Evaluation of the effect of ambient temperature variation on the calibration of a Large Helmholtz Coils System, employed for the calibration of space qualified magnetometers

摘要

Large 3-axes Helmholtz Coils are used as research as well as calibration equipment for the calibration of magnetic instruments. Systems containing magnetic sensors can be inserted into the coil and the magnetism of the system or dynamic platform can be measured for compensation in hardware or software. A Large Helmholtz Coils system (average side-length 2.4 m) is located at the SANSA (South African National Space Agency), Space Science facility in Hermanus, South Africa. The facility is also an INTERMAGNET (International Real-time Magnetic Observatory Network) Magnetic Observatory (HER), therefore the area is magnetically clean and quiet to observatory standards. At SANSA Space Science the Large Helmholtz Coils are used regularly for the calibration of space qualified magnetometers. Predetermined magnetic fields are created in 3 axes in the center of the coils by application of predetermined currents to the coils. However, coil non-orthogonality errors, orientation of the coil relative to the ambient magnetic field, coil levelling errors and fluctuations in the ambient magnetic field have to be compensated for. Thus the coils system has to be adjusted and calibrated annually to absolute magnetic field standards. Since the accuracy of the magnetic sensor calibration is directly dependent on effectiveness and accuracy of the coil calibration procedure, the coil calibration needs to be executed with the highest possible precision. At SANSA this calibration was executed annually using a laborious manual process requiring various magnetic observatory equipment and specialized staff. Man-hour cost is significant and the coils system is non-operational for at least a week, adding loss of possible income to the cost of calibration. A new semi-automatic method could be executed by a less-experienced person using less demanding equipment, with only 6 hours downtime. The new semi-automatic calibration procedure has proven to be relatively repeatable; however, there remains a major uncertainty in terms of the stability of the generated field due to the possible variation in the ambient temperature during calibration and subsequent use. Therefore, the aim of this study was to evaluate the new semi-automatic calibration procedure in terms of repeatability to determine the effect of variations in the ambient temperature on the calibration constants of the coil. Evaluations were executed in autumn ambient temperatures at the location in South Africa. The calibration procedure was executed 50 times spanning a temperature range of 14 °C to 25 °C inside the building (12 °C to 30 °C degrees outside). Analysis of the coil constants have shown that the coil constants exhibit change of-1.4 nT/°C at 60 000 nT applied field. This is significant as magnetic sensors are often calibrated up to 60 000 nT, and moreover, the magnetometer calibrations are not specifically executed at the same temperature as coil calibration. A method of compensating for temperature dependence of the coils, or significant temperature insulation of the building, will have to be investigated in the near future.