Maximizing the Lifespan of Type K and Type N Thermocouples: A Guide to Sulphur Exposure Prevention
Temperature measurement is a critical aspect of various industrial applications, demanding reliable and accurate solutions. Type K and Type N thermocouples are commonly employed for this purpose. However, their longevity can be compromised when exposed to sulphur, a prevalent element found in natural gas, lubricating oils, and greases. This blog post explores the reasons behind the degradation of Type K thermocouples and introduces Type N thermocouples as an alternative, offering insights on how to make them last longer.
Understanding the Common Failure Mode:
The most prevalent reason for Type K thermocouple failure is exposure to sulphur. Sources of sulphur include natural gas used for burners, new refractory materials, lubricating oils, and greases—all of which can contain sulphur. The impact of sulphur exposure is accelerated surface corrosion on the Nickel-Chromium (NiCr) thermocouple wire, resulting in a layer of scaly green corrosion on that wire.
The corrosion process advances, leading to oxidation of the chromium in the NiCr leg. As a result, the normally non-magnetic NiCr leg becomes magnetic, causing a decrease in thermoelectric voltage at the reference junction (cold junction). The more extensive the corrosion, the larger the negative voltage drift becomes, resulting in significant temperature measurement errors.
The phenomenon known as “green rot” also makes the NiCr leg more brittle, eventually leading to complete failure of the thermocouple.
Recommendation #1: 8 Gauge Type N Thermocouple:
Type N thermocouples go a long way to solving the problem of oxidization. The absence of aluminum and manganese in the negative conductor increases the stability of Type N thermocouples. Superior oxidation resistance is achieved due to the combination of higher levels of chromium and silicon in the positive leg. Similarly, the higher level of silicon and magnesium in the negative leg form a diffusion barrier of magnesium oxysilicates to protect it from rapid oxidation.
Recommendation #2: 8 Gauge Type K Thermocouple from 540°C to 1250°C:
For Type K thermocouples, it is recommended to use 8 Gauge within the temperature range of 540°C to 1250°C. This choice ensures the thermocouple is better equipped to withstand the harsh conditions associated with sulphur exposure.
The Role of Protection Tubes:
To protect both Type K and Type N thermocouples from sulphur exposure, the use of protection tubes is crucial. The protection tube acts as a barrier, shielding the thermocouple from corrosive atmospheres and preventing sulphur from reaching the thermocouple wire. It’s essential to regularly check protection tubes for any compromise to prevent sulphur exposure and subsequent corrosion.
Performing a Leak Down Test:
Before replacing a corroded thermocouple, it is recommended to perform a leak down test on the protection tube. This test helps identify any hairline cracks or leaks that could allow sulphur to penetrate, causing corrosion.
Choosing the Right Protection Tube Material:
While metal protection tubes offer resistance to physical damage, they are permeable to gases at medium and higher temperatures. In contrast, Hexoloy Silicon Carbide protection tubes are gas-tight, effectively isolating the thermocouple from the atmosphere. However, they may have limited physical impact resistance. The choice between metal and Hexoloy Silicon Carbide protection tubes depends on the specific application and the level of protection required.
In conclusion, both Type K and Type N thermocouples can offer superior resistance to oxidization, providing prolonged stability over time. Understanding and mitigating the impact of sulphur exposure is crucial for enhancing the longevity of these thermocouples. By implementing the recommended measures, businesses can ensure the reliability and accuracy of their temperature measurements, ultimately contributing to the longevity of their processes and equipment.
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