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I'm heating a ferronickel wire, and I need a reliable measurement of temperature, I was using a thermocouple to do this measurement but the wire is cooled when it's touched by the thermocouple so now I want to estimate the temperature by the change of the wire resistance , I'm using the four-wire method to measure so it should be good enough.

Now I need to find 't' from this equation

RT = R20 [ 1 + alfa (t-20)]

I found the resistivity of the ferronickel to be:

0.86 ohm mm2/m at ºC

Which is quite accurate, but I can't find alfa

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    \$\begingroup\$ Can you calibrate it? For example, by placing a sample in a hot oil bath with a co-located thermocouple or RTD. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 2, 2016 at 20:37
  • \$\begingroup\$ can you link and example of that? Although I will really prefer to find the value of alfa. \$\endgroup\$
    – Luis Ramon Ramirez Rodriguez
    Commented May 2, 2016 at 20:42
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    \$\begingroup\$ No, I can't. It's an experimental determination of the value of \$\alpha\$. Measure the resistance at two widely separated temperatures and calculate. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 2, 2016 at 20:47
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    \$\begingroup\$ It may not be linear. Many metals do not have a constant alfa. There's a full table of Fe-Ni -- unspecified alloy -- at thermometricscorp.com/PDFs/… that specifies a 0.00518 coefficient \$\endgroup\$
    – Scott Seidman
    Commented May 2, 2016 at 21:16
  • \$\begingroup\$ "Balco" may prove a useful search term -- precisionsensors.meas-spec.com/pdfs/rtd.pdf specifies 0.518 to 0.527 % per degree C, which matches the previous comment \$\endgroup\$
    – Scott Seidman
    Commented May 2, 2016 at 21:18

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Balco is 70% Ni and 30% Fe. The alloy has a temperature coefficient of resistance of 0.00518, with better linearity than pure Ni. I don't know what your alloy is, but consider that a starting point.

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