Thermocouples and Magnetic Fields

Thermocouples under Magnetic Fields

Some individuals attempt to measure temperature in magnetic components using various thermocouples without considering how static (DC) or alternating (AC) magnetic fields might affect them. Accurately measuring temperature is essential, as it can greatly influence the performance and lifespan of magnetic components within different magnetic and electromagnetic devices. Research has been conducted to explore the effects of magnetic fields on various types of thermocouples [1]-[6]. From references [1] to [6], it is evident that selecting the appropriate type of thermocouple is crucial, depending on whether it is exposed to DC or AC magnetic fields. These magnetic fields can cause significant fluctuations in the voltage and temperature readings from the thermocouples. For this reason, it's important to know the characteristics of the different type of thermocouples, their material composition and their behavior under different magnetic field environments.

Thermocouples Types

The accuracy of temperature measurements taken with thermocouples can be influenced by magnetic fields, depending on the materials of the thermocouples [8]-[10]. Some thermocouples are made of conductive and non-magnetic materials and other thermocouples are made of magnetic and conductive materials. The thermocouple type and its materials must be considered for temperature measurements under different magnetic field conditions, for example:

Type - J: Made of 100% Iron (Fe) and Constantan (45% Ni, 55% Cu). They are particularly sensitive to oxidation, which can be a concern in magnetized environments. Moreover, Type - J has a cable made of 100% Iron (Fe) which is a conductive and magnetic material, and it can attract magnetic fields, and some eddy currents could be induced in the Iron cable under high frequency magnetic fields presence. These thermocouples should present large fluctuations of temperature under the presence of DC and AC fields.

Type - K: Made of Chromel (90% Ni, 10% Cr) and Alumel (95% Ni, 2% Mn, 2% Al). They are commonly used due to their wide temperature range and durability. Type - K has a cable made of 95% Nickel (Ni) which is a conductive and magnetic material, and it can attract magnetic fields, and some eddy currents could be induced in the Ni cable under high frequency magnetic fields presence. These thermocouples should present large fluctuations of temperature under the presence of DC and AC fields.

Type - T: Made of Copper (100% Cu) and Constantan (45% Ni, 55% Cu). They are less affected by oxidation, which is beneficial when used near permanent magnets. Type - T has a cable made of 100% Copper (Cu) which is a conductive material, and it can permit the induction of eddy currents under high frequency magnetic field tests. These thermocouples could present fluctuations of temperature under the presence of high frequency magnetic fields.

Type - N: Composed of Nicrosil (84.6% Ni, 14.2% Cr, 1.4% Si) and Nisil (95.5% Ni, 4.4% Si, 1% Mg). The Type-N thermocouples provide stability and are suitable for high-temperature applications. They are less influenced by magnetic fields compared to other types. Type - N has cables have a high content of Nickel (Ni) which is a magnetic and conductive material, and it can attract some magnetic field, and it could permit the induction of eddy currents under high frequency magnetic field tests. These thermocouples could present fluctuations of temperature under the presence of high frequency magnetic fields.

Type - E: Made of Chromel (90% Ni, 10% Cr) and Constantan (55% Cu, 45% Ni) and suitable for cryogenic use. Both conductors in this thermocouple are not magnetic. Type - E has a cable with a high content of Nickel (Ni) which is a magnetic and conductive material, and it can attract some magnetic field, and it could permit the induction of eddy currents under high frequency magnetic field tests. These thermocouples could present fluctuations of temperature under the presence of high frequency magnetic fields.

AC and DC Magnetic Fields

The presence of AC and DC magnetic fields can affect the accuracy of temperature measurements taken with thermocouples [8]-[10]:

DC Magnetic Fields: Permanent magnets or DC coils create static magnetic fields that can affect the voltage in thermocouples during temperature measurements, potentially leading to erroneous temp readings or considerable delta temperature readings respect to ambient temperature. This is particularly relevant for thermocouples made from magnetic materials, which might experience changes in their thermoelectric properties.

