In various industrial applications in which a liquid is sprayed into a gas-fluidized bed such as Fluid Cokers (TM), fluid catalytic crackers and gas-phase polymerization reactors, parts of the bed or even the complete bed might become defluidized due to high liquid loading, agglomeration or sintering of particles. For instance, in Fluid Cokers, operating at lower temperatures increases the yield of liquid products and reduces sulphur emissions, but local defluidization may result in some bed regions with a disastrous impact on reactor operation. This loss of bed fluidity is called "bogging". This study examines a novel method employing electrical conductance to detect local bogging in a large scale pilot fluid bed of about 7 tonnes of silica sand. Bogging is induced by increasing the liquid load as well as by changing the atomization performance of a spray nozzle. Several other experimental methods, such as differential and static pressure measurements, image processing, and temperature measurements are evaluated. When liquid is injected into a fluidized bed, a fraction forms liquid solid agglomerates while the remainder forms free moisture, consisting of individual particles coated with a thin layer of liquid. Liquid present as free moisture can be beneficial as it increases the mass and heat transfer processes and increases yields of valuable products. The results indicated that conductance measurements could be used for detecting bogging phenomena online. Bogging is shown to be directly associated with the local free moisture rather than the total moisture level. Increasing the atomization gas flowrate and, consequently, the free moisture, increases the bogging risk. A measurable critical, local free moisture value above which localized bogging occurs is identified. (C) 2015 Elsevier B.V. All rights reserved.