Liquid level definition
Level measurement and verification - Introduction

Locating plane: locating a level implies the presence of a reference plane, chosen in a purely conventional way. This could be, for example, the base of a tank, the zero level of a factory floor, or the so-called "normal" level of a balloon, from which fluctuations in the actual level of the liquid will be assessed.

Liquid in static equilibrium: a liquid level can only be defined if it is in perfect static equilibrium, with a smooth, mirror-like surface.

In practice, the accuracy of level measurement must take account of fluctuations around a statistical mean value, with a standard deviation and a distribution law associated with local and global variations in the liquid "plane". Usually, only the measurement of this mean level is industrially meaningful. High-frequency disturbances (ripples) can be eliminated...