The choice of thermocouple type is dependent upon the temperatures and conditions of use. Whatever the choice, it is usually necessary to provide the thermoelement with suitable protection from the often harsh conditions in which temperature measurements must be made.
There are any number of ways a thermocouple can fail. It can short out, the sheath can fail, it can be damaged during operation or installation, it can be improperly installed, etc. The key is to perform a thorough root-cause analysis and prevent the failure again in the future, if possible.
Protecting tubes, sheaths, and even thermowells can fail due to corrosion or mechanical damage. Processes can go over temperature and expose thermoelements to higher than anticipated temperatures. If a sensor controlling a process drifts low in its output, the process, in response to its controller may as a result be forced to temperatures higher than intended.
Base metal assemblies are vulnerable to attack by a number of chemical agents. They can also be altered by unfavorable operating conditions.
As supplied, noble metal thermocouple wire of good quality has very low impurity levels. Consequently, it is quite susceptible to contamination that can affect its thermoelectric properties. Platinum is especially sensitive to the presence of free silicon, with which it can combine to form a eutectic alloy that will melt at or below normal service temperatures. High-purity insulators and protecting tubes for precious metal assemblies as well as careful attention to cleanliness in handling are therefore essential to help prevent this.
Human error can be a contributing factor as well. Controls may be improperly set, connections may be improperly made, and inappropriate action in response to the operating conditions may be taken by mistake. Redundancy in instrumentation combined with training and responsibility are the usual means to combat these kinds of errors.
In the event of failure of thermocouples, table below has been included to aid analyzing the cause.
Table 1 – Thermocouple failure mode and cause analysis.
|Thermocouple failure mode||Possible causes of failure|
|Open circuit at room temperature||Brittle wires, excessive elongation in manufacture, improper heat treat or excessive cold work: initially defective wires|
|Open circuit at high temperature or during cycling||Brittle wires, non-uniform elongation causing necking of conductors, improper heat treat, excessive temperature for conductors, differential thermal expansion between conductors and sheath: defective initial wires|
|Short circuit||Loose insulation, voids, insulation contamination, conductors twisted, conductor decentralization, moisture|
|Low insulation resistance||Insulation contamination, moisture absorption, improper sealing at ends|
|High potential breakdown||Contamination of insulation conductor decentralization, loose insulation, insufficient sheath wall thickness, voids, excessive potential|
|Sheath fracture during forming||Bend radius too small, brittle sheath material, insufficient sheath wall thickness, inadequate or proper heat treat|
|Sheath fracture under vibration||Support points too far apart, insufficient sheath wall thickness, brittle sheath due to inadequate or improper heat treat, excessive cold work|
|Sheath burn-through||Temperature reaction with atmosphere lowering effective melting point, insufficient sheath wall thickness, improper sheath material for application|
|Loose insulation material||Improper design and insufficient sheath wall thickness|
Table 2 – Failure mode and cause analysis.
|Component||Mode of failure||Possible Causes of Failure|
|Sheath||Longitudinal splits||Excessive cold work, improper heat treat, improper drawing speed, or insufficient wall thickness|
|Rupture at high temperature||Excessive vapor pressure due to presence of helium after leak check, moisture in insulation, or insufficient wall thickness|
|Galling, inclusions and pits||Incomplete lubrication, improper reduction method, improper die configuration, contaminated lubrication|
|Discoloration||Improper heat treat time or temperature, improper cleaning of sheath prior to heat treatment|
|Brittle material, carbide precipitates, large gain size||Improper heat treat time or temperature or quenching or all three|
|Insulation||Low insulation resistance||Moisture, contamination or excessive migration of conductors|
|Corona, acing or breakdown at dielectric potential||Voids, moisture, contamination, conductor decentralization, excessive dielectric potential, loose pack|
|Loose pack||Inadequate sheath reduction, poor initial design, low tensile sheath material, or insufficient sheath wall thickness|
|Discoloration||Inherent contamination, contamination due to reaction with sheath or conductors or both, improper or unclean manufacturing facilities|
|Conductors||Open circuit||Excessive elongation, improper initial design, improper heat treatment: defective starting wires|
|Short circuit (conductor to conductor or conductor to sheath)||Loose pack, voids, band radius too small, improper assembly, conductor decentralization and contaminated insulation|
|Embrittlement||Excessive cold working, improper heat treatment, improper initial condition of conductor|
|Open circuit-high temperature||Non-uniform reduction of area, temperature above wire melting point: defective initial wire (micro cracks)|
|Out of calibration||Improper heat treatment, initial conductor out of calibration, cold work effects not removed, nonhomogeneous section|
|Poor conductor finish||Improper heat treatment on initial conductor, excessive insulation grain size, insulation crushability, insulation grain configuration and hardness, excessive swaging or rolling|
|Temperature/emf variation with length||Nonhomogeneous conductor, non-uniform heat treat or unrelieved cold working|
|Oblated conductors||Worn swaging or rolling dies, or improper die setup|
|Microscopic conductor fractures||Temperature above melting point, excessive cold work and improper heat treat, defective|
– Supply of thermocouples and temperature control system both made to order and standard features
– Maintenance, repair and rebuilt
– Calibration by ISO/IEC 17025 accredited laboratory
– Uniformity evaluation test on site
PHI Thermocouples & Temperature Control Systems
Manual on the Use of Thermocouples in Temperature Measurement, Fourth Edition, ASTM MNL-12, American Society for Testing and Materials, Philadelphia, 1993.
Randy Clarksean and Tom Blanchard, Troubleshooting Thermocouple Failures in High-Temperature Applications, Industrial Heating, 2016.
Richard M. Park, Thermocouple fundamentals, Course #tech temp 2-1