THM1176-LF
 Low fields as you have never seen them before

The measurement range of Metrolab's three-axis Hall magnetometer is expanding… downwards! With the new THM1176-LF (“Low Field”), the Magnetic Endoscope can now measure fields in the range of ±7 mT with a resolution of ±2 µT.
 

Less than a year after launching its breakthrough THM1176 Three-axis Hall Magnetometer, Metrolab is fitting it with a new low-field probe, capable of measuring fields in the range of ±7 mT with a resolution of ±2 µT. "The prototype is operational," announces Metrolab's Philip Keller. "The THM1176-LF will be on sale by next September."

It is the perfect instrument for safety applications, to measure the fringe field of magnets situated in public areas – MRI scanners, train motors, industrial magnets, etc. For example, to avoid injuries to patients or personnel with implants, hospitals usually mark the danger zone around an MRI scanner, where the field exceeds 5 Gauss (0.5 mT).

Higher sensitivity in the same dimensions
From the outside, the THM1176-LF is identical to the standard instrument (0.1 to 20 T). The USB interface offers the same plug-and-play capabilities, and can be directly connected to a PC, or a PDA for handheld use. The THM1176 software can now control either instrument; a free update is available for those who already own the existing THM1176.

On the inside, the LF probe differs dramatically from the original. Instead of one integrated circuit, it consists of three sensor chips, one per axis. These are mounted on a pair of orthogonally assembled printed circuit boards, yielding an active volume of 6 x 3.4 x 3 mm3. For this reason, one cannot remove the probe cap.

The high-sensitivity Hall sensor, coupled with the low-noise THM1176 electronics, can resolve field changes as small as ±2 µT. In a static field, this performance can be further improved by averaging: as usual, N measurements improve the resolution by a factor of √N. Thus, 100x averaging – still allowing a measurement time of well under 200 ms – yields a resolution of 0.2 µT – astonishing for a portable teslameter!

Flux concentrator
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.