Under normal conditions, the loop resistance of thermocouples in service will gradually change over time. However, it is not uncommon to see the resistance of thermocouples start to quickly rise or even spike.
An unusual increase in thermocouple resistance can be caused by various factors including:
- Poor junction connections with T/C extension wire
- Extended use in oxidizing atmospheres
- Wire thinning
- Hot end junction failure
This sensor degradation can compromise thermocouple measurements and eventually lead to sensor failure. This can be avoided when using the THZ3 and TDZ3 Dual Input Smart HART Temperature Transmitters and the STZ Functional Safety Dual Input Smart HART Temperature Transmitters. They can be configured to monitor the thermocouple loop for resistance changes and alert the user before the sensor fails.
On installation, the transmitter captures a baseline resistance level and continuously monitors the thermocouple resistance. As shown above, the THZ3 and TDZ3 can be configured to send warnings or alarms once the resistance passes set levels. Detecting these changes in resistance levels can help detect and reduce sensor failures and a subsequent halt to processes.
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