There are now a number of types of portable magnetometers based on different physical principles:
- Instruments based on Magnetoresistive and Hall-effect sensors are not used to measure the earth magnetic field even if they are sensitive to the value of magnetic fields. They are used in the industry to detect ferromagnetic objects at short distances or to measure higher magnetic fields. We shall not go in more details about them here.
- The ‘fluxgate’ sensor has a special magnetic core (high-Mu material) being saturated within the magnetic field to measure a single axis vector of this field. The basic principle is to compare the drive-coil current needed to saturate the core in one direction as opposed to the opposite direction. The difference is due to the external field. Full saturation is not necessary; any non-linearity will do. As the core approaches saturation, the signal picked up in the sense coil will show the non-linearity. If the sensor is oriented to measure the vertical component of the earth magnetic field, it serves as magnetometer while oriented to measure the horizontal vector of the earth field, it serves as compass. The typical sensitivity of these sensors is 1 Hz/nT i.e. the frequency picked-up at the sense-coil varies by 1 Hz for each variation of 1 Gamma (1nT) of the vertical vector of the field. Fluxgate mags are quite easy to build by hobbyists since all the necessary active components can be bought from Speake and Co in UK and from Fat Quarters in USA. They usually are built in the ‘Gradiometer’ Configuration to measure Vertical Field Gradients. In actual practice, they can detect field gradients of around 10nT in continuous measurement status. Actually, practical experience has shown that this type of mag is extremely sensitive to variations of temperature and the theoretical sensitivity is far from being given in practical implementations.
- The standard ‘Proton Precession’ magnetometer is to be discussed in more details in the following chapters. In short, the protons of a proton-rich fluid are magnetized to align in the same direction and stop their normal precession. When they are released, they resume their precession in phase and induce a voltage at a frequency depending upon the ambient magnetic field. This frequency is measured and the corresponding field value is calculated. This type of mag can give sensitivities better than 1 nT with a relatively low sampling rate of 1 measurement every 1 to 3 seconds.
- An Overhauser PPM uses a proton-rich fluid mixed with a very small quantity of a special chemical component containing free radicals. A constant magnetic field of RF frequency (around 60MHz) is applied on the fluid. This produces what is called the ‘Overhauser effect’ providing the same polarization effect from a polarizing field much smaller than in standard PPM and in much less time. The net effect is that Overhauser mags take 4 times less power consumption than normal PPM’s and can give up to 100 times better signals and faster sampling rates (more than two measurements per second). However, the chemical component looses its free radicals over time and should be regularly replaced.
- Cesium is the most widely available optically-pumped alkali vapor magnetometer. It can give sensitivities better than 0.1 nT with a very fast sampling rate but this technology remains completely out of reach from standard hobbyists for its complexity and the cost of required special material and professional adjustment instruments.
In summary, the types of mags that can practically be built by hobbyists are essentially the Fluxgate and the PPM.