This is an excerpt from a recent educational article by Pepperl+Fuchs:
There are two types of level measurement and level control:
Point level detection – also known as level limit detection
A level limit switch or point level is usually a probe that only provides a discrete output signal when the level reaches the probe or responds to it. It only signals to the operating personnel that a certain level has been reached, but does not indicate the height of the level. Level limit switches protect the pumps from overflowing or running dry.
Continuous level measurement
With continuous level measurement, the measuring probe indicates the exact height of the fluctuating level in the tank. Devices for continuous level measurement can be set so that the level in the tank is always indicated. Switch points for low-low or high-high level can be added as alarm points. This setting indicates to the operator that the level has reached a critical height.
The continuous measuring probe can be set up for both continuous level measurement and point level monitoring. Although this setting is convenient, it can lead to problems if the probe and/or the technology used provide incorrect measurements due to insufficient feedback, for example, due to an extended foam layer.