Soldering thermocouple wires together is OK?

Hello,

I wish to measure the temperature of a MOSFET case with a thermocouple.

If the thermocouple junction that is glued to the FET is held together with solder, then
how much does such soldering affect the accuracy of the temperature measurement?

After all, the thermocouple measures temperature, by measuring the small voltage created when two disimilar metals are in contact with each other....

...these metals are in contact due to being twisted together, but one has to then solder them together to ensure that they stay together.

...when the soldering is done, you would think that the solder metal would short out the small potential difference that one is trying to measure.(?)

So, is it OK to solder the ends of thermocouple wires together when doing such temperature measurements?

By the nature of thermoelectric effect, the voltage doesn't change if you place a third metal between both thermocouple wires. Thermocouples are mostly welded to achieve maximum temperature resistance. But brazing or soft oldering works as well. You'll need a suitable flux to solder NiCr wires, e.g. diluted phoshoric acid, or solder wire for stainless steel.

Thankyou FvM,

So are you saying that nrmal Rosin-flux cored solder could act to corrode the thermocouples.

I believe that all solder that i get off RS or farnell has flux in it, and it is not the type that contains the special flux's that you mention.

-do you know where i can get some of that solder that you kindly specify?

-or should i just try and get flux-less solder and use that?

grizedale said:

So are you saying that nrmal Rosin-flux cored solder could act to corrode the thermocouples.

Click to expand...

I think he means exactly the opposite, the rosin flux is very weak so you need a much stronger corrosive substance to make soldering possible.
Unfortunately I have never used any of these and I have no idea where you can find them.

Alex

Yes, Alex has understood the point correctly. You can just try, if the solder joint looks good with regular solder, you don't need special flux.

A decent twisting together of the wires with suitable pliers usually does the job of holding them together well enough to function accurately as a thermocouple - solder can then be used just to hold the twisted bits in place on the component leg or similar.
For good isolation without losing temp accuracy we used some DCB substrate 3mmx3mm pieces (Al2O3 - alumina 0.5mm with copper on each side) with the thermocouple soldered to one side and the other side soldered to the point of interest, this works well with most hand held temp meters as the common mode noise transferred to the instrument is now very much reduced - you can have a lot of these and feed them into a low speed scanning mux for automated temp readings all over your psu etc - as they are all now suitably isolated. Regards, Orson Cart.