A paddle-type flow detector comprising: a housing, a vane assembly and a biasing mechanism adapted to bias said vane assembly to a first position, the vane assembly having a trip stem with a paddle at its distal end, the trip stem being connected toward its proximal end to the housing about an axis of rotation, wherein said trip stem is biased to said first position and said flow detector is adapted to indicate flow when said trip stem is in a second position; and a self-test system comprising: an arm for displacing the trip stem from said first position to said second position; a first sensor for detecting when said trip stem is in said first position; a second sensor for detecting when said trip stem is in said second position; the arrangement being such that, in use, when said trip stem is in said second position, said arm is adapted to release said trip stem and said trip stem is adapted to return to said first position because of said biasing; wherein said self-test system is adapted to determine the amount of time between when said second sensor stops detecting said trip stem and said first sensor detects said trip stem after said arm releases said trip stem, and based on said amount of time, determine if said paddle is attached to said trip stem. a housing, a vane assembly and a biasing mechanism adapted to bias said vane assembly to a first position, the vane assembly having a trip stem with a paddle at its distal end, the trip stem being connected toward its proximal end to the housing about an axis of rotation, wherein said trip stem is biased to said first position and said flow detector is adapted to indicate flow when said trip stem is in a second position; and a self-test system comprising: an arm for displacing the trip stem from said first position to said second position; a first sensor for detecting when said trip stem is in said first position; a second sensor for detecting when said trip stem is in said second position; the arrangement being such that, in use, when said trip stem is in said second position, said arm is adapted to release said trip stem and said trip stem is adapted to return to said first position because of said biasing; wherein said self-test system is adapted to determine the amount of time between when said second sensor stops detecting said trip stem and said first sensor detects said trip stem after said arm releases said trip stem, and based on said amount of time, determine if said paddle is attached to said trip stem. an arm for displacing the trip stem from said first position to said second position; a first sensor for detecting when said trip stem is in said first position; a second sensor for detecting when said trip stem is in said second position; the arrangement being such that, in use, when said trip stem is in said second position, said arm is adapted to release said trip stem and said trip stem is adapted to return to said first position because of said biasing; wherein said self-test system is adapted to determine the amount of time between when said second sensor stops detecting said trip stem and said first sensor detects said trip stem after said arm releases said trip stem, and based on said amount of time, determine if said paddle is attached to said trip stem.