The Effect of Construction Material on the Thermal Gain Dependence of a Fluxgate Magnetometer Sensor

Fluxgate magnetometers are an important tool in geophysics and space physics but are typically sensitive to variations in sensor temperature. Changes in instrumental gain with temperature, thermal gain dependence, are thought to be predominantly due to changes in the geometry of the wire coils that sense the magnetic field. Scientific fluxgate magnetometers typically employ some form of temperature compensation, and support and constrain wire sense coils with bobbins constructed from materials such as MACOR machinable ceramic (© Corning) which are selected for their ultra-low thermal deformation rather than for robustness, cost, or ease of manufacturing. We present laboratory results comparing the performance of six geometrically and electrically matched fluxgate sensors in which the material used to support the windings and for the base of the sensor is varied. We use a novel, low-cost thermal calibration procedure based on a controlled sinusoidal magnetic source and quantitative spectral analysis to measure the thermal gain dependence of fluxgate magnetometer sensors at the part-permillion per degree Celsius level in a typical magnetically noisy university laboratory environment. We compare the thermal gain dependence of sensors built from MACOR, polyetheretherketone (PEEK) engineering plastic (virgin, thirty percent glass filled, and thirty percent carbon filled), and Acetal to examine the trade between the thermal properties of the material, the impact on the thermal gain dependence of the fluxgate, and the cost and ease of manufacture. We find that thermal gain dependence of the sensor varies as one half of the material properties of the bobbin supporting the wire sense coils rather than being directly related as has been historically thought. An experimental sensor constructed from thirty percent glass filled PEEK (21.6 part-per-million per degree Celsius) had a thermal gain dependence within 5 part-per-million per degree Celsius of a traditional sensor constructed from MACOR ceramic (8.1 part-per-million per degree Celsius). If a modest increase in thermal dependence can be tolerated or compensated, then thirty percent glass filled PEEK is a good candidate for future fluxgate sensors as it is more economical, easier to machine, lighter, and more robust than MACOR.