DESCRIPTION FIELD OF INVENTION
Embodiments of the present disclosure relate generally to a thermocouple and more specifically to industrial thermocouples with superior creep resistance. RELATED ART
A thermocouple is an electrical device consisting of two dissimilar electrical conductors that are fixed together at one end forming electrical junctions at differing temperatures. The electrical junction creates a voltage with a change in temperature at the junction and the voltage is measured which is then referenced to determine the temperature. Thus, thermocouples are being used to measure temperature in a number of processes.
FIG. 1A through ID are the diagrams illustrating conventional thermocouple in an example. It comprises a plurality of different conducting wires (102A and 102B) aligned within a tubular protective sheath material (104) wherein a porous ceramic material (106) is filled within the tube providing both electrical and thermal insulation for accurate sensing of temperature at the thermocouple junction (108). In an example, the conducting wires may comprise two or more lead wires made of at least one of chromel, alumel, platinum, platinum-rhodium, constantan, iron-constantan, nicrosil, nisil, copper, nickel-chromium and the like whereas the tubular protective sheath material comprises stainless steels, inconels, incolly and the like. The porous ceramic material comprises magnesium oxide (MgO) that acts as a mineral insulation providing both electrical and thermal insulation.
I However, conventional thermocouples fail to sense the temperature and gets damaged in a system with a regular usage on industrial scale due to corrosion attack and creep cavitation. Conventionally, various studies have been conducted on the frequent failures of thermocouples on industrial scale and those failure analyses concluded that there are four degradation mechanisms involved in the failure of thermocouples. The four degradation mechanisms comprise thermal degradation by grain growth and/or spheroidization, corrosion attack by sulphur, pure creep cavitation damage and creep-corrosion interaction. Though the corrosion attack by sulphur and other corrosive species is well addressed in conventional thermocouples, creep and creep-corrosion interaction are not well recognised in the industry across the world. [0005] FIG. 2A through FIG. 2D represents microstructural degradation of thermocouple wires by creep cavitation damage and creep-corrosion interaction in an example. FIG. 2A illustrates the

longitudinal microstructures of a thermocouple lead wire depicting O-type cavities coalesced to form W-type cracks predominantly along transverse grain boundaries leading to the failure of chromel/alumel wires in the thermocouple. In FIG. 2B, 210A and 210B illustrates the scanning electron microscopic images depicting creep cavities in the thermocouple lead wires whereas 210C and 210D represents coalescence of cavities to form macro cracks leading to transverse fracture of the thermocouple wires. FIG. 2C illustrates longitudinal microstructures of a thermocouple wire indicating intergranular corrosion attack aided by creep cavitation. FIG. 2D illustrates scanning electron microscopic images of a service used thermocouple sample from hydrogen cracker unit reactor bed wherein 220A represents fractured thermocouple lead wire and the sheath material and 220B illustrates skewed dimples depicting tearing mode of sheath failure. In FIG. 2D, 220C and 220D illustrates leopard skin morphology on faceted fracture surface suggesting creep cavitation damage of leads in the thermocouple. Thus, there is a need of an industrial thermocouple with superior creep resistance to prevent the conducting wires from corrosion and creep damage.
SUMMARY
[0006] According to an aspect of the present disclosure, a thermocouple (401) comprising a first and a second conducting wire (402A and 402B) forming at least a pair of conducting wires to sense a temperature in a system, an electrical junction (408) formed by joining the first and second conducting wires (402A and 402B) at one end, an outer sheath (404) to protect the thermocouple from external environmental factors, and a mineral insulation (406) providing both electrical and thermal insulation, wherein the plurality of first and second conducting wires (402A and 402B) are wavy in structure such that the surface area is increased with increase in the length of the conducting wires protecting the thermocouple (401) from creep and corrosion damage at higher temperatures in the system.
[0007] Several aspects are described below, with reference to diagrams. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the present disclosure. One who skilled in the relevant art, however, will readily recognize that the present disclosure can be practiced without one or more of the specific details, or with other methods, etc. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the features of the present disclosure.
BRIEF DESCRIPTION OF DRAWINGS

