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Temperature measurement reliability challenges: Part 2

Hydrocarbon Engineering,

Degradation of sensor performance

According to Honeywell, temperature measurement reliability is directly linked with temperature sensor health. Sensors start degrading the moment they are manufactured. They are made by combining dissimilar metals and are prone to corrosion and degradation. When sensors are installed in tough industrial process environments, sensor degradation accelerates.

There are thousands of temperature sensors in difficult to reach locations in a process facility. This makes it critical to keep a continuous watch on their health. Operating the plant based on temperature readings with a poor sensor will result in faulty measurements leading to poor yield, and in extreme conditions, a plant shutdown.

Current temperature transmitters have the intelligence to learn the resistance characteristics of all their thermocouple sensors. This intelligence can be customised based on the unique baseline determined for every installation. This gives the users a real time view of sensor health instead of just sensor status information such as ‘good’ or ‘bad’.

An alert can be generated when the loop resistance crosses the trigger level. It can be made available locally in the device display as well as in the instrument asset management system to identify and implement necessary thermocouple maintenance or replacement. The sensor health trend can also be shown in the local display on a continuous basis, just like the process temperature display. This helps not only avoid bad PV measurements, but also optimises sensor health to minimise the risk of a plant shutdown.

Ambient temperature variation

Ambient temperature is one of the critical parameters impacting transmitter performance. Current transmitters employ an exclusive temperature sensor closer to the electronic module to monitor the ambient and cold junction temperature. This is used to adequately compensate for the variation in the process temperature measurement. In addition to this, the higher and lower limits are monitored to ensure that the device is not subjected to abnormal limits. Alerts are set for the higher ambient or cold junction temperature so that a bad PV condition is eliminated. This alert helps the plant to take timely action to avoid conditions that will degrade the life of the device.

Adapted from a report by Emma McAleavey.

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