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Re: gEDA-user: Design Lab Equipment


On Mar 31, 2007, at 2:08 PM, John Griessen wrote:

Steven Michalske wrote:

There's another reason for no plating. What is that shiny plating on those commodity test leads? Hmmm....
Laws of intermediate metals don't apply to mystery metals, do they?
It applies to all metals, even alloys. It is all to do with the vacuum levels of the atoms and the alignment of the actual conduction bands. This also works with semiconductors, Thermoelectric coolers use P and N doped semiconductors to increase efficiency ( higher temperature per volt or amp )

The case where they saw errors was noticing statistics using different kinds of leads
rather than noticing on one measurement. They were measuring a kelvin bridge, so they
were taking measurements many inches apart, sometimes several feet apart. So, if one
test lead was one alloy and one another, and they had a one degree temperature
difference, wouldn't they see a difference due to the one non- copper test lead and the temp difference
from it to the meter?

Making the definition of the lead as the plating on the tip, no.
the junction that the plating has is only a few thousandths of an inch. Now if the wire of the probe was made with different material than its counterpart probe, making the junction spread across the wire, this allows for temperature gradients, that generate voltages.


Would the difference go away when only copper is used throughout?

Maybe, but it's the junctions of the connectors that matter where the plating is concerned, those junctions will be generating verrrry little thermoelectric voltages if any.
Going out on a limb here I suspect that if they tested two different probes where the alloy of the probe's wires were different. With the wires being of different alloys, if you apply the law of intermediate metals, and audit the connections and junctions with there relative temperatures. showing that the ends of the probe wires form a thermocouple between probe A's wire and probe B's wire. Where all of the contacts weren't causing problems, but the variance in the probe wires alloys.


The testpoints they measured were copper also, and the system under test was connected by copper everywhere.

John Griessen

PS I like Kai-Martin's amplifier idea better for some GPL kit hardware.

How about a version where you add an A2D
and ethernet to the diff amp!

And make a more expensive, but +/- 300V one. The a +/- 700V one.... all GPL.
Selling as kits with howto's published under creative commons or GPL license
making all the liability severed from "consultant advice" and instead being zero liability
published information.


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