RTD Resistance Temperature Calculator

Calculator Inputs

Example Data Table

Formula Used

Platinum RTDs are commonly modeled using the Callendar-Van Dusen relationship, referenced to the resistance at 0C (R0). The coefficients A, B, and C define the curve shape for a given standard.

• For t >= 0C: R(t) = R0 x (1 + A·t + B·t^2)
• For t < 0C: R(t) = R0 x (1 + A·t + B·t^2 + C·(t - 100)·t^3)

To compute temperature from resistance, the calculator uses an analytic inversion for the t >= 0C region when possible, and otherwise applies a Newton-Raphson iteration on the full equation.

How to Use This Calculator

1. Select whether you need temperature from resistance or resistance from temperature.
2. Choose a standard preset, or select Custom coefficients for A, B, and C.
3. Enter R0 for your sensor (typically 100 or 1000 ohms).
4. Set the connection type, then enter lead resistance per wire if applicable.
5. Provide the measured resistance or the temperature input, depending on your mode.
6. Click Calculate to display results above the form, then export CSV or PDF.

Professional Article

1) Why RTDs stay the accuracy benchmark

Platinum resistance temperature devices deliver repeatable measurements because platinum is stable and its resistance changes predictably. A Pt100 is 100 ohms at 0C, while a Pt1000 is 1000 ohms. Higher nominal resistance often improves resolution on long cable runs.

2) The Callendar-Van Dusen curve in practice

Industry conversions typically use the Callendar-Van Dusen model with coefficients A, B, and C. For temperatures at or above 0C, the equation is quadratic, which enables fast inversion. Below 0C, the cubic correction term improves fit for cold-chain, environmental chambers, and cryogenic test ranges down to about -200C.

3) Wiring choices and lead compensation

Two-wire installations add lead resistance twice, pushing computed temperature higher than reality. Three-wire circuits cancel one lead if the wires match closely, which is common in industrial transmitters. Four-wire sensing separates current and voltage paths and is preferred for calibration benches and long, high-accuracy runs.

4) Sensitivity and resolution expectations

A key metric is dR/dT, the resistance change per degree. Near 0C, a Pt100 changes by about 0.385 ohms per C for the common alpha value. If a meter resolves 0.01 ohms, the temperature step is roughly 0.026C.

5) Using R0 and coefficient selection correctly

R0 is not always exactly 100 or 1000 ohms after installation. Element tolerance classes and aging can shift it slightly. For best results, measure resistance at a known reference point such as an ice bath and update R0 accordingly. Use the preset that matches your sensor standard, or apply certified coefficients if provided.

6) Verifying results with back-calculation

When converting from resistance to temperature, it is good practice to compute resistance again from the resulting temperature and compare. A small mismatch indicates rounding or convergence limits; a large mismatch usually signals wrong wiring assumptions, incorrect lead values, or a sensor not matching the selected standard.

7) Calibration workflows and documentation

Calibration records typically include measured resistance, corrected RTD resistance, computed temperature, and sensitivity at each point. CSV exports support traceable logs, while PDF summaries are convenient for quality documentation. Consistent rounding and units reduce reporting errors.

8) Common pitfalls and quick checks

If resistance is below R0, temperature is typically below 0C. If temperature seems impossible, check units, connection type, and cable resistance. Confirm the range: most platinum RTDs operate from -200C to 850C. For fast troubleshooting, compare your value against typical points like 138.505 ohms at 100C for Pt100.

FAQs

1) What is the difference between Pt100 and Pt1000?

Pt100 is 100 ohms at 0C, while Pt1000 is 1000 ohms at 0C. Pt1000 provides a larger voltage signal for the same excitation current, often improving resolution and reducing relative lead-wire impact.

2) Why does the equation change below 0C?

Below 0C, platinum resistance deviates from a simple quadratic fit. The additional cubic correction term improves accuracy for subzero operation, which matters in refrigeration, environmental testing, and low-temperature laboratories.

3) How do I estimate lead resistance per wire?

Measure the loop resistance of a known cable length with a multimeter, then divide by two to estimate per-wire resistance. Alternatively, use manufacturer data in ohms per meter and multiply by installed length.

4) Is the three-wire correction always accurate?

Three-wire compensation assumes the two current-carrying leads have nearly equal resistance. If the leads differ significantly or the circuit is nonstandard, errors remain. For highest accuracy, use four-wire sensing.

5) What does sensitivity dR/dT tell me?

Sensitivity is the resistance change per degree. It helps translate resistance noise into temperature noise. Higher sensitivity means better temperature resolution for a given measurement uncertainty in ohms.

6) Why might my computed temperature disagree with a probe display?

Differences can come from probe class tolerance, self-heating due to excitation current, lead-wire effects, or the transmitter using a different standard curve. Verify R0, wiring method, and coefficients used by your instrument.

7) Can I use this for non-platinum RTDs?

The implemented model and presets are intended for platinum RTDs. Other materials can use different coefficients and ranges. If you have published A, B, and C values for your element, custom mode may approximate behavior, but validate with reference data.