Existing theories for open-core fluxgate are not fitting experimental results in the case of ring-core sensors. Simple description based on transfer functions which takes into account the interaction between two half-cores is presented. Measured inner loops and dynamical transfer functions justified this approach. Flux-evaluating fluxgate was proposed for simple instruments. Its main advantage is its insensitivity to excitation current field parameter variations, but it has a threshold level of about 100 nT in present time. Methods of calibration and noise evaluation of sensors are also mentioned.
