is the stated values of the cells, which is established by comparison with
some other standard, and the Tech Note 430 procedures provide a means of
monitoring the behavior of the cells over time.  If, using the statistical
methods described in the text, the resulting cell voltages remain within a
reasonable range of values, one can have confidence that the cells have not
drifted, or otherwise deteriorated since the last comparison with primary
standards.

Somehow I don't feel misled by NIST's process.

regards,

Tom







----- Original Message ----- 
From: "wolfbob" <wolfbob at csnsys.com>
To: "Tom Dawson" <wb3akd at earthlink.net>; <test-equipment at mailman.qth.net>
Sent: Tuesday, June 01, 2004 12:22 PM
Subject: Re: [Test-Equipment] STD Cells----NBS Monograph 84


> I did not read the Monograph in detail but scanned it looking specifically
> for any indication that the variations in the emf of an individual cell
was
> possibly random and if they had any clues as to the distribution of the
> randomness. I found NOTHING that would indicate anything but a systematic
> change in emf due to a number of factors (temperature, age, shock history,
> etc).
>
> I think the world has been misled by the FACT that the NIIS uses the
average
> of a number of cells to DEFINE the STANDARD volt. This is a definition. It
> is a good definition as it solves some of the problems germane in
> determining a fundamental standard. But we poor folk are not trying to
> establish and maintain an ABSOLUTE standard, but trying to establish and
> maintain a secondary standard that has a voltage stamped on it side and
that
> is that. Averaging a bunch of these is no more accurate than the value of
> any one of the calibrated secondary standards. In fact the average is
worse
> as it is contaminated by your local differential measurement errors. Using
a
> number of standard cells is useful in determining if one is bad or changed
> from its original, but the average is definitely not as accurate as what
is
> stamped on the side of a good cell.
>
> Either that or I'm wrong...
>
> I hope this discussion will bring out the experts (I am definitely nothing
> but a cranky old fat man) and this forum can expand into one of
measurement
> methodology as well as the interests for the equipment to support that
> methodology.
>
> WBob
>
>
> ----- Original Message ----- 
> From: "Tom Dawson" <wb3akd at earthlink.net>
> To: <test-equipment at mailman.qth.net>
> Sent: Tuesday, June 01, 2004 5:02 AM
> Subject: [Test-Equipment] STD Cells----NBS Monograph 84
>
>
> >
> >
> > Monograph 84 regarding all you ever wanted to know about Weston standard
> > cells is at
> >
> >
> > http://www.radio-science.com/std_cells/mono_84.pdf
> >
> >
> > If you want it, download within the next couple of weeks because its
about
> > 6MB and consumes quite a bit of that site's storage.
> >
> >
> > regards,
> >
> > Tom
> >
> >
> > _______________________________________________
> > Test-Equipment mailing list
> > Test-Equipment at mailman.qth.net
> > http://mailman.qth.net/mailman/listinfo/test-equipment
> >
> >
> >
>
>
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