Abstract: A marine seismic streamer section for a towed underwater streamer containing a buoyancy regulating fill liquid and the usual hydrophones, strain cables, signal wires and the like housed in a generally cylindrical tubular jacket sealed at its ends against loss of fill liquid and containing a depth sensor for producing electrical signals indicative of the depth of the streamer. The sensor is in a rigid housing whose interior is connected by a tube to a valved calibrating chamber having a valve member normally communicating the interior of the depth sensor housing to the fill liquid pressure conditions in the jacket. The chamber has an external access port normally closed by a plug, and the valve member is movable by a probe inserted through the port when the plug is removed and a source of gauged pressure connected to the access port to subject the interior of the depth sensor housing to the gauged pressure source.