Part of the reason that the new sensor can achieve such high sensitivity is that it contains a microscopic piece of soft iron, acting as a flux concentrator. Strong magnetic fields will saturate this flux concentrator, but the process is reversible and the sensor is not damaged.
The hysteresis curve of the flux concentrator does, however, affect the instrument accuracy, partly because of the relatively strong nonlinearity and partly because of the remanent field. It took careful experimentation and analysis to be able to quantify these effects, and additional firmware development to be able to correct for them in an optimal fashion. In the end, Metrolab expects to be able to achieve ±1% accuracy, regardless of the field direction, down to ±20 µT.
Assembly and calibration
Having understood this subtle phenomenon, there was still the serious problem of geometry to be solved. The THM1176 was nicknamed the "magnetic endoscope" on account of its exceptionally compact design – "and we were keen that the LF should be no bigger," says Pascal Sommer from Metrolab. "But this time it had to contain not one but three integrated circuits, while leaving enough room for two perpendicular circuit boards, stiffeners, soldering and gluing!" It just goes to show how extremely intricate the 3D assembly process is.
Further innovations involved the calibration process. Since NMR teslameters cannot measure such small fields (yet), Metrolab uses a fluxmeter as reference. This in turn suggests using an AC field instead of the DC fields usually used for calibration – and in fact, this allows collecting thousands of calibration points, covering the entire range, in a matter of seconds. All it takes is a large Helmholtz coil, a precision AC power supply, a new jig, and lots of new software…
But there is every chance that future THM1176-LF owners won't give this the slightest thought. Armed with their magnetometer, they must spend just a few seconds zeroing their probe in the zero-Gauss chamber before beginning their measurements.