Hydrostatic and differential-pressure level
Liquid height creates hydrostatic pressure related to density and gravity. Pressure or DP instruments can therefore infer level when density and reference conditions are adequately known. Changes in density can appear as false level change.
Radar
Radar level instruments use electromagnetic energy and time-of-flight or frequency-related processing to infer distance to a surface. Their performance can be affected by vessel geometry, internal structures and dielectric properties.
Guided-wave radar
Guided-wave radar sends electromagnetic energy along a probe. It can be useful in applications where a guided path improves measurement, though coatings, interfaces and probe mechanics still matter.
Ultrasonic
Ultrasonic level measurement uses acoustic time of flight. Vapor composition, temperature, foam, turbulence and obstructions can influence the acoustic path.
Point-level switches
Not every application needs a continuous level value. Point-level devices answer a discrete question such as high, low, present or absent and are often used for operational alarms or control functions.