Miniature thermocouple attaching by soldering yes or no? [solved]

Thread Starter

frpr666

Joined Feb 2, 2010
50
Hi,
I have a miniature thermocouple type K (http://www.omega.com/pptst/5TC.html) and my intention is to attach it to PCBA (temporary for the temp. measurement).
Please is it possible to attach it with classic soldering method (solder wire SnPb)? Will the soldering affect the output mV ?
Please do you have any idea how to attach it to DPS?
 

shortbus

Joined Sep 30, 2009
10,045
It depends on the actual thermocouple end point. If it is in a "sheath" or cover that is solderable I'd say OK to solder. But if the end point is just open wire I'd say no. The termocouple works by the two different wire types being in contact with each other, and if they're not sheathed and protected, the solder could migrate between them and not allow them to work as intended. If you go back to your link to the thermocouple and 'click' on the specs it takes you to a PDF, on that PDF they show a TAP(thermocouple adhesive label) to do what you want, hold the thermocouple to a item for measuerment.
 

Thread Starter

frpr666

Joined Feb 2, 2010
50
Hi Shortbus,
thank you for reply. It seems that the TC joint is without any insulation. Attaching by TAP tape is possible but it seems to me that between the TC head and the PCB surface (object of temp. measurement interest) will remain air gap.
Maybe someone have experience with attaching TC by soldering. I'm asking bcz I'm not sure whether the tin drop between the Cr & NiCr wires will cause short-cut or not.
 

Attachments

Yea, most thermocouples work by two differing metals, welded together, not soldered. The weld is where the voltage differential occurs. This is dependant on temperature, and on how these are welded. Soldering will not work on these kinds of thermocouples.
 
Last edited:

jpanhalt

Joined Jan 18, 2008
11,087
Agreed that most thermocouples are welded at the tip. Disagree that an intermediate material, e.g., solder will affect the voltage -- even if they aren't welded (so long as the tip is at a homogeneous temperature).

It is called the law of intermediate materials, which is presented as, "Law #2" here: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwj34-Xxy4DQAhVMxoMKHfj4CygQFggeMAE&url=http://www.met.sjsu.edu/~clements/met163_lecture/met163_lecture_5.ppt&usg=AFQjCNHMOfz_pfJhbxC-csoMu-gNpc2uNg&sig2=NM7AfgQR1_WBsmeO_qbl_g&bvm=bv.136811127,d.eWE

Don' fight the law. You can solder the tip.

John
 

Thread Starter

frpr666

Joined Feb 2, 2010
50
Thank you. After today's searching I got that the soldering of the welded joint doesn't affect the thermocouple measurement.
There is a popular, and very misleading, misconception of how a thermocouple operates. In this
erroneous ‘model’, it is imagined that the thermocouple's 'hot' (measuring) junction somehow
functions as an electrical source, and that the junction itself produces the thermocouple's small
signal voltage. This concept is simply not true. The actual thermoelectric effect is an extended
and continuous one that is distributed along the entire length of the thermocouple conductors.
The process is driven by the temperature differences, or gradients, through which these
conductors pass.
source:
http://www.advindsys.com/ApNotes/tcfundamentals.pdf
This point can be good for MythBusters I think. https://en.wikipedia.org/wiki/MythBusters
 
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