[volt-nuts] 732A and Prologix received

[Don@True-Cal]

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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