[volt-nuts] 732A and Prologix received

Tue Aug 26 00:28:35 EDT 2014

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 at 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-bounces at febo.com] On Behalf Of Mike S
> Sent: Monday, August 25, 2014 11:03 AM
> To: volt-nuts at febo.com
> Subject: Re: [volt-nuts] 732A and Prologix received
>
> On 8/25/2014 11:02 AM, Don at 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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