[volt-nuts] Traveling Standards

[Charles Black]

   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
   On 1/8/2012 8:59 PM, Bob Smither wrote:

Bob Smither wrote:

After way too long (boy, having to work really cuts into my hobby time)
the first Traveling Standard is on its way to the first volt-nut that
signed up.

I will be reporting results as I get them.

The first measurements are in - see [1]http://www.c-c-i.com/node/160 .

My measurements averaged around 6.99697 for the LM199A and 5.00026 for the MAX63
50.

The measurements from a fellow volt-nut with an HP 3458A are 6.997326 and
5.000541 respectively.

The TS will be on its way back to me this week.  After I spend some time with
it, I will get it mailed to the next volunteer.

Thanks to all who offered to measure the TS.

If this first result holds my HP 3455A is within its 90 day spec limit after at
least 25 years of not being calibrated.

My 3455A died some years back and has been in storage.  I recently got it out
and was able to repair it using the excellent HP documentation that came with it
.


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References

   1. http://www.c-c-i.com/node/160
   2. mailto:volt-nuts at febo.com
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