An agricultural baler for producing rectangular bales comprising: » A pre-compression chamber, • a crop material supply means configured to supply harvested crop material to said pre-compression chamber, • a baling chamber configured to receive pre-compressed material from the pre-compression chamber, the baling chamber comprising: ∘ four sidewalls (4,5,6,7) defining a rectangular enclosure extending in a longitudinal direction between a first section at the inlet side of the baling chamber and a second section at the outlet side of said chamber, at least one of said sidewalls being displaceable in a direction perpendicular to said longitudinal direction, so as to increase or diminish the cross-section of said enclosure, ∘ a plunger and a driving mechanism adapted to reciprocally move said plunger inside said enclosure, to thereby move crop material in said longitudinal direction, thus progressively forming a rectangular bale in a bale forming process, ∘ one or more reinforcement means (15,20,25) arranged in connection with at least one of said sidewalls, said reinforcement means forming an obstruction that locally neutralizes or limits deformation of said sidewalls by bulging outwards with respect to the interior of the baling chamber, said deformation being a consequence of said bale forming process, • actuation means arranged for exerting a pressure on said displaceable sidewall(s), to thereby actuate displacement of said sidewall(s); • at least one friction or moisture sensor mounted on one of said displaceable sidewalls of the baling chamber, said sensor is located at the location of or in the immediate vicinity of one of said reinforcement means; said reinforcement means comprise support beams attached to said displaceable walls, said support beams being arranged transversally with respect to said longitudinal direction, said actuation means being arranged for exerting a pressure on said displaceable sidewall(s) at the location of said support beams, and wherein said at least one sensor is located at the location of or in the immediate vicinity of one of said transversally arranged support beams; characterised in that, when the friction or moisture sensor is located between said transversally arranged support beams and the baling chamber's first section or between said support beams and the baling chamber's second section, the distance between the friction or moisture sensor and said transversally arranged support beams is maximum one fourth of the distance between said transversally arranged support beams and said first section or second section respectively. » A pre-compression chamber, • a crop material supply means configured to supply harvested crop material to said pre-compression chamber, • a baling chamber configured to receive pre-compressed material from the pre-compression chamber, the baling chamber comprising: ∘ four sidewalls (4,5,6,7) defining a rectangular enclosure extending in a longitudinal direction between a first section at the inlet side of the baling chamber and a second section at the outlet side of said chamber, at least one of said sidewalls being displaceable in a direction perpendicular to said longitudinal direction, so as to increase or diminish the cross-section of said enclosure, ∘ a plunger and a driving mechanism adapted to reciprocally move said plunger inside said enclosure, to thereby move crop material in said longitudinal direction, thus progressively forming a rectangular bale in a bale forming process, ∘ one or more reinforcement means (15,20,25) arranged in connection with at least one of said sidewalls, said reinforcement means forming an obstruction that locally neutralizes or limits deformation of said sidewalls by bulging outwards with respect to the interior of the baling chamber, said deformation being a consequence of said bale forming process, ∘ four sidewalls (4,5,6,7) defining a rectangular enclosure extending in a longitudinal direction between a first section at the inlet side of the baling chamber and a second section at the outlet side of said chamber, at least one of said sidewalls being displaceable in a direction perpendicular to said longitudinal direction, so as to increase or diminish the cross-section of said enclosure, ∘ a plunger and a driving mechanism adapted to reciprocally move said plunger inside said enclosure, to thereby move crop material in said longitudinal direction, thus progressively forming a rectangular bale in a bale forming process, ∘ one or more reinforcement means (15,20,25) arranged in connection with at least one of said sidewalls, said reinforcement means forming an obstruction that locally neutralizes or limits deformation of said sidewalls by bulging outwards with respect to the interior of the baling chamber, said deformation being a consequence of said bale forming process, • actuation means arranged for exerting a pressure on said displaceable sidewall(s), to thereby actuate displacement of said sidewall(s); • at least one friction or moisture sensor mounted on one of said displaceable sidewalls of the baling chamber,