Making Sense of Sensors Part 1- Service Life

Service Life

Useful thermocouple life is a difficult prediction to make, even when most of the details of an application are known. And unfortunately, such information is often very hard to determine. Below, Pyrosales address some of the conditions to be aware of when attempting to determine replacement schedules. The best test for any application is to actually install, use, and evaluate the in-use performance of a design that is thought likely to succeed. The recommendations, and non-recommendations, listed under the thermocouple type descriptions are a good place to start when first selecting an assembly style to install in a process.

This Series will explore several types of thermocouples including: Type B, Type C, Type E, Type J, Type K, Type N, Type R, Type S and Type T. 

De-calibration and Drift

All thermocouples are subject to calibration drift with use. It is just a matter of how much, and how fast this may happen. Thermocouple performance is critically dependent upon absolute uniformity of both physical and chemical properties along the entire length of the circuit. When thermo-element materials are produced, careful steps are taken to assure that this uniformity (or homogeneity) is achieved. In use, different parts of the circuit experience different conditions of heat, chemical exposure, etc., and as a result, such parts actually do change in physical structure and composition from the original thermo-element wire. 
 

What can go wrong?

Mechanical Damage – Protecting tubes, sheaths, and even thermowells, can fail vibration, thermal shock and mechanical wear.

Temperature Overshoot – Processes can go over temperature and exposure thermo-elements to higher than anticipated temperatures, damaging the elements.

Human Error – Controls may be improperly set, connections may be improperly made, and inappropriate action in response to the operating conditions may be made by mistake.
 

Troubleshooting

The best approach to solving a problem is to question system performance: Is the performance reasonable for the conditions? Do changes in the controls produce a logical result? What about the product? Does its condition correspond with what the instruments are saying?
 

How to test a used thermocouple

Redundancy – ‘Probe’ the location by placing a new thermocouple that has a known output alongside the suspect one in an operating process and compare the readings.

Remove and replace – If it is not practical to have two sensors in place at the same time, remove the suspect probe and replace it with another one known to be in working order, and compare readings.
 

Tips

To achieve long and reliable thermocouple life, the usual strategy is to operate the device comfortably under its maximum temperature, and provide it with the cleanest possible environment in which to work. Enclosures, such as sheaths, protecting tubes, and thermowells, are the usual means of controlling the conditions that actually surround the thermo-elements themselves. Redundancy in instrumentation combined with training and responsibility, are good process practices! 

Continue reading… Part 2