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October 2007

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Wed, 3 Oct 2007 10:18:36 +0300
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There are two things here, at least.

Most low-temp (<500°C) couples use chromium-nickel and aluminium-nickel 
alloys. The precision depends on the accuracy of the alloys and it would 
be foolish to rely on new ones as better than ±2°C. Then, after time, 
there is a migration of intermetallics and this will reduce the accuracy 
further. This is more rapid at high temps (thanks, Mr Arrhenius!) and 
time/temp is more important than cycling. Thermocouples are never used 
for precision measurements at <250°C; platinum resistance thermometers 
(Pt100) are usually better than ±0.1°C, but their physical size and 
thermal inertia rules them out for profiling. With thermocouples, there 
is also the problem of cold junction compensation, which can further 
reduce the accuracy.

For reference, although irrelevant here, only platinum (Pt/PtRh13) 
thermocouples are considered accurate to better than 1°C, provided the 
cold junction is in melting ice. Their downside is that they are very 
insensitive so are really useful only for high temperatures (>800°C).

Brian

Steve Gregory wrote:
> 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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