A construction system for a capacitive sensor comprises a source electrode (210), a screening element (220) with partition (221) which forms a first and a second screened chamber (220.a, 220.b), a field sensor (230), a circuit (250), a spacing member (260) with a through-duct, and a screw (270). The partition (221) is provided with a hole (224) and said spacing member (260) is positioned inside the first chamber (220.a) with the axis (260.y) of the duct arranged coaxial with the axis (224-y) of said hole (224). The same spacing member (260) is positioned between the proximal face (223) of the partition (221) and the distal face (232) of the field sensor (230) and said field sensor (230) is provided with a threaded hole (233). The head (271) of the screw (270) is arranged inside the second chamber (220.b) and its shank (272) is arranged passing axially through said hole (224) and across said through-duct of said spacing member (260) with its end portion which mates with the threaded hole (233) of said field sensor (230).