An agricultural baler for producing rectangular bales, having a reciprocating plunger moving back and forth in the enclosure formed by walls, thereby compressing crop material and forming rectangular bales, and further including at least one actuator for adapting the cross-section of the enclosure depending on the required pressure that needs to be exerted on the advancing bale. Actuators can apply their force on the sidewalls along transversal support beams or brackets forming the ‘density belt’. The baling chamber is provided with one or more friction or moisture sensors which are placed at the location or in the immediate vicinity of one or more side wall reinforcements.