[volt-nuts] Traveling Standards

[Bob Smither]

On 05/08/2012 12:52 PM, Charles Black wrote:
>    Hi Bob,
>    What a great job you are doing by organizing this cooperative field
>    comparison! It sounds like a lot of work though!
>    Looking at the first lab photo brought back memories of where I started
>    with my new (to me) 3458A three years ago! I hope it is OK to offer
>    suggestions that will make measuring microvolt levels on a 3458A more
>    accurate . Similar techniques can be used on any meter though.
>    I have been periodically monitoring my Datron 4910 and 4912 10V
>    standards for about 3 years looking at measurement  drifts mainly
>    caused by temperature. I have many hours of recorded data looking at
>    0.1 PPM resolution drifts in my system. In order to improve data
>    stability I  had to change my measuring technique from a BNC cable with
>    dual banana adapters (what is shown in the photo) to twisted pair
>    copper wires or shielded twisted pair wires (22 or 24 gauge) to
>    minimize input thermal offsets and settling time. Tin plating on the
>    wires measures about the same offset voltage as copper so it hasn't
>    been a problem to use it so far. If I put a huge thermal mass on my
>    input terminals (plastic dual banana plug) it can take many minutes to
>    stabilize and even then air currents will keep my meter from
>    stabilizing as completely as it can. Today my meter has about -240nv
>    offset voltage from a copper wire short but I see -1.2 microvolt with a
>    shorted dual banana plug, even a gold plated one. I have a shielded
>    dual banana to BNC adapter that's even worse.
>    My 3458A is very temperature sensitive so I constantly monitor the
>    temperature right in front of the input jacks to 0.1 degree C. I try to
>    keep mine at 23 degrees and do frequent ACAL's and repeat the ACAL if
>    more the terminal temperature varies much over a degree from 23C. The
>    TEMP? is also very important to watch and could be used to predict
>    temperature offset error. The TEMP? creeps up over a few months due to
>    dust in the filter but it is easy to clean with a little tap water.
>    The 3458A has a variable resolution feature that can be a problem at
>    high resolutions. Full DC resolution is acheived at NPLC = 1000. Lower
>    NPLC levels can be used to speed up measurements if loss of accuracy is
>    not a concern.
>    My shop has distinct thermal stratification so I try to keep the test
>    unit  I am measuring at the same height as the meter and  close to the
>    meter when doing accurate work.
>    As far as the portable standard goes - it would be great if it had gold
>    plated dual banana spaced terminal posts similar to the 3458A posts for
>    best repeatability and flexibility but, of course, any jacks will work
>    if they have enough time to stabilize.
>    Charlie

Thanks Charlie - great suggestions.

My goal has always been to know the values from the two references in the TS to
with 10 ppm.  For the two refs (nominal 7 Volt and 5 Volt) this is 70uV and 50 uV.

I have one meter here that I can easily see thermal effects on (an old Fluke
845AR - 1uV full scale on most sensitive setting) so I appreciate your notes
about the use of copper wires.

Even though only trying for 10 ppm I appreciate that any source of error that
can be eliminated needs to be considered.

With your permission I would like to add your notes to the page here:

  http://c-c-i.com/ts

so I refer folks that are taking measurements on the TS to them.

Best regards,
-- 
=========================================================================
Bob Smither                                               bob at smither.net

If Tyranny and Oppression come to this land, it will be in the guise of
fighting a foreign enemy.
        -James Madison
=========================================================================
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