Magnetic Resonance (MR) Imaging was used to measure the time-averaged voidage and particle velocity in a 3D gas-solid fluidized bed. Two different distributors were used. The mass-flux through a horizontal plane was calculated by combining the local voidage and particle velocity measurements. Based on the conservation of mass it was possible to give an error in the combined voidage and particle velocity measurements. It was found that the error in the mass flux was usually small (\u3c 5%), albeit increasing with increasing fluidization velocities
