The invention uses microwaves in the spinning industry in order to determine the band mass of an elongated, substantially non-twisted fiber structure. In addition, a method for measuring the sliver mass from the frequency detuning and the damping of resonance signals of a microwave sensor while taking into consideration the moisture of the fiber structure with a computer is suggested. Likewise, a corresponding spinning preparation machine and a laboratory device for measuring the sliver mass are part of the invention. Lastly, a method and a spinning preparation machine for recognizing foreign matter in a moved fiber structure with the aid of microwaves is suggested.