The probe forms a capacitance with the metalic tank-wall.
The capacitance is sum of three capacitance:-
C(air)=ε(air) x P x (H-L)
C(material)=ε(material) x P x L
C(residual) is due to device itself.
Where ε(air) is the dielectric constant of air ≈1.
ε(material) is dielectric constant of material.
P is the constant of probe and installation, H is the active length of probe and L is the level of material.
Capacitance to level translation is performed with the aid of on-site calibration also called “wet-calibration”.