How do the sensors for water level measurements generally work?

There are two basic principles on which are water level measurements sensors operated. The first one is the hydrostatic pressure of water under the sensor. The second one is ultrasounds ray transmitted by sensor and receipted by it after its reflection by the water level. The first group of the sensors are of the submersion construction and is equipped by the pressure balance capillary to balance the unexpected changes of atmospheric pressure, the second one is constructive as pendant and must be fixed with some additional construction.