CRM has developed a technique to measure the hydrostatic pressure in the blast furnace hearth. Three measuring campaigns have been performed at Arcelor Cockerill-Sambre BF B. Complementarily, a mathematical model computing the liquid levels in the hearth and the position of the bottom of the dead man has been developed. It takes into account the movement of the dead man. Its results are consistent with the pressure measurements. The combined use of the model and the measurements allow estimating the coke bed voidage in the hearth and concluding if the dead man is floating or not. For most casts, it begins to float a few minutes after plugging. The estimated coke bed voidage is very well related to the quality of the coke used during the measuring campaigns.