AC Magnetic Fields: Alternating (AC) magnetic fields can induce eddy currents in the metallic and magnetic components of thermocouples, causing heating that may inaccurately reflect the temperature being measured. The frequency and intensity of the AC field can exacerbate this effect.

Static Magnetic Fields

Temperature tests were conducted using various types of thermocouples (-K, -J, -T) subjected to a static magnetic field generated by a square sintered NdFeB magnet. The thermocouple tips were positioned on the surface of the square NdFeB magnet.

Thermocouples Type- K, -J, and -T were evaluated with the square magnet magnetic field, and both the ambient temperature and the final stable temperature on the magnet were recorded using a thermocouple meter.

Thermocouples Type - J and -K showed a significant temperature difference compared to the ambient temperature, exceeding a delta T = 2°C. In contrast, the Type -T thermocouple exhibited a minor temperature difference from the ambient temperature, less than delta T = 0.5°C.

° °C

In addition, we noted that the cables and tips of thermocouples Type -K and -J are attracted by the magnets. Indicating that they are made of a magnetic material.

Other experiments were performed to analyze the magnetic field effects on different thermocouples produced by shielded static fields. A thin low carbon steel plate was attached to the square magnet, and the tips of the thermocouples were located on the plate where the magnet pole is located (black mark on surface of plate see the following photos).

Thermocouple Type - J showed a significant temperature difference compared to the ambient temperature, exceeding a delta T = 1.5 °C. In contrast, the thermocouples Type -K and -T exhibited a minor temperature difference from the ambient temperature, less than delta T = 0.6°C.

Conclusion

Using thermocouples to measure temperature on magnetic field environments requires careful consideration of the type and material of the thermocouples. Additionally, understanding the influence of magnetic fields on thermocouple performance is essential for obtaining accurate temperature readings. By selecting the appropriate thermocouple type, it is possible to mitigate the effects of magnetic fields and ensure reliable temperature measurements in magnetic applications.

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References

[1] A.V Inyushkin, K Leicht, and P Esquinazi, "Magnetic field dependence of the sensitivity of a type E (chromel–constantan) thermocouple," Cryogenics, Vol. 38, No. 3, pp. 299-304, 1998.

[2] M. Chatterjee and P. Y. Nabhiraj, "Capturing the temperature of the workpiece in an induction heating system using thermocouple with minimal error," 2021 5th International Conference on Electronics, Materials Engineering & Nano-Technology (IEMENTech), Kolkata, India, 2021, pp. 1-5.

[3] L. Qiu, W. Chen and L. Zhang, "Research on Measurement Technology of Internal Temperature Rise of High Frequency Magnetic Components," CPSS Trans. Power Electronics and Applications, vol. 7, no. 1, pp. 28-36, March 2022.

[4] S. Beguš, J. Bojkovski, J. Drnovšek, and G. Geršak, "Magnetic effects on thermocouples," Measurement Science and Technology, Vol. 25, pp. 1-11, 2014.

[5] W.F. Schlosser and R.H. Munnings, "The effect of a magnetic field on a copper-constantan thermocouple at low temperatures," Cryogenics, Volume 12, Issue 4, 1972, pp. 302-303.

[6] Shir, Farhad, Mavriplis, Catherine, and Bennett, Lawrence, "Effect of Magnetic Field Dynamics on the Copper-Constantan Thermocouple Performance," Instrumentation Science & Technology - INSTRUM SCI TECHNOL. vol. 33, pp. 661-671.

[7] Thermocouple Types. Technical Article: https://www.thermometricscorp.com/thertypav.html

[8] Ambrell Company, Technical Article: Will a thermocouple temperature reading be affected by the electromagnetic field of induction heating?

[9] Technical Article: Tech Brief: Environmental Effects on Thermocouples | Electronics Cooling

[10] Omega Engineering, Technical Article: Measuring Temperature in Electromagnetic Environments