Type N Thermocouple - technical Information

Thermocouple Type N, is a relatively new thermocouple type and was billed as a replacement for the Type K thermocouple (the most common in industrial use), but without its drawbacks - Type N (Nicrosil-Nisil) exhibits a much greater resistance to oxidation-related drift at high temperatures than its rival, and to the other common instabilities of Type K in particular, but also the other base metal thermocouples to a degree. It can thus handle higher temperatures than Type K (1,280°C, and higher for short periods).

Oxidation resistance is superior in Type N because of the combination of a higher level of chromium and silicon in the positive Nicrosil conductor. Similarly, a higher level of silicon and magnesium in the negative Nisil conductor form a protective diffusion barrier. Type N also shows much improved repeatability in the 300°C to 500°C range where Type K’s stability is more lacking (due to hysteresis induced by magnetic and/or structural inhomogeneities). High levels of chromium in the NP conductor and silicon in the NN conductor provide improved magnetic stability. Beyond this, it does not suffer from other long term drift problems associated with transmutation of the high vapour pressure elements in mineral insulated thermocouple assemblies (mainly manganese and aluminium from the KN wire through the magnesium oxide insulant to the KP wire). Transmutation is virtually eliminated since the conductors contain only traces of manganese and aluminium. Finally, since manganese, aluminium and copper are not used in the NN conductor, stability against nuclear bombardment is much better.

Standardised in 1986 as BS EN 60584-1 Part 8 and subsequently published in IEC 60584, this relative newcomer to thermocouple thermometry has even been said to make all other base metal thermocouples (E, J, K and T) obsolete. It can be said to provide many of the rare metal thermocouple characteristics, but at base metal costs. In fact, up to a maximum continuous temperature of 1,280°C, depending on service conditions, it can be used in place of Type R and S thermocouples (which are between 10 and 20 times the price).

Although adoption of this thermocouple type was slower than anticipated, it is now seeing ever greater use. There is no doubt that it is a fundamentally better thermocouple than its base metal rivals.