My post on this thread earlier today seems to be lost (maybe the system
change) so I am re-posting some of it (corrected) since it's apropos to
the discussion.
Anyone who has calibrated (CAL 0 anyway) a 3458A has enough information
to deduce that lowest input short voltage is going to be a copper wire
since the meter is set to zero volts during calibration using a heavy
gauge (14 to 16) copper wire. This is very convenient since it is so
easy to duplicate in the field and makes simple inexpensive test leads
best for high precision measurements. In order for the 3458A to make
full accuracy measurements (8 digit) NPLC must be set to 1000 (according
to the User's Guide). If you use a lower NPLC value there is a table in
the User's Guide that can be used to determine how accurate your
measurements are going to be.
Just for fun I ran several shorts for NPLC 1000 on my 3458A. It has been
about a year since I clid my last CAL 0 so it was going to be
interesting at least for me. The first shunt was my test "U' shaped
shunt that I used for my last CAL 0. Note: STP = Shielded Twisted Pair.
Calibration shunt -0.00021mv ± 10nV Equilibration time 5
minutes. 14 gauge per Calibration Manual.
"U" heavy wire -0.00021mv ± 10nV Equil. time 5 minutes.
Used the through holes in the Input banana posts only.
Copper wire -0.00019mv ± 10nV NAPA PVC covered
automobile wire at same contact points as CAL shunt
Standard Ground Plate -0.00040mv ± 10nV Equil. time 5 minutes. Gold
plated ground plate from my Datron 4910
Copper wire -0.00019mv ± 10nv Equil. time 2
seconds. Used the banana through holes.
STP 2 meter test lead -0.00021mv ± 10nV Equil. time 2 seconds.
M27500 24 gauge STP Tefzel insulation.
Banana plugs -0.00021mv ± 30nV Equil. time 20
minutes. My best "Perfect" gold plated plugs with copper wire.
Note that all my test shorts equilibrate to virtualy zero volts. There
is almost no Seebeck voltage for any of my test shorts as long as the
system is allowed to equilibrate. Equilibation times varied between less
than 2 seconds up 20 minutes with longer times due to excessive thermal
mass. Excessive thermal mass also caused some voltage instability.
Charlie
On 8/25/2014 6:54 PM, Don@True-Cal wrote:
Why?
Let me count the ways. You can never count on any Seebeck voltage to be immediately
offset, there are far too many variables. Best example I can think of...why is
there an Ohms Offset Compensation feature on any good high resolution DMM. 1) Try
measuring a 1 or 10 Ohm resistor with your 3458A in 4-wire mode using inexpensive
nickel-plated leads and even allow plenty of time for everything to thermally
stabilize. Using Ohms Offset Compensation, enable and disable it and observe the
difference. If the Seebeck voltages were all immediately offset, as you say, there
would be no difference. But there most certainly is. Or simply, why is there a need
for ohms offset compensation feature if all Seebeck voltages cancel each other out.
Sure, nickel-plated is a horrible choice but if it all canceled, what difference
would it make how bad is. 2) The cal lab workhorse calibrator is the 5700A/5720A.
In between trips back to Fluke for full calibration, there is an interim external
calibration procedure using the 732B, 742A-1 & 742A-10k. If someone used a set
of gold-plated interconnects for this procedure, they would be laughed out of the
lab and the calibrator would be useless until recalibrated properly. A set of
5440A-7002 (banana plug) cables comes with this calibrator (5440A-7003 spade lugs
for 5720A) and recommended for the calibration procedure but other Beryllium Copper
or pure Copper cables are also acceptable. 3) Lab air drafts will never allow true
thermal symmetry around the DMM or DUT terminals. To convince yourself, place an
oscillating fan several feet back from the DMM and DUT terminals and using the 1 or
10 Ohm setup from above, again with the nickel-plated leads, watch the variations.
Sure the fan and the nickel-plated exaggerates the issue but it quickly dispels the
notion that all the Seebeck voltages are canceled out.
BTW, the plating layer temperature on a plated terminal will be somewhere
between the temperature of the base metal and mating terminal it's connected to.
This is not just theory, my 40+ years in the cal lab is driving my arguments
but it never hurts to have physical-science on your side.
Don Johnson
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Mike S
Sent: Monday, August 25, 2014 11:03 AM
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] 732A and Prologix received
On 8/25/2014 11:02 AM, Don@True-Cal wrote:
Silver or Gold plating on the terminal or wire will introduce the
undesirable dissimilar metal properties, both at the plating junction
and at the plating metal to DUT terminal.
Why?
Any Seebeck effect is immediately offset in the opposite direction, since both
junctions are (under normal conditions) at essentially the same temperature
(e.g. there's a copper-gold thermocouple, the minimal thermal resistance of a
micron of gold on the contact(s), then a gold-copper thermocouple). It seems to
me that the improved consistency of the contact outweighs any loss from the
thermocouples.
A more typical contact would be copper-nickel plate-gold plate, but the concept
is the same. Unless there is heat flowing through the entire assembly so one
thermocouple is warmer than the offsetting one (e.g.
shortly after plugging in a banana plug warmed by body heat), they simply
cancel.
Even if connecting gold plated to nickel plated contacts, it works out the same
- a copper-nickel-gold-nickel-copper connection is completely offset. It's when
the offsetting thermocouples occur across a temperature gradient that you have
problems.
--
Mike
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