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I just found out on Wikipedia (also here and there) that the voltage in a thermocouple in not generated by the both the cold & hot junction, but by the temperature gradient in the wires which generates a wire type + temperature dependent voltage gradient.

This quite surprised me because I completely ignored it, but it makes sense since in the Peltier effect, it is even reversed between P-type and N-type bars.

However, the question is then “why do we bother documenting couples by pairs, instead of specifying the gradient function for each material and just list the materials / pairs that are compatible or not compatibles with various condition (temperature, atmosphere type, etc). Wouldn't it make a more powerful information with a reduced volume of data ?”

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    $\begingroup$ People don't buy two separate samples of different materials and fabricate a thermocouple themselves, unless they have some very special requirement. They buy a ready-made device made from a standardized pair of materials. "Type K" (and the corresponding international color codes) tells you all the properties you might want to know. But if you are using a standard temperature probe, you set the meter to "Type K" and it "just works". $\endgroup$
    – alephzero
    Aug 22 at 15:03
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    $\begingroup$ Why does the weather forecast just give you a few numbers and not all the data so you can decide for yourself what the weather will be? $\endgroup$
    – DKNguyen
    Aug 22 at 19:19
  • $\begingroup$ @DKNguyen Exactly. The question reminds me of the (probably apocryphal) story of a car maker who hired a physicist to do an engineering job. Needing to know how much the exhaust system would expand when running hot, the guy spent a whole week researching how to calculate the expansion from the atomic structure of the various metal alloys involved, instead of just looking up the expansion coefficients in a materials handbook... $\endgroup$
    – alephzero
    Aug 22 at 20:14
  • $\begingroup$ It would only make sense if all couplings produced a valid thermocouple and you could get the manufactured combination. $\endgroup$ Aug 23 at 5:31
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    $\begingroup$ @Camion But does it really? You think that because you aren't a forecaster, but it's also a lot of work to rediscover the metal pair with the optimal output, chemical tolerance, and accuracy. Remember that the curves are not actually linear. Metrology, in particular, is always complicated. The point is that it's a bunch of needless work for people who don't have the time or expertise to do the job or will ever need that for any other task. Then you just end up buying one off the shelf anyways because an expert has already figured out all the optimized combinations and all their characteristics $\endgroup$
    – DKNguyen
    Aug 24 at 3:43
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For precision applications, the traces of residual elements in each individual heat of metal may make a tiny difference. So, there are different levels of certification depending on the variability of EMF output. I worked for a company that did a lot of in-house heat-treating ; I welded many chromel /alumel thermocouples together from coils of ordinary certified wire. I do not have any hard data, but for example when our thermocouples indicated 1000 F, it may have had a +/- 1 F accuracy; fine for heat-treatment. But precision calorimeters used in research need greater accuracy, possible to +/- 0.01 F accuracy, for that they need the high precision certified pairs.

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