Types of Thermocouple Designs
2 main types of thermocouple designs are largely used M.I. thermocouple design i.e. mineral insulated thermocouple & the non M.I. thermocouple design.
Mineral Insulated Thermocouple
This is a thermocouple design which is a magnesium-oxide insulated one, also known as the MgO thermocouple. Such types of thermocouples are mainly utilized in multiple processes & lab applications. Available in all types of thermocouple elements, in many diameter ranges and many types of materials, they are inherently durable and flexible, and their quite high temperature rating makes them a standard selection for many temperature measurement applications. Many looked-for features make them an ideal selection for general as well as special use. An MgO sensor is constructed by setting the element(s) in the sheath of suitable material as well as size, thus insulating those elements from themselves and the sheath with loosely-filled/breakable magnesium-oxide powder/insulators, and after that pulling down /drawing the filled sheath to the final diminished size. The result of the swaging procedure produces the element(s) with largely-impacted MgO insulation, & also gives raised dielectric strength insulation among the elements themselves as well as their sheath. An M.I. thermocouple consists of thermocouple wire, which is embedded in tightly-packed refractory oxide powder insulation, all of these enclosed in a seamlessly drawn metal sheath (that is mostly of stainless steel).
Advantages of M.I. Thermocouples
- Small size & great flexibility; this allows temperature measurement in places with poor accessibility
- Nice mechanical strength
- Protection of the wires of the thermos-element against any corrosion or oxidation or even contamination
- Rapid thermal reaction
M.I. Thermocouples Tip Models
M.I. Thermocouple’s junction tip could be of 3 main types. This tip can either be isolated type or grounded type or even of the reduced type.
- Insulated Tip: The insulated hot-end junction is best-suited for many applications, specifically when low EMF reception is mandatory. The high insulation resistance is increased by the maximum density of ultra-pure MgO powder insulation
- Grounded Tip: Bonded/grounded junction provides quite a speedier temperature reaction than the other insulated types. This is not advisable for multipoint instrumentation.
- Reduced Tip: The reduced tip junction is best-suited for applications wherein low mass & very speedy reaction time is needed and high mechanical strength is required. Such a tip can be supplied for thermocouples from 1.0 to 6.0 mm in diameter.
Non M.I. Thermocouple
In such a thermocouple, the wires are either insulated ones with ceramic beads/after ceramic insulation, that is covered with metal jacket (mainly of stainless steel) & some type of termination (such as an extension, connector end, or plug). In such a sort of design, the leads or wires of the thermocouple are shielded from the measurement medium when protection is provided. The material of the sheath largely depends on the medium of measurement; mostly stainless steel is utilized. Depending on the corrosive environment, selection of the sheath is altered. This design doesn’t offer flexibility, and is not available in small sizes. This one does not have very good mechanical strength. In such types of thermocouples, the construction sheath could be ceramic or even metal depending on the fit. All types of open, earthed and unearthed junctions are created in the construction of both M.I. thermocouples & non-M.I. thermocouples, alike.
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