Hi Steve,
You are absolutely right. Thermocouples can degrade, depending on what
environment they meet when used. Complicated, very complicated matter. I
have a paper about it somewhere, may find it. I remember the degradation may
be caused by diffusion of copper in the case of CuFe thermocouples. Or
corrosion due to gases in ovens. Or other causes which I don't remember.
We use thermocouples in our firing ovens for thickfilm technology. They are
exposed to nearly 1,000 Centigrades, and they degrade. In your case we speak
of less than 300 Centigrades, so all processes should be much slower.
Degradation after 5 cycles is not possible, because the reactions I point at
are too slow at the low temperature. But, again, thermocouples degrade
slowly. Brazed thermocouples degrade faster than thermocouples that are
joined with one lasershot.I've done numbers of thermocouples myself with YAG
laser. See if I can find that paper, if there is a need.
/Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Steve Gregory
Skickat: den 2 oktober 2007 20:04
Till: [log in to unmask]
Ämne: [TN] Thermocouple Longevity?

Hi Everyone!
 
How many reflow profiles do ya'll put the same thermocouples through
before you think about replacing them?
 
I've typically used them until they quit working or the insulation
deteriorates so bad that I don't want to use them anymore. I was told
that was not a good practice. That your typical K thermocouple starts
drifting after only 4 or 5 thermal cycles. I said; "Really?" I didn't
want to debate a whole lot because it was a customer who said this, so I
just agreed and went on.
 
So, do any of you know this to be a fact, and do any of you trash a
thermocouple after only 4 or 5 profiles and replace them with new ones?
 
Kind regards,
 
Steve Gregory

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