An apparatus and method for determining fluid flow (e.g., sterile media flow, filtrate flow, etc.) through tubing is provided. The apparatus includes an air source, a flow valve, a first sensor, a second sensor, and a media tubing section extending between the first sensor and the second sensor. In operation, air from the air source is introduced by the flow valve into a fluid flow to create a lead line of fluid immediately following the introduced air. As the lead line of fluid passes each sensor, the sensors are used to determine a time between when the lead line passes the first sensor and the second sensor and that determined elapsed time is used, along with a determined volume of the media tubing section to determine the rate of flow of the fluid flow.