What is duplex thermocouple?

Duplex temperature sensors are ones that have two actual elements and separate wiring available for each, but in one sheath. This allows an operator to switch over from an element that has gone bad to a good one, without having to remove the bad sensor from a thermowell or similar. There are also transmitters that can utilize both sets of wires from this type of device, use one as a primary, and then automatically switch over to the backup in case of failure. This would be used for very critical measurement points. You would use the standard extension wire for each type of sensor, or better yet a local transmitter, especially in the case of a thermocouple, as they are prone to noise effects.