Timeline for Will thermocouples sharing a common wire create inaccuracies during measurement?
Current License: CC BY-SA 4.0
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| Dec 25, 2022 at 13:07 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Jul 29, 2022 at 7:02 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Sep 3, 2020 at 18:00 | history | tweeted | twitter.com/StackElectronix/status/1301580724001218560 | ||
| Oct 25, 2018 at 6:31 | comment | added | KalleMP | Loop/branch currents may cause volt drops and become a significant source of error. I feel for you, this is an off label use of thermocouples. I would look to research where people have implemented single wire junctions onto the work piece and read how they have managed it. They have the advantage that the work piece is essentially 0 Ohms where you have the additional possible (small) volt drops in your common wire. | |
| Oct 25, 2018 at 5:43 | comment | added | jonk | I wish I had written better and I wish I knew for sure. Your question requires physics, or perhaps what may also be called "microelectronics." The free electron Drude model and solid state lattice screening effects are driving some thoughts. Regardless, the upshot I think is that the forces of electric repulsion are unimaginably strong and have very very long range effects on the conduction band (Fermi exclusion forces a band) electrons. So effects at one junction are almost instantly felt at other junctions and modify an otherwise undisturbed measurement. | |
| Oct 25, 2018 at 3:31 | comment | added | user43655 | Thank you @jonk, I appreciate the explanation. I'm glad the measurements are working properly then :) | |
| Oct 25, 2018 at 0:45 | answer | added | Jasen Слава Україні | timeline score: 1 | |
| Oct 24, 2018 at 23:38 | comment | added | jonk | This means that there are very long ranging effects, as the electrons redistribute throughout the entire system. I almost cannot see a way that the highest temperature joint would not affect the others. | |
| Oct 24, 2018 at 23:36 | comment | added | jonk | Bonding them together locks the Fermi levels together, so to speak. So their "energy distance" relative to "vacuum, far away from the nucleus" moves about, instead. Since the Fermi levels are locked tight now, it's instead the difference between one metal's energy distance to vacuum, vs the other metal's, that the thermocouple measures. The sea of electrons in the conduction band will distribute immediately throughout all of the metal structures, regardless of kind of metal, such that they maintain a cloud with its electron particles as far apart from each other as can be achieved. | |
| Oct 24, 2018 at 23:36 | comment | added | jonk | As I understand it, how this works is that the dissimilar metals, bonded, force their Fermi levels together all the time. Physicists use \$\infty\$ as their "zero-reference" for energy. For atoms, this means "vacuum, far away from the nucleus." The work functions and Fermi levels and so on are all referenced against this as the default zero-energy point. Differing metals have different Fermi levels. And these Fermi levels for a given metal are different at any given different temperature. | |
| Oct 24, 2018 at 23:07 | comment | added | user43655 | @jonk We are purchasing pre-made cables which are like this. My best guess as to why is for lower cost because they will get broken and need replacement somewhat often. I know the company will join these two metals together (from what I can tell they are soldered together). | |
| Oct 24, 2018 at 21:31 | comment | added | jonk | I think I know what's happening, but I'm struggling to describe the physical model to you. Before I try, are you creating your own combo system by fusing the dissimilar metals yourself at those junctions? (Not buying pre-made thermocouples, but actually using a supply of the right kind of both metals and forming a junction by some kind of weld you make yourself?) Or is this a different setup? | |
| Oct 24, 2018 at 21:09 | history | asked | user43655 | CC BY-SA 4.0 |