Mineral Insulated Thermocouples
The most popular insulation and thermocouple protection style for industry today is the mineral insulated, metal sheathed type (MI or MIMS). These are comprised of a seamless metal sheath enclosing highly compacted mineral insulant powder (typically, magnesium oxide) which supports and electrically insulates the thermocouple wires held inside.

Temperature ranges covered are from -200°C to +1,250°C, and the assemblies provide a very high integrity, compact, hermetically sealed, self-armoured, yet quite flexible construction, suitable for very arduous operating conditions. MIMS cables available range from two to six cores, and diameters from 0.25 to 10.8mm. There are many advantages with this construction. They include small size, ease of installation (they can be bent, twisted and flattened without impairing performance), good mechanical strength, excellent isolation of the junction from hostile environments, high long term accuracy and stability, fast response and good insulation resistance. They are also readily available off the shelf and are reasonably priced. They are thus ideal for accurate measurement in a very wide range of applications, including extreme environments, like high vibration and high pressure/vacuum.

Additionally, they allow the use of a wide range of outer sheaths and seal termination styles to suit tremendously diverse operating conditions. Sheath materials offered (usually 15% of the overall probe diameter in length) include mild and stainless steels, inconel and the Nicrobell alloys, selection being on the basis of application temperature range and environment. Finally, platinum-rhodium alloy sheaths are often used with platinum thermocouples. The finished assembly can be anything from a few millimetres long to tens of metres or more. Beyond this, all of the usual thermocouple alloy combinations are available as MI thermocouples - both rare and base metal types. Also, the measuring junction can be exposed, insulated from the sheath material or bonded to it (grounded - welded to the sheath tip). The former provides for fastest response; insulated versions (offering insulation resistance over 100Mohms) obviate ground loops on associated instrumentation by providing a floating output; and the grounded types offer earthed output with faster response to temperature changes.

On the down side, limitations can include problems due to the different thermal coefficients of expansion of the stainless steel sheath variants, for example, as compared particularly to the Type K and N thermocouple materials - sometimes leading to premature mechanical fatigue failure. Also ironically, with both the stainless steels and Inconel 600 sheaths, there are possibilities of material contamination problems due to vapour diffusion of the elements, leading to actual contamination of the thermocouple wires by the sheath material itself. There can also be problems relating to the ingress of water vapour, resulting in reduced insulation resistance and hence calibration instability and, possibly, again premature failure. This latter phenomenon, however, is really a matter of care in manufacture and repair.

Special sheathing alloys have been developed to deal with these limitations, particularly for higher temperature applications with Type K and N thermocouples. These include the Nicrobell B and C alloys, which take their cue from the inherent advantages of Type N thermocouple materials. Basically, Nicrobell B is Nicrosil with added magnesium and niobium which improve the oxidation and high temperature strength properties respectively of the alloy. Also, since it is inherently Nicrosil, the sheath has a much more closely compatible thermal coefficient of expansion especially with Type K and N thermocouple wires. In fact, there is a ten-fold improvement.

Together, this means that MI thermocouples using Nicrobell B can last four to six times longer than their stainless steel based alternatives. And, remembering the transmutation reductions achieved using Nicrobell B, sheaths constructed from this material also out-perform Inconel 600 in terms of long term drift due to thermocouple wire contamination.

A high chrome Nicrobell (Nicrobell C) is also available specifically designed for carburising environments. Although this has slightly poorer high temperature strength than Nicrobell B, it arrests chromium carbide embrittlement effects, and features all of the other advantages of Nicrobell.

In general, it is recommended that the smallest diameter mineral insulated metal sheathed thermocouples should be avoided if possible for very high temperature or corrosive environment measurements. There does seem to be a correlation between MIMS cable diameter and its survival and long term performance.


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