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I'm working on attaching to a thermocouple harness that had six 16AWG chromel wires and one 12AWG Alumel. The alumel would T off with each of the chromel wires to make a pairing and so the common was the same for each to give six measurements around the engine. I need to make a cable that will hook into this and go off to my box to do measurements. I'm having a hell of a time finding 12AWG of any flavor of type k thermocouple wire and 16AWG is possible but rare so expensive.

My question is would dropping the gauge to 16AWG or 20AWG for my cable affect the measurement significantly? It's only about 10ft of cable that I need and from what I've read the change in gauge adds about less than 10ohms/1000ft so that shouldn't be affected too much. Is there anything else to consider or avoid for this setup?

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https://solutions.borderstates.com/what-are-the-different-types-of-thermocouple-wire/

https://www.omega.ca/en/resources/thermocouple-wire

These websites state that the reason for a larger AWG is to reduce the wire resistance in longer runs. The resistance needs to be kept sufficiently low so that noise currents induced onto the wires do not produce a noise voltage that drowns out the thermocouple junction voltage from the Seebeck effect.

The longer the run or the more EMI in the area, the larger the AWG has to be.

Rule of thumb is to try to have a loop resistance of less than 100 Ohms. 20AWG works fine up to 100 feet in an area with no EMI.

12AWG would be expensive. That's a lot of nickel and chromium. Your thermal response would also be very slow due to high mass.

Note that you can filter thermocouples with resistors and capacitor. A series resistor on each line, and on the far side of the resistor away from the junction put a capacitor between the lines and a capacitor from each line to ground. Use C0G capacitors if possible since these are in the signal path (no piezo effects or DC bias effects). All placed as close as possible to the input, of course.

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  • \$\begingroup\$ So based on what you're saying is that as long as the total loop resistance is under 100 ohms then a change in gauge shouldn't affect the thermal reading? I'm only planning on making a 10 ft extension and using standard values for wire gauge if I dropped it to a more common 16AWG would be fine since the total loop resistance is less than 100 ohms. \$\endgroup\$ – BrownKuma Apr 24 at 16:22
  • \$\begingroup\$ I would not be worried about the length from the sounds of it. I would be more worried about the EMI but I don't have a feel for the noise around engine spark plugs. Can you just get a 6 foot piece of 20AWG To test? \$\endgroup\$ – DKNguyen Apr 24 at 17:17
  • \$\begingroup\$ You can also filter thermocouples BTW with resistors and capacitors. \$\endgroup\$ – DKNguyen Apr 24 at 17:29

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