The Pt100 curve
A Pt100 is a platinum resistance element with 100 Ω at 0 °C (a Pt1000 has 1000 Ω). Its resistance rises by roughly 0.385 Ω per °C, but not perfectly linearly, so the Callendar–Van Dusen equation is used:
R(T) = R₀ × (1 + A·T + B·T² + C·(T − 100)·T³) for −200 to 0 °C
A = 3.9083 × 10⁻³, B = −5.775 × 10⁻⁷, C = −4.183 × 10⁻¹²
Quick reference (Pt100)
| Temperature (°C) | Resistance (Ω) |
|---|---|
| -200 | 18.52 |
| -100 | 60.26 |
| -50 | 80.31 |
| 0 | 100.00 |
| 25 | 109.73 |
| 50 | 119.40 |
| 100 | 138.51 |
| 150 | 157.33 |
| 200 | 175.86 |
| 300 | 212.05 |
| 400 | 247.09 |
| 500 | 280.98 |
2-, 3- and 4-wire connections
The transmitter measures the element and the lead wires. With a 2-wire connection, lead resistance adds directly to the reading: 1 Ω of total lead resistance on a Pt100 reads about 2.6 °C high. A 3-wire connection compensates for the leads as long as all three wires have equal resistance, and a 4-wire connection eliminates lead resistance from the measurement entirely.
Field check
To check an RTD, disconnect it and measure its resistance with an ohmmeter (subtracting the lead resistance on 2-wire runs), then compare with the calculator at the known process or bath temperature. An open element reads infinite resistance; a shorted element reads near 0 Ω, which a transmitter will interpret as far below −200 °C.