a fluid level controller having a thermal expansion element in the form of cylinder made from polytetrafluoroethylene . the cylinder is located in a container which is connected to a supply of cryogenic fluid by a ball valve . when the cryogenic fluid in the container reaches the level of the cylinder and cools it , the cylinder shrinks and causes a lever to close the ball valve . when the cryogenic fluid falls below the level of the cylinder , the cylinder expands and the lever opens the ball valve and permits cryogenic fluid to flow through the valve and into the container . the cylinder has a heating element disposed in its interior to hasten the response of the cylinder to a change in the fluid level .