A nuclear method for continuously monitoring the coalbed and ash levels and void formation in commercial-scale, fixed-bed, pressurized gas producers is proposed. Measurement of gamma ray transmission through the coalbed is the basis of the method. The coalbed level is monitored by detecting the transmission of gamma rays from a 1 to 2 ci co-60 source positioned on the outside wall of the vessel. Coal bridging or, alternatively, ash level is monitored by a 10 ci co-60 source at the center of the coalbed. Source activities and geometries are calculated for a 12-foot-id producer designed to operate at 350 psi. Source capsule and detector specifications and operational safety are also discussed.