[0008] FIG. 1A through 1D are the diagrams illustrating conventional thermocouple in an
example.
[0009] FIG. 2A through FIG. 2D represents microstructural degradation of thermocouple wires by
creep cavitation damage and creep-corrosion interaction in an example.
[0010] FIG. 3 is a diagram illustrating a thermocouple with enhanced creep resistance in another
example.
[0011] FIG. 4A is a diagram illustrating a thermocouple with planar wavy conducting wires in an
embodiment of the present disclosure.
[0012] FIG. 4B is a diagram illustrating an electrical junction within the thermocouple with planar
wavy conducting wires of the present disclosure.
DETAILED DESTRIPTION OF THE PREFERRED EXAMPLES
[0013] FIG. 3 is a diagram illustrating a thermocouple with enhanced creep resistance in another example. In order to increase the creep resistance of the conventional thermocouples, the conducting wires (102A and 102B) are replaced with conducting spring like structures (302A and 302B) within the outer protecting sheath (304). Due to thermal expansion in the system at higher temperatures, the outer sheath material (304) expands naturally while tendency of the natural expansion of the conducting wires is restricted as it is pulled at one end at an electrical junction (308) where they are welded together. Thus, there is a tension in the wire which brings high stress at above the natural operating temperatures such as 500°C. Also, as there is a corrosive atmosphere in the system, the thermocouple gets failed to sense the temperature in the system and the conducting wires gets permanently damaged due to creep-corrosion effect. The spring structure of the conducing wires facilitate maximum length of the conducting wire to take the stress thereby reducing the overall stress applied on the conducting wires from the outer sheath (304). This leads to the reduction of corrosion-creep damage of the thermocouple. [0014] However, the time required by the thermocouple to sense the temperature using the spring like conducting wires is marginally increased as the length of the conducting wires are increased for approximately 2 to 3 times to that of the linear conducting wires in conventional thermocouples. Achieving the spring structure in the conducting wires is also difficult and is more complex in retaining the spring structure while in use. Besides the difficulty in achieving the spring structure in the conducting wires, it is also difficult to insert the spring structured conducting wires into the sheath material of the thermocouple when plurality of wires is being

used during the fabrication time. The fabrication of thermocouples with plurality of spring structured conducting wires poses difficulty as the spring structure lacks good bonding between the wires and the magnesium oxide insulation (306) when induced into the outer sheath (304) with good bonding between them to provide both electrical and thermal insulation. The poor bonding between the porous magnesium oxide ceramic material and the conducting wires leads to the damage of thermocouple when higher temperatures are received in the system. [0015] FIG. 4A is a diagram illustrating a thermocouple with planar wavy conducting wires in an embodiment of the present disclosure. As shown there, the thermocouple (401) of the present disclosure comprises a plurality of conducting wires (402A and 402B), a mineral insulation (404) and an outer sheath (406) wherein the conducting wires (402A and 402B) are having a planar waviness throughout their length coupled at one end forming an electrical junction. The plurality of conducting wires comprises at least two different metal composites such as Chromel/Alumel, Platinum/Platinum-Rhodium, Constantan/Iron-Constantan, nicrosil, nisil, copper and nickel-chromium in an example. The plurality of wavy conducting wires (402A and 402B) are embedded within the mineral insulation (404) covered by the outer sheath (406) as an outer protecting layer. The planar waviness of the embedded conducting wires (402A and 402B) accommodates thermal expansion mismatch and thereby reduces stress acting between the thermocouple materials comprising the plurality of conducting wires (402A and 402B), the mineral insulation (404) and the outer sheath (406).
[0016] Conventionally, outer sheath material possesses larger cross-sectional area and higher coefficient of thermal expansion (CTE) relative to the conducting wires and the mineral insulation. Exposing the thermocouple to a high operating temperature in a system leads to a significant expansion of the outer sheath material when compared to the conducting wires and the mineral insulation. The significant expansion of the outer sheath material imposes a high tensile stress on the thermocouple conducting wires in an axial direction and high compressive stress in a radial direction. However, the amount of tension in the conducting wires is restricted due to frictional drag between the conducting wires and the mineral insulation which result in a constant tensile stress on the conducting wires at elevated temperatures. This eventually leads to a creep damage and a slow degradation of the conducting wires. If thermocouple is using in a corrosive atmosphere, that corrosive species may accelerate the failure process of the thermocouple.

