RTDs (Resistance Temperature Detectors) are offered with 2, 3, or 4 lead configuration. The best configuration for a specific application depends on a number of factors, however the sensor configuration must match instrumentation, otherwise lead-wire resistance cancellation circuitry may be ineffective.
Factors to consider:
- Cost of installation – more wires generally means higher cost
- Available space – more or larger wires require more space
- Accuracy requirements
2 wire RTD configurations may provide the required accuracy, especially with high resistance elements
2-lead constructions result in lead-wire resistance getting added to the element resistance. Consequently, the temperature reading is artificially high. The graph below shows the temperature error, from 2 leads of various sizes and lengths, for a 100-ohm platinum RTD at 100°C.
3-lead RTD constructions result in canceled lead-wire resistance error only if the instrumentation can measure true 3-wire resistance.
Lead-wire resistance error cancellation is most effective when all the lead wires have the same resistance. Using 3 wires of the same AWG, length, and composition will typically result in lead-wire resistances matched within 5%. The graph below shows the temperature error from lead-wire of various sizes and lengths, for a 3-lead 100-ohm platinum RTD at 100°C.
4-lead RTD constructions result in canceled resistance only if the instrumentation can measure true 4-wire resistance. True 4-wire resistance measurement will effectively cancel leadwire resistance error even if all 4 wires are not the same AWG, length, and/or composition…