Type N - A solution to Instability

Instabilities in thermocouples come in several forms. Firstly, there is long-term drift due to compositional changes caused by oxidation (or neutron bombardment in nuclear applications). For example, above 800°C, Type K thermocouples exposed to air may suffer oxidation effects that alter conductor homogeneity and introduce measurement errors of several percent. When mounted in sheaths with limited air volume, “green rot” can occur due to preferential chromium oxidation. Nuclear applications pose an additional challenge with transmutation of materials under neutron bombardment, again leading to EMF instability.

Secondly, short-term thermal EMF hysteresis is an issue in base metal thermocouples, particularly Type K, when cycled between 250°C and 600°C. Errors of 5°C or more are common here, especially around 400°C. These are typically caused by magnetic and structural inhomogeneities. Thirdly, in mineral insulated thermocouple assemblies, EMF shifts can result from migration of manganese and aluminium from the KN (negative) wire through the magnesium oxide insulant to the KP (positive) wire.

Type N materials were specifically engineered to combat these instabilities. Thanks to their alloy structure, Type N thermocouples offer better resistance to drift, hysteresis, magnetic effects, and even neutron-induced transmutation. The NP (Nicrosil) conductor contains higher levels of chromium and silicon, while the NN (Nisil) conductor contains elevated levels of silicon and magnesium. Together, these form a diffusion barrier that significantly enhances long-term performance.

Type N’s stability in nuclear applications is another major advantage — the absence of manganese, aluminium, and copper in the NN conductor eliminates the transmutation issue almost entirely. Temperature cycling hysteresis is also reduced dramatically. Over the range 200°C to 1,000°C (with peak deviation around 750°C), hysteresis errors are reduced to just 2–3°C, making Type N far more stable than its base metal counterparts.