A flow sensor sub-assembly for sensing flow of a fluidic medicament includes a flow tube having a flow tube inlet and a flow tube outlet, and an acoustical transmission rate. The medicament flows through the flow tube. A first piezo element is arranged at an upstream position of the flow tube and a second piezo element is arranged at a downstream position of the flow tube, such that the first piezo element and the second piezo element are mounted apart a pre-selected distance from each other. An absorber sheath encircles the flow tube. The absorber sheath has an upstream end and a downstream end. The absorber sheath is comprised of a material with an acoustical transmission rate different than the flow tube.