Industrial Heating Magazine: How Long Should My Thermocouple Last?

William Nash and Eric Yeager – Cleveland Electric Labs; Twinsburg, Ohio

As a designer and manufacturer of thermocouples, the most commonly asked question we receive is: How long should my thermocouple last?

Unfortunately, the answer to this question is quite complex and is dependent on a host of variables, not all of which may be obvious. At the root of the complexity are often unseen or unanticipated factors that may have a major effect on the sensor’s usable lifespan. Essentially, every application has its own intricacies, and their impact on operational and environmental factors are not always considered or understood to specify a proper thermocouple design. To underscore the impact of often unseen or unknown factors, the following are a few application scenarios of which we have collaborated with our customers.

Application Example 1

Our first example is from an emergency call we received from a large company with multiple locations. They were suddenly seeing massive product deviations from their box-anneal operation. Their downstream product users were reporting their materials to be overheating by more than 100°F. The financial loss was substantial and growing by the day.

Responding immediately, we spent two days inspecting rows of furnaces, sensors and operational details. One of the questions asked of different operators was: “How often do you change out your Inconel protection tubes?” The operators’ repeated response was: “Our preventive-maintenance schedule calls for change-out once every 12 months.”

The tubes appeared to be in good condition, but upon closer examination they exhibited signs of unusual age. One of the metal protection tubes was removed from service and was cut off about 3 inches from the hot end. Looking into the cut piece, the tube appeared as a clogged artery lined with plaque. There was a very thick lining of inky-black carbon. This indicated that the tube had not been replaced in several years.

Upon reviewing the maintenance logs, it was discovered that there was a lapse in tube replacement going back over three years. In effect, new thermocouples were installed into contaminated tubes, causing the degradation of the sensors and generating greater than a 100°F shift in accuracy. In this scenario, thermocouple life, which should have been close to one year, was shortened to within a week.

Another example of unseen forces impacting the accuracy and life of a thermocouple occurred in a large steel operation. Out of the blue (or so it seemed), our customer was experiencing rapid failures of their high-value, Type-R platinum thermocouples. The situation was promptly investigated, and it was found that the internal conductor wires were melting near the junction. This is an unmistakable sign of silica contamination causing eutectic formation, which allows platinum to melt at very low temperatures (Fig. 1).

Fig. 1. Silica contamination and grain boundary slippage

Upon inspection, we found zero evidence of silica (SiO2 ) in the furnaces, and our customer believed they had made “no changes” in their operation in over a decade. Through a collaborative, open dialogue, we identified they were in fact now treating a product containing silica. As it turned out, the steel composition had seen a major change, which included a significant silicon content. Based on that revelation, a thermocouple was designed that was better suited to resist the infiltration of the silica, thereby protecting and extending the life of the assembly.. View More

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