a method and a measuring device measure the density of fluid media with a sensor having a density measuring device . a frequency oscillator with mass balance is used as the density sensor and all components related to the frequency oscillator with mass balance for the oscillation behavior , oscillation excitation and oscillation evaluation , as well as the frequency oscillator with mass balance directly associated with the measuring and sensor electronics , are enclosed in a housing or cartridge . these components are used with the frequency oscillator with mass balance in the cartridge , and / or the frequency oscillator with mass balance together with the cartridge is adjusted or calibrated using measurement standards and adapted as necessary to the specific application . the cartridge with the frequency oscillator with mass balance is releasably or replaceably connected with the measuring device or its base body before the beginning of the measuring operation to investigate the media .