Useful thermocouple life is a very 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. The very 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.
Decalibration and Drift - Stability
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 thermoelement 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 thermoelement wire.
Because the thermoelectric emf resulting from a given temperature difference is sensitive to changes in the chemical and metallurgical properties of the wire, the total emf produced by a used probe can be different from an otherwise identical new one under the same conditions. The changes are usually small (often negligibly small) over appreciable periods of time. But under adverse conditions, it is possible to realise large drifts at rapid rates.
Achieving Long Reliable Thermocouple Life
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 thermoelements themselves.
What Can Go Wrong With Thermocouples
Protecting tubes, sheaths, and even thermowells can fail due to corrosion or mechanical damage. Processes can go over temperature and exposure thermoelements to higher than anticipated temperatures. If a sensor controlling a process drifts low in its output, the process, in response to its controller may as a result be forced to temperatures higher than intended. Base metal assemblies are vulnerable to attack by a number of chemical agents. They can also be altered by unfavourable operating conditions. As supplied, noble metal thermocouple wire of good quality has very low impurity levels. Consequently, it is quite susceptible to contamination that can effect its thermoelectric properties. Platinum is especially sensitive to the presence of free silicon, with which it can combine to form a eutectic alloy that will melt at or below normal service temperatures. High-purity insulators and protecting tubes for precious metal assemblies as well as careful attention to cleanliness in handling are therefore essential to help prevent this.
Human error can be a contributing factor as well. Controls may be improperly set, connections may be improperly made, and inappropriate action in response to the operating conditions may be taken by mistake. Redundancy in instrumentation combined with training and responsibility are the usual means to combat these kinds of errors.