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I'm currently using a cubic fit formula I found online to derive the temperature of a PT100/385 RTD using resistance. The formula is -247.29 + 2.3992*R + 0.00063962 *R^2 + 0.0000010241 *R^3 where R is Ohms. Does anyone know where I could find a similar formula for a 39250 RTD? I've tried to find one online, but I have had no luck.

I need to use this instead of the standard Linear one because I need more accurate readings at high temperatures.

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  • \$\begingroup\$ Where's the link to the datasheet? Does it give you three or more R-T values? You also need to specify whether the formula gives you °C or °F. There's an edit link under your question. Welcome to EE.SE. \$\endgroup\$ – Transistor Feb 28 '19 at 19:13
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Assuming you're talking about the old-fashioned so-called "American standard" RTD, they're seldom used these days. You can find Callendar-Van Dusen equation coefficients for those, and some other types, on this National Instruments web page.

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If your RTD is 50 ohms, just scale it (double the measured resistance) so it fits the 100 ohm equation.

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  • \$\begingroup\$ Thank you! Would you happen to know the variation of the Callendar-Van Dusen equation which solves for temperature with an input resistance? \$\endgroup\$ – Trubsworth Mar 1 '19 at 0:11
  • \$\begingroup\$ Ah, I would use a search to invert it but you could fit it to a polynomial for least squares error. I don’t think the results would be as good with the latter. \$\endgroup\$ – Spehro Pefhany Mar 1 '19 at 0:24

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