•Separation of bubbles from a gas cavity in a magnetic liquid is studied.•Analysis of acoustomagnetic oscillations gives the bubble’ s radius.•The gas bubble radius does not depend on the gas influx rate.•Possible application as a gas counter for microfluidic devices. In this work, we report results of the experimental study of the dynamics of a gas bubble separating from the air cavity in a magnetic fluid placed in an inhomogeneous magnetic field created by a ring magnet. Two experiments were carried out for samples with different concentrations: video recordings of the interphase boundary in a flat channel and recordings of electromagnetic vibrations caused by the gas bubble oscillations in a magnetic fluid. It is shown that the bubbles have a stable size. The respective dependencies of the bubble size on the magnetic fluid concentration and the air supply argue that it is possible to use the considered setup as a microdoser or a gas meter in microfluidics applications.