Abstract: A device for controlling "U" tubing in the flow of a fluid such as cement in oil well casings comprises a body which is inserted in the casing towards its lower end. The body provides a passage so that cement can pass through the body and out of the end of the casing before passing up round the exterior of the casing, displacing mud in front of it. A member is mounted in the passage which normally does not impede the flow of cement through the passage. However, when the pressure differential at spaced points along the flow of cement increases beyond the predetermined level as a result of `U` tubing, the member moves to restrict severely the area of the passage. This halts `U` tubing. The member can move back to the full flow position once `U` tubing has been controlled.