Thermocouple Type N: Type N (Nicrosil/Nisil) thermocouple has a Nicrosil (nickel-14%, chromium-1.5%, and silicon) positive leg, and a Nisil (nickel- 4.5%, silicon-1%, and magnesium) negative leg.
With a general application range of -270 to 1300°C (-450 to 2372°F), the type N thermocouple was developed to overcome several problems inherent in type K thermocouples.
This thermocouple has an outstanding thermoelectric stability that can be compared with platinum-based thermocouples. It has an excellent resistance to high temperature oxidation phenomena, and is ideally suited for accurate measurements in air up to 1300°C. In vacuum or controlled atmosphere, it can withstand 1300°C depending on sheath materials and diameter. Various experiments show that type N thermocouples behave similarly to platinum-based thermocouples, and they drift 10 to 20 times less than type K thermocouples between 1000 and 1200°C.
Also, type N has been found to be more stable than type K in nuclear environments, where type K has been the sensor of choice. High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B, R, S) types. It has better stability in nuclear applications than type K, and is becoming more popular.
In the temperature range 300 – 600°C, a crystalline phenomenon occurs in nickel-chromium alloys, which leads to a reversible thermocouple sensitivity drift. Even if a thermocouple is used in the 900 – 1300°C range, part of it is submitted to that 300 – 600°C range in the application temperature gradient (e.g. entrance of a furnace). Type N thermocouples are subject to this drift, but are at least 50 percent less sensitive to ageing than type K.
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Continue reading… Part 8