In a gravitational field, the vertical upward force exerted on an object in a fluid due to its density difference from the surrounding fluid is called buoyancy.

An object with an overall density lower than that of a liquid will eventually float on the surface of the liquid, and this object can be called a float. Under constant conditions, the buoyancy experienced by the float remains constant (constant buoyancy).

When a float is subjected to an upward force that is less than its own weight or a downward force that is less than the weight of a liquid equivalent to its own volume, the characteristic of the float eventually floating on the surface of the liquid will not be changed.

The buoyancy experienced by the float in a stable state is constant.

When there is a vertical change in the position of the liquid surface, the vertical position of the float will change synchronously with the position of the liquid surface under constant buoyancy.

A device that expresses the vertical position of a float in a form that can be directly read by humans through various mechanical, electromagnetic, optical, and other physical means is a float level gauge.