A freely rotatable turbine is placed in the path of flowing fluid to drive the turbine at an angular velocity that, at steady state, is dependent upon the linear velocity of the flowing fluid. A transient condition is created in the angular velocity of the turbine. The time constant of the transient condition is determined. The time constant is representative of the density of the flowing fluid--the shorter the time constant, the higher the density and vice versa. The transient condition is created by disturbing the free rotation of the turbine, namely, by imnpairing free rotation to reduce the angular velocity of the turbine to zero, and then permitting the turbine to return to free rotation.