[0017] As the conducting wires (402A and 402B) in the thermocouple (401) of the present disclosure are having planar waviness in its structure throughout their length, the length of the conducting wires (402A and 402B) increases which accommodates axial thermal expansion and increased surface area for increased frictional drag when compared to the conventional thermocouples with linear shaped conducting wires (102A and 102B). Thus, thermal stress is significantly reduced thereby significantly increasing creep life of the thermocouple. [0018] FIG. 4B is a diagram illustrating an electrical junction within the thermocouple with planar wavy conducting wires of the present disclosure. In an embodiment, the mineral insulation (404) comprising magnesium oxide is filled within the tubular outer sheath (406) with plurality of holes throughout the length of the outer sheath. The wavy conducting wires (402A and 402B) are then placed inside the holes and then the holes are filled with the ceramic porous magnesium oxide material so that the bonding between the wavy conducting wires (402A and 402B) and the mineral insulation material (404) is increased. Once the wavy conducting wires and the magnesium oxide particles are tightly packed within the outer sheath, one end of the wavy conducting wires are joined together and crimped to form an electrical junction (408) whereas the other ends of the wavy conducting wires are coupled to a source of heat in the system. In an example, the end of the conducting wires may be welded together to form the electrical junction (408). The temperatures from the source are then fed to the wavy conducting wires and generates a voltage at the electrical junction (408) based on the difference in the electromotive force of the wavy conducting wires. The voltage determined by the electrical junction is again fed to the system which converts and digitalises it into a temperature. The thermocouple with wavy conducting wires provides an increased creep resistance with just 5% of increase in length of the conducting wires when compared to the conventional thermocouples. As the quantum of material used for the wavy conducting wires is nearly same as that of the conventional thermocouples, the response time is not varied as such in the thermocouples with spring structured conducting wires. [0019] Further, the waviness of the conducting wires offers a two dimensional i.e., surface or planar structural change in the conducting wires while the entire volume keeps changing in the spring structured conducting wires. The increase in the volume of the conducting wires in the spring type thermocouples results in failure in the systems at higher temperatures whereas the wavy conducting wires remain intact at the same higher temperatures in the system as the surface area gets increased with increase in the length.

[0020] In an embodiment, the thermocouple with wavy conducting wires comprises stainless steel as the outer sheath (404) with a high thermal coefficient of substantially ten times more cross-sectional area than that of the conducting wires. A porous ceramic magnesium oxide core is used as the mineral insulation (406) and is twice in cross-sectional area than that of the conducting wire. In another embodiment, the wavy conducting wires comprises two different metallic materials such as chromel and alumel with lower thermal expansion coefficient than that of the magnesium core and the outer sheath material.
[0021] In another embodiment, the outer sheath material (404) of the thermocouple of the present disclosure comprises an outer diameter of 8-12 mm with a typical thickness of 1.5-2.5 mm. In an example, the wavy conducting wires (402A and 402B) comprises the chromel and alumel wires with a diameter of 0.5-1.5 mm having a length of 0.5-5 m and wavelength of 5-15 mm. In another example, the thickness of the conducting wires is directly proportional to the wavelength of the wires in which the diameter of the wire is three times to that of the amplitude. [0022] While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-discussed embodiments but should be defined only in accordance with the following claims and their equivalents.

CLAIMS
I/We Claim,
1. A thermocouple (401) comprising:
a first and a second conducting wires (402A and 402B) forming at least a pair of conducting
wires to sense a temperature in a system;
an electrical junction (408) formed by joining the first and second conducting wires (402A
and 402B) at one end;
an outer sheath (404) to protect the thermocouple from external environmental factors; and
a mineral insulation (406) providing both electrical and thermal insulation,
wherein the plurality of first and second conducting wires (402A and 402B) are wavy in
structure such that the surface area is increased with increase in the length of the conducting
wires protecting the thermocouple (401) from creep and corrosion damage at higher
temperatures in the system.
2. The thermocouple as claimed in claim 1, wherein the plurality of first conducting wires (402A) comprises at least one of chromel, nickel-chromium alloy, iron, nicorsil, platinum-rhodium alloy and copper.
3. The thermocouple as claimed in claim 2, wherein the plurality of second conducting wires (402B) comprises at least one of alumel, platinum-rhodium, Constantan, nisil and platinum.
4. The thermocouple as claimed in claim 3, wherein the outer sheath comprises stainless steel, inconels and incolly and the mineral insulation comprises porous ceramic magnesium oxide powder.
5. The thermocouple as claimed in claim 4, wherein the wavelength of the conducting wires (402A and 402B) is ten times to that of the diameter of the conducting wires which ranges between 5-15 mm in that the amplitude is three times to that of the diameter of the conducting wires (402A and 402B).
6. The thermocouple as claimed in claim 5, wherein the cross-sectional area of the outer sheath (404) is ten times more than that of the conducting wires (402A and 402B).
7. The thermocouple as claimed in claim 6, wherein outer diameter of the thermocouple (401) ranges between 8-12 mm such that the conducting wires (402A and 402B) has higher thermal expansion coefficient than that of the mineral insulation (406) and the outer sheath (404).

8. A method, system and apparatus providing one or more features as described in the paragraphs of this specification.