Abstract: A method and apparatus for monitoring the density of a flowing fluid and controlling the flow thereof. The apparatus comprises at least one vertical tube, having an inlet at the bottom and an outlet at the top for establishing a fluid flow through the tube. A movable member, having a weight to volume ratio substantially equal to the density of the fluid, is disposed to move freely in the tube between the inlet and outlet, and adapted to close either the inlet or outlet when the density deviates substantially from said ratio. Closing of the inlet or outlet is used to stop the fluid flow.