[volt-nuts] Traveling Standards
cblack at centurytel.net
Tue May 8 17:52:04 UTC 2012
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.
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
I will be reporting results as I get them.
The first measurements are in - see http://www.c-c-i.com/node/160 .
My measurements averaged around 6.99697 for the LM199A and 5.00026 for the MAX63
The measurements from a fellow volt-nut with an HP 3458A are 6.997326 and
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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