Choosing the best industrial thermocouple for your application

01 September 2007

Ken Jefferies of Labfacility explains the differences between Type K and N thermocouples and how to select the appropriate one.

The majority of industrial applications still specify the Type K thermocouple for temperatures up to 1100oC but there is an alternative. The type K thermocouple, suitably sheathed is a stalwart amongst industrial temperature sensors; it has been the thermocouple of choice for decades and remains a sound, versatile option for higher temperatures.

Many existing installations utilize type K wiring (extension or compensating cable), connectors and appropriate instrumentation. Also, type K is specified historically in many cases. For these reasons, a change to an alternative sensor type would be unviable in many cases.

Type K material is available in standard Class 2 and Class 1 tolerance and, in MI (mineral insulated) form; with various sheath material options including stainless steels, Inconel 600 alloy (™) and others to best suit the particular application. The documented problems with type K relate to the thermoelectric instability with long exposure to elevated temperatures and sensitivity to neutron irradiation in nuclear reactor environments causing molecular changes. Basically, a drift in “output” (thermal emf) can be observed over time and this is explained later.

An alternative type N thermocouple (Nicrosilnisil) provides a similar (not identical) thermoelectric characteristic with much improved stability and therefore a much longer operational lifetime. The use of a type N sensor, like any other type does require the use of specific cables, connectors and instrumentation all of which are readily available.

The Nicrosil-Nisil thermoelement (Nickel-Chromium – Silicon / Nickel Silicon) is typically sheathed in Inconel or Pyrosil which offer good protection against many media. The assemblies are ideally suited for accurate measurements in air up to 1200oC. In vacuum or controlled atmosphere, they can withstand temperatures in excess of 1200oC. The sensitivity of 39µV/oC at 900oC is only slightly lower than that of Type K (41µV/oC). Interchangeability tolerances are the same as for Type K.

Thermocouple drift known as thermoelectric instability is a feature of some base-metal thermocouples; it is displayed in various forms (Ref 1).

i) A gradual and often cumulative drift in thermal emf on an “ageing” basis i.e. repeated and/or sustained use of the thermocouple at elevated temperatures. It is due largely to molecular changes in the alloys caused by oxidation.
ii) A short-term, cyclical change in thermal emf during heating and cooling in the mid to high temperature range 200oC to 700oC. The cause of this is believed to be molecular change due to magnetic effects.
iii) Non-cyclical, short and long term variations in the transfer characteristics of the thermocouple (emf versus temperature) due to structural magnetic variations.

Various research indicates that type K thermocouples appear to be significantly more unstable as the applied temperature rises through 1000oC. Type N on the other hand, especially in oxidizing atmospheres, displays a far superior thermoelectric stability characteristic similar to that of the rare metal types (R&S) up to 1200oC.

The high stability of Type N alloys relative to other base metal thermocouple alloys is due to the fact that their molecular structures substantially reduce the main causes of thermoelectric instability described above.

This is achieved mainly by increased levels of chromium and the addition of silicon to the nickel base to modify the oxidation modes and by the use of silicon and magnesium which form oxidation inhibiting films¹

The extension cable insulation for Type N is IEC-584 colour coded as pink outer, pink positive and white negative. Labfacility offers a wide range of Type N thermocouple assemblies, cables and connectors from stock.

Ref 1 N. Burley, Nicrosil/Nsil Type N Thermocouple (1983)

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