RTD (Pt100 / Pt1000) Resistance to Temperature
The temperature a platinum RTD's measured resistance corresponds to, by the IEC 60751 Callendar-Van Dusen relation R = R0(1 + A T + B T^2) with A = 3.9083e-3 and B = -5.775e-7 per C, solved for T. R0 is the ice-point (0 C) resistance: 100 ohms for a Pt100, 1000 ohms for a Pt1000. A Pt100 reading 119.40 ohms is at 50 C, 138.51 ohms is 100 C, and 100.00 ohms is exactly 0 C. The inverse is exact at or above 0 C; below 0 C it drops the C(T-100)T^3 term, staying within about 0.02 C to -40 C. Assumes a lead-compensated (3- or 4-wire) reading -- uncompensated 2-wire lead resistance adds to R and reads hot. The sensor's calibration, tolerance class (A/B), and self-heating govern the field accuracy.
Formula and source
R = R0 (1 + A T + B T^2), A = 3.9083e-3, B = -5.775e-7 (IEC 60751), solved for T: T = (-A + sqrt(A^2 - 4B(1 - R/R0))) / (2B). R0 = 100 (Pt100) or 1000 (Pt1000).
IEC 60751 platinum RTD Callendar-Van Dusen resistance-temperature relation with the standard A/B coefficients, by name (not reproduced from a table); the sensor calibration and tolerance class govern.
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