The level trol is used for measuring level 0 liquids in a closed vessel.
PRINCIPLE. : It works on archimedes principle "The loss in weight of a troly immersed in a liquid is equal to amount of liquid displaced by the body". The level trol basically consists of the following :

DISPLACER : It is consists of a cylinderical shape pipe sealed and filled inside with sand or some weight.The purpose of this is to convert change in level to primary motion. The variation in buyoancy resulting from a change in liquid level varies the netweight of the displacer increasing or decreasing the load on the torque arm. This change is directly propertional to change in level and specific gravity of the liquid.

RELAY : Amplifies pressure variations at the nozzles.

REVERSING ARC : It is used for the following purposes.
1. Motion take of from Torque tube.
2. Means of rever control action.
3. Adjustment for specific gravity.

PROPERTIONAL UNIT. : Converts primary motion to a propertional output air pressure.

CONTROL SETTING UNIT : Provides a motions of varying the set point.   _{Your Comment}