When long conductors are connected with sensor electronics, which type of sensor is affected by the added resistance?

When long conductors are connected with sensor electronics, which type of sensor is affected by the added resistance?

• A. RTD
• B. Thermistor
• C. Thermocouple
• D. Hygrometer

Answer: (B)

When sensor leads are long, their resistance becomes part of the sensing circuit and can shift the measurement shown by the electronics. For resistive sensors, the readout often depends on the current through the element or on a voltage divided across it, so any extra series resistance from the leads changes that relationship. A thermistor is a high-resistance, nonlinear resistor whose value changes strongly with temperature. In typical readout schemes (like a simple current-excitation or voltage-divider setup), the lead resistance adds in series with the thermistor. Because the thermistor’s resistance can be large and changes nonlinearly with temperature, the added lead resistance can significantly distort the voltage (or current) that the converter interprets as temperature, producing notable errors, especially as the lead length increases. The other sensors behave differently: thermocouples produce a voltage that is largely referenced to the measurement system’s input impedance, so with a high-impedance readout the impact of lead resistance is minimal. RTDs, while also resistive, are commonly measured with techniques (such as four-wire/Kelvin methods) that cancel the effect of lead resistance. Hygrometers typically don’t rely on a simple resistive signal in the same way, so lead resistance is not their primary concern. So, the added resistance from long leads most strongly affects the thermistor in typical configurations.

When sensor leads are long, their resistance becomes part of the sensing circuit and can shift the measurement shown by the electronics. For resistive sensors, the readout often depends on the current through the element or on a voltage divided across it, so any extra series resistance from the leads changes that relationship.

A thermistor is a high-resistance, nonlinear resistor whose value changes strongly with temperature. In typical readout schemes (like a simple current-excitation or voltage-divider setup), the lead resistance adds in series with the thermistor. Because the thermistor’s resistance can be large and changes nonlinearly with temperature, the added lead resistance can significantly distort the voltage (or current) that the converter interprets as temperature, producing notable errors, especially as the lead length increases.

The other sensors behave differently: thermocouples produce a voltage that is largely referenced to the measurement system’s input impedance, so with a high-impedance readout the impact of lead resistance is minimal. RTDs, while also resistive, are commonly measured with techniques (such as four-wire/Kelvin methods) that cancel the effect of lead resistance. Hygrometers typically don’t rely on a simple resistive signal in the same way, so lead resistance is not their primary concern.

So, the added resistance from long leads most strongly affects the thermistor in typical configurations.