[volt-nuts] Thermal EMF - more results

[Don@True-Cal]

I will argue that this issue is extremely relevant, especially in low voltage and low resistance measurements. You are correct in your position if the entire junction is in thermal stability. If however, the junction is exposed to a sudden change in temperature, air drafts being the most likely, then your example of copper wire soldered to copper terminal will indeed exhibit thermal EMF due to the difference in thermal mass of the copper wire to solder vs. the solder to copper thermal. Air drafts in the metrology lab is typically the concern in thermal EMF. A simple oscillating fan on the interconnect terminals during a precise, low (1 to 10) ohms, 4-wire, 1 to .01 PPM measurement will convince you. It is also easier to see if graphing the measurement results. The generated thermal EMF increases as the difference in thermal mass increases between the soldered principles in the junction. A #12 copper wire soldered to another #12 copper wire - no concern, even in a draft. A #12 copper wire soldered to a #22 copper wire - no concern if at thermal stability but blow on it and observe.

Regards,
Don

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Neville Michie
Sent: Friday, July 08, 2016 7:08 PM
To: Discussion of precise voltage measurement <volt-nuts at febo.com>
Subject: Re: [volt-nuts] Thermal EMF - more results

All this talk about thermal EMFs may not be particularly relevant.
Solder is used in connections between two metallic objects, like wire and a terminal. The thermal EMFs between a soldered copper wire and copper terminal have exactly equal and opposite values between the solder and the metals.
I first met this in mid 60s making thermocouples, I was told that they should be welded not soldered because of the solder metal EMFs. I did both soldered and welded over the years, mainly for practical considerations, and although some situations were quite critical, I have never found evidence that solder matters. Even if you consider a metal like tin diffusing into the copper and changing its EMF, adjacent to the area of diffusion is a reverse gradient of tin concentration and thermodynamic considerations dictate that there is no effect for static conditions.
The worst effect is the response of the junction to a persistent thermal gradient, but this is hard to maintain over the distance of solder thickness.
cheers,
Neville Michie


> On 9 Jul 2016, at 8:56 am, Herbert Poetzl <herbert at 13thfloor.at> wrote:
> 
> On Wed, Jul 06, 2016 at 08:53:19PM +0200, Andrea Baldoni wrote:
>> On Wed, Jul 06, 2016 at 03:42:18PM +0200, Herbert Poetzl wrote:
> 
>>> Any plans on testing Sn42/Bi57.6/Ag0.4 ?
> 
>> Hello Herbert.
> 
> Hey Andrea,
> 
>> I don't have any source of it. My supplier (Heraeus) hasn't it, at 
>> least, it wasn't included in the list of samples they are able to 
>> give me on request.
> 
>> If you have it and you could ship it (a meter of 0.5-1mm wire or so 
>> would be sufficient for me and I will give it back after), I will be 
>> happy to test it.
> 
> Send me your address (off list) and I'll send you a meter for testing.
> 
> Best,
> Herbert
> 
>> Today I received the Sn99.3/Cu0.7 and I will post result soon.
> 
>> Best regards,
>> Andrea Baldoni
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