Magnetic effects on thermocouples

Abstract

Thermometers in laboratory environment and industrial applications are often subject to extraneous, usually unwanted and uncontrolled magnetic fields. Magnetic field influence can be minimized, but cannot be fully cancelled out. Even more, in most cases, there is no awareness of the existence of magnetic fields, let alone their effect on measurement instrumentation. In the past, sensitivity to high dc magnetic fields has been investigated in cryogenics and at high temperatures. More recently, the magnetic effect on weak dc magnetic fields was presented. The goal of this paper was to analyse and empirically and experimentally prove the magnetic sensitivity of thermocouples exposed to low magnetic fields: both dc and ac. Precision and uniform alternating and direct magnetic flux densities were generated by means of permanent magnets and power amplifiers with air-cored coils. The magnetic effect on ferromagnetic and non-ferromagnetic thermocouples at liquid-nitrogen-boiling point (−196 °C), ice point (0 °C), in water (17 °C) and at melting point of gallium fixed point cell (29.7646 °C) was investigated. Magnetic-field-dependent temperature errors of up to 700 mK (at 5.3 mT: dc) and up to 1 °C (at 10 mT: ac 50 Hz magnetic fields) were detected. From the results, it can be concluded that, ideally for temperature measurements of the highest accuracy in the above-cryogenic temperature range, magnetic sensitivity should be estimated and taken into account either as the correction of an error and/or as an additional source of measurement uncertainty. Special consideration should be given to thermocouple orientation relative to the magnetic field direction, influence of metal enclosures and magnetization effects on ferromagnetic components of thermocouples.

Design and experimental results of the 1-T Bitter Electromagnet Testing Apparatus (BETA)

“…However, there was significant measurement error resultant of both the magnetic field and electrochemical effects which adds to the standard error, ±2.2 • C, of thermocouples. 19,20 Analytical outlet water temperature of the cooling channels falls within the thermocouple error. New methods of DAQ for experimentally measuring magnet and water temperatures are being investigated with thermochromic liquid crystal paper imaging.…”

Comparative Investigation on the Quality of Sensitivity of Six Different Types of Thermocouples

“…A study [11] 12.2. A study [12] has performed an experiment about magnetic field sensitivity of thermocouples, the aims were to determine errors of temperature measuring and sensitivity to magnetic field at various temperature ranges, thus it has presented a table about the influence of magnetic field on sensitivity of encapsulated thermocouples of J-type and T-type. The study worked on sensitivity related to both DC and AC magnetic fields by positioning thermocouples in a stable and an controlled temperatures, then the sensitivity to magnetic field was determined at various temperatures measuring, then, it has analyzed the final results according to each of metal properties, magnetization of metal sensors, and mechanical effects, then, it has proved that:  Thermocouples in the metal enclosure has different errors due to magnetic sensitivity than the ones without metal enclosure.…”

D-Mag — a laboratory for studying plasma physics and diagnostics in strong magnetic fields

“…The investigation of various materials and diagnostics in strong magnetic fields plays an important role in many different scientific and industrial fields. For example, magnetization effects of dusty plasmas [2]- [4] are relevant to astrophysics, as well as accurate temperature measurements using thermocouples [5], where a magnetic field can induce voltage in the wire and cause inductive heating, are of great significance. In particular tests in strong magnetic fields are necessary to develop and validate diagnostics for current and future experimental fusion devices, such as Wendelstein 7-X (W7-X), ITER and DEMO.…”