Level Measurement Equipment

Level of liquid in a vessel should be maintained above the exit pipe because if the vessel empties the exit flow will become zero, a situation that would upset downstream processes and could damage pumping equipment that requires liquid.  Also, the level should not overflow an open vessel nor should it exit through a vapor line of a closed vessel, which could disturb a process designed for vapor.  In addition, level can influence the performance of a process; the most common example is a liquid phase chemical reactor.  Level is usually reported as percent of span, rather than in length (e.g., m).
 Level sensors can be located in the vessel holding the liquid or in an external “leg” which acts as a manometer.  When in the vessel, float and displacement sensors are usually placed in a “stilling chamber” which reduces the effects of flows in the vessel. 
Float :  The float of material that is lighter than the fluid follows the movement of the liquid level.   The position of the float, perhaps attached to a rod, can be determined to measure the level.  

Displacement:  By Archimedes principle, a body immersed in a liquid is buoyed by a force equal to the weight of the liquid displaced by the body.  Thus, a body that is more dense than the liquid can be placed in the vessel, and the amount of liquid displaced by the body, measured by the weight of the body when in the liquid, can be used to determine the level. 

Differential pressure:  The difference in pressures between to points in a vessel depends on the fluids between these two points.  If the difference in densities between the fluids is significant, which is certainly true for a vapor and liquid and can be true for two different liquids, the difference in pressure can be used to determine the interface level between the fluids.  Usually, a seal liquid is used in the two connecting pipes (legs) to prevent plugging at the sensing points. 
Capacitance : A capacitance probe can be immersed in the liquid of the tank, and the capacitance between the probe and the vessel wall depends on the level. By measuring the capacitance of the liquid, the level of the tank can be determined.