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Sparky tools

RTD Lead Error Calculator

How much error does the cable add to this RTD reading?

Data: Pt100 sensitivity of about 0.385 ohm per °C and the 3-wire mismatch model as published by ICS Schneider. IEC 60751 edition not confirmed. About standard editions

For reference only. Verify calculations against your instrument specifications and site requirements. Assumes matched copper lead conductors.

RTD Configuration

How far apart the two compensated leads are. 0 means matched leads, which cancel.

2-wire Wiring

    RTD Sensor          Transmitter
   ┌───────┐          ┌───────────┐
   │  Rt   │──────────│ I+        │
   │       │  Lead R   │           │
   │       │──────────│ I-        │
   └───────┘          └───────────┘

Both lead resistances add directly to the RTD reading. No compensation.

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Questions

Why does cable length matter on a 2-wire RTD?
On a 2-wire connection the instrument cannot tell the leads from the sensor, so both leads add to the reading. A Pt100 changes about 0.385 ohm per degree, so every 0.385 ohm of lead reads as about one degree high.
Does a 3-wire connection remove the error?
It cancels the lead resistance when the two compensated leads are matched. What is left is the difference between them. Enter a lead mismatch to see that error. 0% means perfectly matched.
When should I use 4-wire?
When the reading has to be as good as the sensor itself, or the run is long. The tool treats a 4-wire connection as having no lead error.
Pt100 or Pt1000?
A Pt1000 changes ten times as much per degree, about 3.85 ohm. The same lead resistance gives a tenth of the temperature error.
What cable length do I enter?
The one-way length from the sensor to the instrument. The tool counts both conductors of the loop.
Why does the tool ask for the ambient temperature?
Copper resistance rises with temperature, about 0.39% per degree. The tool works out the cable resistivity at that temperature from 17.24 mΩ·mm²/m at 20 °C.