A Performance Assessment of Current Formulations of Bare-Wire and Mineral-Insulated-Metal-Sheathed Type N Thermocouples

Abstract

The Type N thermocouple, at its introduction in the early 1980s, was intended to radically improve base-metal thermocouple measurements and would render other base-metal types obsolete, or so it was claimed. Almost 40 years on, Type K persists in being the thermocouple of choice, despite adequate opportunity for the uptake of Type N. The reasons for this may be many; however, recent research at is showing Type N, at least at low temperatures, is not nearly as stable as early claims made out. This study reports on the inhomogeneities in Type N thermoelements, which develop as a function of temperature and time in a selection of mineral-insulated-metal-sheath (MIMS) and bare-wire samples sourced from a range of manufacturers. Measurements were made using a linear-gradient furnace and high-resolution homogeneity scanner. It was found that Type N thermocouples, in both the bare-wire and MIMS format, are susceptible to significant deviations in Seebeck coefficient from the reference functions at temperatures between 100 °C and 950 °C. In fact, use at temperatures below 500 °C can result in measurement errors equal to or worse than for equivalently specified Type K. Consequently, the benefits of using as-supplied Type N, in terms of accuracy and longevity, are only fully realized at temperatures greater than 900 °C. Despite these findings, additional experiments revealed drift rates can be reduced by about a factor of four if thermal preconditioning between 600 °C and 900 °C is used on MIMS Type N, which is similar to the improvements seen for thermally preconditioned MIMS Type K.