Two-Wire vs. Four-Wire RTD Systems

Two-wire and four-wire RTD (Resistance Temperature Detector) systems are both used to measure temperature based on the change in electrical resistance of an RTD sensor. However, the key difference lies in how they handle lead wire resistance, and this fundamental distinction leads to inherent errors in two-wire systems compared to their four-wire counterparts.

In a two-wire RTD system, the sensor is connected to the measurement instrument using only two wires. This poses a measurement accuracy challenge because the resistance of the lead wires themselves adds to the overall resistance measurement. This added resistance can introduce errors, especially when dealing with long lead wires or when the resistance of the leads is significant compared to the RTD sensor's resistance.

It is important to note that since CTC's TR series RTD sensors are dual output vibration and temperature sensors, they actually require a four-pin connector and a four-conductor cable (minimum) to accommodate a vibration, common, and two RTD terminals. 

CTC connectors which are compatible with two-wire RTD systems have the following pinout: 

1 = Vibration Signal (+) (Red)
2 = Common (-) (Green)
3 = RTD (White)
4 = RTD (Black)

Shown Above: CTC's M4TR2 Connector with five-socket M12 molded polycarbonate connector, for use with two-wire RTD sensors

The temperature measurement error introduced per foot is static and can be calculated based on the following approximate values:

CB119 Error (°C) / ft. ≈ 0.235984

CB218 Error (°C) / ft. ≈ 0.336688

The below table can be used as a reference for the approximate error in (°C) introduced by using a two-wire RTD system at varying lengths:

It is important to note that only the temperature measurement will be impacted by this error value, and the vibration readings will not be impacted. Further, the above values are only an approximation and apply to a sensor at room temperature. The actual error could be more significant depending on the actual environmental temperature since the RTD element is not perfectly linear and temperature will affect the resistance of the cable conductors.

On the other hand, a four-wire RTD system uses two additional wires also connected to RTD terminals. The purpose of these additional conductors is to allow a compatible system to fully compensate for the described cable conductor resistance. There are a few different ways this is done and will depend on the system being used, but the four-wire configuration in essence eliminates the errors introduced by the resistance of the wires. This makes four-wire RTD systems more accurate and reliable, particularly in applications where precise temperature measurements are crucial. It is important to note that this functionality only applies to RTD measurement systems/equipment that support four-wire configurations. Simply using a six-conductor cable with a system that only supports two-wire RTD configurations will not result in any benefit.

To accommodate this kind of configuration, CTC uses a four-pin connector with a six-conductor cable, where each RTD output has two conductors tied together.

CTC connectors compatible with four-wire RTD systems have the following pinout:

1 = Signal (+) (Red)
2 = Common (-) (Black)
3 = RTD (L1, L2) (Green and Black)
4 = RTD (L3, L4) (Blue and Brown)

Shown Above: CTC's M4TR4 Connector with five-socket M12 molded polycarbonate connector, for use with four-wire RTD systems

This six-conductor setup can accommodate three-wire RTD measurement configurations as well, by simply omitting the use of one of the extra conductors (white or brown). Since all of the conductors within the above CTC cable are identical, a three-wire configuration can produce results very close in accuracy to a four-wire configuration. So, if you have a system that supports a three-wire configuration instead of a four-wire configuration, this can be a perfect solution. 

In summary, an inherent error in two-wire RTD systems is primarily due to the combined measurement of both sensor and lead wire resistance. Four-wire RTD systems overcome this limitation by fully compensating for the additional lead wire resistance, resulting in more accurate and reliable temperature measurements. CTC recommends that two-wire RTD systems be used only if the online monitoring system cannot accommodate a four-wire RTD system. In such a case, CTC also recommends keeping the cable length as short as possible. If the online monitoring system can accommodate a four-wire RTD system, this method is always preferred.