RTD Two-, Three- and Four-Wire Connections
Compare RTD lead functions and lead-resistance compensation, identify a matching instrument input, and keep Pt100/Pt1000 sensor data separate from wiring topology.
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Compare lead arrangements
An RTD measures temperature through resistance. Two-, three- and four-wire arrangements change how the instrument deals with lead resistance; they do not change the sensor into a different nominal resistance type. Identify both the sensor characteristic and the input wiring mode.
| Arrangement | Measurement relationship | Instrument requirement |
|---|---|---|
| Two-wire | Both lead resistances contribute to the measured path. | An input and accuracy assessment appropriate to the complete lead resistance. |
| Three-wire | Instrument compensation uses the extra lead and assumptions about lead resistances. | A matching three-wire input; compensation depends on the instrument method and lead balance. |
| Four-wire | Separate excitation and high-impedance voltage-sense paths reduce lead-resistance influence. | A true supported four-wire measurement arrangement and correct terminal mapping. |
| Pt100 / Pt1000 | Nominal platinum sensor resistance at 0 °C. | Matching nominal resistance, characteristic curve and input configuration. |
Trace function before color
The conceptual four-wire path sends excitation through the sensor and senses voltage across it with a separate pair. Two leads may connect to each end of the sensing element. Follow the exact sensor and input-module diagram; color and lead count are not a universal terminal assignment.
- Read sensor type, resistance curve, tolerance and lead arrangement.
- Identify excitation, sense and return functions on the actual input.
- Confirm supported wiring mode and excitation settings in the instrument manual.
- Record any extension, junction or unequal lead condition.
Record the measurement limitations
Lead compensation does not remove every measurement error. Sensor tolerance, lead imbalance, self-heating, mounting and input configuration can still matter. Do not infer a passing calibration from a plausible room-temperature display.
Use test records to keep the sensor characteristic and instrument settings beside the reading. A transmitter converting an RTD signal to 4–20 mA introduces a separate interface; read current-loop wiring for that output side.