# PT100 - 39250 Cubic fit formula

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.

• 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. – Transistor Feb 28 '19 at 19:13

## 1 Answer

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

• Thank you! Would you happen to know the variation of the Callendar-Van Dusen equation which solves for temperature with an input resistance? – Trubsworth Mar 1 '19 at 0:11
• 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. – Spehro Pefhany Mar 1 '19 at 0:24