Under normal conditions, the loop resistance of a thermocouple will gradually change with temperature. However, exposure to extreme high temperatures can cause the resistance to increase at a substantial rate. This can be a sign of several issues within your sensor. For example, high temperatures can release impurities and chemicals within the thermocouple wire insulation that diffuses into the thermocouple metal and changes its characteristics. This can possibly lead to resistance increases as the thermocouple insulation material is breaking down.
An increase in thermocouple resistance can be caused by other factors including:
- Wire Thinning
- 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 below, 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 to reduce sensor failures and a subsequent halt to processes.
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