An ozone-based disinfecting device is provided comprising a mixer having a generally hollow body with a water inlet for water under pressure, a spray nozzle for generating a generally conical spray of water introduced by way of the water inlet, a contact chamber communicating with a gas inlet for ozone rich gases, and an outlet aperture from the contact chamber that is coaxial with the spray nozzle and spaced apart therefrom. An electronic flow sensing device senses the extent of the flow of water through the spray nozzle according to vibration caused by water flowing through the mixer. The electronic flow sensing device is preferably located in a pocket formed in the mixer and preferably comprises a piezoelectric sensor embedded at least around its periphery in a settable material. A preferred construction of the mixer is also described.