|Pneumatic Modulevel Operating Principle
The key elements of the Modulevel® pneumatic control are magnetic coupling, which allows the controller to be mechanically isolated from the sealed sensing unit; the range spring, which translates the change in buoyancy force to motion of the attraction ball; and the controller head, which provides a modulated pneumatic signal in direct proportion to the input from the vertical motion of the attraction ball.
As the liquid level in the vessel increases or decreases, the buoyancy force on the displacer increases or decreases. The motion created by the change in force acting on the range spring, causes movement of an attraction ball within an enclosed pressure-boundary tube.
A magnet encircling the tube follows the attractor ball, transferring the motion to a rotating cam, which in turn operates a flapper against a nozzle increasing or decreasing the pressure within the pneumatic relay. The output pressure signal can be used in a variety of ways to operate a control valve or signal to alarms, indicators, process controls or other devices.
With optional integral control, the pilot nozzle proportional signal is conditioned through an additional metering valve system, which will eliminate offset from the desired control point.