A flow meter, comprising: a coupler adapted to couple to a drip chamber； a support member operatively coupled to the coupler； a first image sensor having a first field of view and operatively coupled to the support member, wherein the first image sensor is positioned to view the drip chamber within the first field of view； a valve in fluid communication with the drip chamber； a background pattern having an array of lines positioned within the field of view of the first image sensor wherein the drip chamber is between the first image sensor and the background pattern； and at least one processor operatively coupled to the first image sensor, wherein the at least one processor is configured to: receive a first image data from the first image sensor, and provide a background image； and determine that a free flow condition exists by: comparing the background image with the first image data； and determining that the liquid distorts the array of lines by changing angles.
