Abstract: A novel method is provided for detection and/or control of the dynamics, e.g. the headspace, in a fluid bed reactor system based on detecting a pre-determined particle density gradient formed within a spatially confined fluid bed of fluidised particles in a carrying medium. More specifically, a method is provided where transmitting means and receiving means are used for detecting the pre-determined particle density gradient formed and subsequently the signal is used for activating or deactivating flow controlling means whereby the dynamics of the system can be controlled.