Connection Impedance
Unwanted electrical impedance in connection wires introduces measurement errors in low-resistance sensor circuits. This parasitic lead resistance arises from the finite conductivity of the cables connecting a resistance temperature detector to a transmitter. The added resistance is interpreted by the transmitter as a higher temperature, which causes a positive measurement bias.
Measurement Distortion
Standard copper conductors of long lengths can add several ohms to the circuit, which alters the sensor reading. For a standard Pt100 sensor, an error of just one ohm equates to a temperature error of nearly two and a half degrees Celsius. This level of distortion is unacceptable in industrial process control.
Compensation Architecture
Multi-wire bridge configurations are used to measure and mathematically subtract the lead effects from the final output. In a three-wire configuration, a separate lead is used to measure the line resistance, which is then subtracted by the transmitter circuitry. A four-wire measurement provides the highest accuracy by separating the current excitation path from the voltage measurement path, which eliminates the influence of parasitic lead resistance entirely.
Cable Length
System designers must minimize cable runs and select appropriate wire gauges to control these electrical errors. This precaution is particularly important when deploying passive sensors over long distances in large-scale plants.
