Type N Thermocouple - Technical Information

Type N - Nickel-Chromium-Silicon vs Nickel-Silicon

Thermocouple Type N, was billed as the revolutionary 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 (see Part 1, Section 2.4). It can thus handle higher temperatures than Type K (1,280°C, and higher for short periods).

Thermocouple Oxidation - Type N Thermocouples

Basically, oxidation resistance is superior 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. The device also shows much improved repeatability in the 300°C to 500°C range where Type K’s stability is somewhat 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 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.

Another claim by the more enthusiastic manufacturers and distributors is that it provides 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 sensor was slower than many anticipated, it is seeing ever greater use and this can only grow. There is now no doubt that it is indeed a fundamentally better thermocouple than its base metal rivals.

Designs for Type N Thermocouples

Mineral Insulated thermocouples are rugged sensors which can generally be bent, twistened or flattened to suit application requirements. The Type N conductors are insulated from one another and the sheath by very tightly compacted magnesium oxide powder. With an insulated junction, the insulation resistance between the conductors and sheath is in excess of 100 MΩ.

Swaged tip, reduced tip thermocouples provide a unique fast response. The reduced end also provides high strength, and low displacement, which can be the solution to an otherwise difficult temperature measurement application. The reduced tip sensors have the advantage of having a rugged large diameter sheath over most of the length and a low thermal mass, fast response reduced tip. Homogeneity and integrity are maintained in the transition from the sensing area to larger cable diameter.

Miniature Thermocouples down to 0.010" satisfy many demanding precision temperature measurement applications due to the response, size, displacement and robustness of these miniature assemblies.