A flow sensor sub-assembly includes a flow tube having a lumen, an outside diameter, a first end, and a second end. An inlet fitting includes a conical orifice sized for insertion in either end of the flow tube, such that an internal passage of the inlet fitting is coaxial and concentric with the lumen and the end of the flow tube abuts a shoulder. An outlet fitting includes a conical orifice sized for insertion in either end of the flow tube, such that an internal passage of the inlet fitting is coaxial and concentric with the lumen and the end of the flow tube abuts a shoulder. A first piezo element integrated with the inlet fitting is arranged at an upstream position of the flow tube assembly and a second piezo element integrated with the outlet fitting is arranged at a downstream position of the flow tube assembly.