Example Data Table
These values show common IEC Pt100 resistance points.
| Temperature (°C) | Pt100 Resistance (Ω) | Typical Use |
|---|---|---|
| -50 | 80.31 | Cold storage checks |
| 0 | 100.00 | Ice point reference |
| 100 | 138.51 | Steam and hot water checks |
| 200 | 175.86 | Oven and process checks |
| 400 | 247.09 | High temperature equipment |
| 600 | 313.71 | Industrial heat systems |
| 850 | 390.48 | Upper platinum RTD range |
Formula Used
The calculator uses the Callendar Van Dusen equation for platinum RTD conversion.
For temperatures at or above zero degrees Celsius:
R = R0(1 + A T + B T2)
For temperatures below zero degrees Celsius:
R = R0[1 + A T + B T2 + C(T - 100)T3]
Here, R is corrected resistance. R0 is resistance at zero degrees Celsius. T is temperature in degrees Celsius.
Two wire correction subtracts two lead resistances. Three wire correction subtracts lead imbalance. Four wire correction assumes lead effect is removed by the instrument.
Combined uncertainty is estimated with this root sum square method:
U = sqrt(sensor tolerance2 + meter error2 + installation allowance2)
How To Use This Calculator
- Enter the measured RTD resistance in ohms.
- Enter the correct R0 value, such as 100 for Pt100.
- Select the RTD standard or choose custom coefficients.
- Select the wiring method used by the measurement.
- Add lead resistance or lead imbalance values when known.
- Choose tolerance class, meter error, and installation allowance.
- Select the output unit and decimal precision.
- Press calculate, then review the result above the form.
- Use CSV or PDF buttons to save the same calculation.
RTD Resistance To Temperature Guide
RTD Basics
An RTD is a resistance temperature detector. It changes resistance as temperature changes. Platinum elements are common because their response is stable, repeatable, and well documented. This calculator converts measured resistance into temperature using practical correction options. It is useful during commissioning, troubleshooting, calibration checks, and panel maintenance.
Why Lead Compensation Matters
A two wire RTD includes sensor resistance and both lead wires. Long cable runs can add meaningful error. Three wire circuits reduce most lead error when wires match closely. Four wire circuits are best for precision work because the measuring current and sensing path are separated. The lead fields in this tool help remove estimated cable resistance before temperature is calculated.
Choosing Standards And Inputs
The default coefficients follow common platinum RTD behavior. Pt100, Pt500, and Pt1000 elements use the same curve shape but different nominal resistance. R0 is the resistance at zero degrees Celsius. A Pt100 has 100 ohms at zero degrees. A Pt1000 has 1000 ohms at zero degrees. Custom coefficients allow special probes, laboratory data, or vendor curves.
Reading The Result
The corrected resistance is the value used by the temperature equation. The resistance ratio shows corrected resistance divided by R0. The slope value estimates ohms per degree near the calculated point. It helps judge how much meter error affects temperature. The uncertainty field combines tolerance class, meter error, and installation allowance. It is an estimate, not a certificate.
Good Measurement Practice
Use a stable current source. Avoid self heating by using the lowest suitable excitation current. Check terminals for corrosion. Confirm the transmitter range before comparing values. Record sensor type, wiring method, ambient conditions, and instrument model. Repeat tests after moving leads or changing connectors.
Using Exports
The CSV export stores the numeric results for spreadsheets. The PDF export creates a compact report for field notes. The example table shows common Pt100 values. Use it for quick comparison, but always trust calibrated instruments for acceptance testing. For harsh plants, compare the calculated value with process history. Sudden differences may show moisture ingress, loose screws, insulation damage, wrong element selection, or a transmitter scaling mistake. Small disciplined checks prevent long outages and unnecessary probe replacement during critical production test runs.
FAQs
What does an RTD measure?
An RTD measures temperature by changing resistance. Platinum RTDs are widely used because their resistance curve is predictable and stable.
What is R0?
R0 is the RTD resistance at zero degrees Celsius. Pt100 uses 100 ohms. Pt1000 uses 1000 ohms.
Why is lead resistance important?
Lead resistance adds to the measured value. If ignored, it can make the calculated temperature higher than the real sensor temperature.
When should I use four wire mode?
Use four wire mode for precision checks, calibration work, and long cable runs. It removes most lead resistance influence.
Can this calculator handle Pt1000 sensors?
Yes. Enter 1000 as R0. The same platinum curve can be used when the sensor standard matches the selected coefficients.
What is the temperature offset field?
The offset field adds a known correction to the calculated Celsius value. Use it only when a valid calibration correction exists.
Why can results fall outside the normal range?
Wrong R0, wrong coefficients, damaged wiring, or incorrect lead correction can push results beyond the expected platinum RTD range.
Is the uncertainty value a calibration certificate?
No. It is only an estimate based on selected tolerance, meter error, and installation allowance. Use certified calibration for compliance decisions.