[Test-Equipment] Corrected link

[John Kolb]

The little bit I remember about standard cells is that
one should never dray ANY current from them. Don't measure
the output with regular voltmeters. They should only be
measured with a voltmeter such as the old style Fluke
or perhaps Wavetek differential voltmeters, which are
infinite input impedance at null.

A secondary voltage standard with low output resistance
would be adjusted to be the same voltage as a standard cell
by placing a high impedance microvoltmeter between the standard
cell adn the secondary standard, and adjusting the sec. standard
until they matched. The secondary standard would then be used
for calibrating voltmeters, etc.

For the differential voltmeters we manufactured at Wavetek
back in the late 60's, we bought a half dozen or ten standard
cells and placed them inside a lead bathtub we had molded
inside a styrofoam container so that any tempurature changes
would occur very, very slowly.

John

On Mon, 31 May 2004, wolfbob wrote:

> I found two or three gross approximations in the paper with a quick scan.
> First is the assumption that standard deviation can be applied. As I
> remember form 40 years ago, the concept for standard deviation only applies
> to distributions that are Gaussian. They make no proof or present any
> evidence as to just what is the error distributions and why they should be
> random with a Gaussian distribution. The phenomena that is causing the
> variation could be defined and it is entirely possible that the errors are
> Gaussian, but I rather doubt it.
>
> Taking the average if a few things can be very misleading and fraught with
> statistical errors. If you run some basic probability test like Student T
> (again only applicable for Gaussian distributions as I recall) or what ever
> to see just what confidence there is that the average is any more accurate
> than any one of the cells.
>
> A very good example that all can appreciate is that of taking the reading of
> time from 6 watches and averaging them and expecting to get more accuracy in
> knowing what time it is. Here is a case where it is obvious that the error
> is not random but arbitrary, a really quite different thing. It should be
> obvious the average reading of the watches is no more accurate than any one
> of the watches as you do not have a clue as to the distribution of the
> errors or their cause, except the are not random, just different. Now if you
> have 10,000 watches, you have another hunk of mathematics coming in to play
> and that is the central limit theorem that says if you got enough
> independent samples of any distribution they will appear Gaussian and all
> the good math applies. Six is not a large enough to qualify under this
> caveat.
>
> Another way to see the error of this thinking is to make a simple
> assumption. One of the 5 cells is dead ass on the correct voltage and the
> other four are wrong. The average will also be wrong. just randomly picking
> one of the 5 cells for the standard will make you right 20% of the time
> whereas the average will make you wrong 100% of the time.
>
> WBob
>
> ----- Original Message -----
> From: "Tom Dawson" <wb3akd at earthlink.net>
> To: <test-equipment at mailman.qth.net>
> Sent: Monday, May 31, 2004 8:59 AM
> Subject: [Test-Equipment] Corrected link
>
>
> > Sorry,
> >
> > Fumble fingered the previous post.
> >
> >
> > Follow the link to the following NBS Tech Note relating to measurements of
> > standard cells, and maintaining the volt with a small group of saturated
> > standard cells.
> >
> >
> > http://www.radio-science.com/std_cells/tn_430.pdf
> >
> > and a dos program to do the arithmetic.
> >
> > http://www.radio-science.com/std_cells/cells.exe
> >
> >
> > regards,
> >
> > Tom
> >
> >
> > _______________________________________________
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> >
> >
>
>
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