Thermocouple wire resistance per metre

Three verified Type K set points: φ0.025 mm at roughly 1,900 Ω/m, φ0.5 mm at about 5 Ω/m, and φ1.0 mm at about 1.2 Ω/m. Those three are consistent with a conductor resistance that scales inversely with the square of the diameter, the relationship the tool uses to reach any other gauge, including the φ0.5 mm anchor it is scaled from.

Within a few per cent, and the tool says so rather than implying more. Run the inverse-square relationship back against the three set points and it returns 1.25 Ω/m where φ1.0 mm measures about 1.2 Ω/m, and about 2,000 Ω/m where φ0.025 mm measures about 1,900 Ω/m, a spread of roughly 4 to 5 per cent. Treat the output as an estimate for sizing and comparison, not as a certificate value.

Because a thermocouple generates millivolts, not volts. A long run of fine wire adds series resistance that attenuates the signal before the instrument sees it, a φ0.025 mm Type K conductor at around 1,900 Ω/m is the extreme case. That is why runs longer than about 10 m should use φ0.5 mm or larger, or switch to extension-grade compensating cable.

For a two-wire thermocouple circuit, yes, the current has to travel out and back, so both legs contribute. The tool multiplies by two for that reason. Instruments also have an input resistance limit, and the total loop figure is what has to stay inside it.

It does. Conductor resistance rises with temperature, so a hot run is a little more resistive than the same wire cold. For checking a run against an instrument's input limit the cold figure is the practical one to start from; where a loop runs very hot over a long distance, treat the estimate as the optimistic end of the range.