Type K (nickel-chromium/nickel-aluminium) thermocouples are widely used in industry for measuring temperature in the range 200 C to 1000 C but drift by more than 5 C in the range 300 C to 500 C due to metallurgical ordering and disordering (the hysteresis effect). Nickel-chromium-silicon/nickel-silicon-magnesium thermocouples (known as Nicrosil/Nisil or Type N) were developed in order to overcome this problem, although they have not yet been widely adopted. Both Type K and Type N thermocouples, when used in their mineral insulated form with stainless steel or Inconel sheaths, can show large calibration drifts above 1000 C due to contamination from the sheaths.
Recently, Type N thermocouples have been developed which are sheathed in alloys similar in composition to Nicrosil and should have lower calibration drifts above 1000 C due to contamination. The project was aimed at investigating the high temperature stability of such thermocouples from two different sources under continuous exposure to progressively higher temperatures and conditions of temperature cycling. Measurements were also made with Inconel-sheathed Type K thermocouples for comparison purposes.
The measurements established the limits of temperature exposure to which mineral insulated metal sheathed thermocouples can be subjected without failing or drifting unacceptably: up to 1300 C for Type N thermocouples sheathed in Nicrosil related alloys or upto 1200 C for Type K Inconel sheathed thermocouples.