There are two types of thermocouple construction used most commonly.These are Mineral Insulated (M.I.) Thermocouples & Non M.I. Thermocouples.
Mineral Insulated Thermocouples:
Magnesium Oxide insulated thermocouples, are used in many process and laboratory applications. They are rugged in nature and bendable, and their fairly high temperature ratings make MgO thermocouples a popular choice for a multitude of temperature measuring applications.
MgO sensors are constructed by placing an element or elements into a sheath of a suitable material and size, insulating the elements from themselves and the sheath with loose filled or crushable Magnesium Oxide powder or insulators, and then swaging or drawing the filled sheath down to its final reduced size. The swaging process produces an element with highly compacted MgO insulation and provides high dielectric strength insulation between the elements themselves and their sheath.
Mineral insulated Thermocouples consist of thermocouple wire embedded in a densely packed refractory oxide powder insulate all enclosed in a seamless, drawn metal sheath
(usually stainless steel).
At one end cores and sheath are welded from a “hot” junction. At the other end, the thermocouple is connected to a “transition” of extension wires, connecting head or connector.
Non M.I. Thermocouples
In Non-M.I. thermocouples, thermocouple wires are either insulated with ceramic beads or after insulation of ceramic, covered by a metal sheath (usually stainless steel) and some form of termination (extension lead, connecting head or connector for example) is provided. In this type of construction thermocouple wires are protected from the measuring environment when a sheath protection is provided. The sheath material is dependent on the measuring environment usually stainless steel is used. According to the corrosive environment sheath selection is changed.
This construction does not provide flexibility & not found in small sizes. Not too good mechanical strength.
In Non M.I. construction sheath may be of ceramic or metal as per suitability.
Exposed, Grounded and Ungrounded all types of junctions are formed in both the M.I, & Non M.I. construction.