Abstract: A valve structured for attachment in-line with a conduit containing a fluid under pressure. The valve provides a flow path for the fluid, with the flow path including an area whereat all of the fluid passing through the valve must pass between a stationary surface and an interior surface of an elastic tubular member. A liquid filled chamber of the valve is isolated from the flow path, and in communication with an exterior surface of the elastic tubular member. A first pipe is attached to the valve in open communication with the liquid filled chamber. The other end of the first pipe is connected to a pressure vessel. A flexible diaphragm is affixed within the pressure vessel to divide the interior cavity thereof into a first and a second chamber. Both the first chamber and first pipe are completely filled with liquid and in communication with each other.