Basically, a mineral insulated cable consists of 3 components: a seamless tube, thermocouple wires and magnesium oxide.
The seamless tubes are properly washed and cleaned before usage. The thermocouple wires are insulated with ceramic MgO beads and after that they are installed in the seamless tubes.
Then, the diameter reduction process is started. This process contains several steps. After reducing the diameter by a drawing dye, the tubes are then annealed (heat treatment) with a mixture of gases in order to restore the metallurgical structure. This process is repeated until the MI cable has the desired outer diameter.
After the manufacturing process, the MI cable is tested extensively on cross-section, outer diameter and surface cracks. After this, the MI cables are calibrated in order to make sure the quality is class 1 or higher according to IEC or ASTM standards.
Why should it be calibrated? The wires that are used in the production are already calibrated, so it seems to be double work. It is not. The wires used to make a certain MI cable are reduced in several steps to create a compact cable. After every production step, the cable is annealed, putting the wires back into the original state of grains, which keeps the wires (and the cable) flexible. Still, all of these reducing steps do affect the EMF values of the wire. After the MI cable has been finished, it is calibrated to make sure that all these reducing steps did not affect the EMF values too much. Calibration is done to ensure they are still in line with IEC 60584 Class 1 or better.
The purpose of calibration is to ensure that the EMF values of the finished product, the MI cable or finished thermocouple element, are in line with the accepted tolerances per the indicated standard. IEC 60584 class 1 for example, or ASTM E 230 special limits.
Okazaki calibrates the finished product at five temperatures: 200°C, 400°C, 600°C, 800°C and 1000°C.
According to the standard to which Okazaki calibrates, IEC 60584 class 1, only three temperatures are mandatory on the bare, unused wire, to ensure that the bare unused wire is within the IEC 60584 class 1.
Read about the advantages of mineral insulated thermocouple cables over other conventional thermocouple cables.