Where levels cannot be observed and hence measured directly, it is common to use a float and to indicate remotely the elevation of this float.
The float must be of an average density between that of the two fluids, the densities of which must be significantly different (such as water and air) to ensure that a sufficient buoyant force is generated on the float, with changing level, to activate the position-sensing mechanism.
The float may generate an analog signal which varies over the whole range of operating level, or may generate an on-off signal as the level rises above or falls below a predetermined elevation. A series of on-off sensors at different elevations can generate a digital type of level indication.
Float with mechanical indicator:
Several simple methods allow the float position and hence level to be observed indirectly from outside the vessel containing the fluids. A vertical rod attached to the float and protruding through a hole in the top of the tank can show the level by the amount of protrusion of the rod, which may be graduated. An external weight attached to the float by a rope or tape running over a pulley at the top of the tank can show the level on an inverse scale outside the tank. A rotating shaft passing through a sealed hole in the side of the tank can be connected to the float by a lever so that any rise or fall in the level rotates the shaft appropriately and so moves a pointer on an external graduated scale.
Float with electrical resistance sensor:
In a closed vessel a float having a lever connected to a variable-resistance sensor can cause a change in the electrical resistance as the level rises or falls. The change in electrical resistance can be used, via a suitable calibrated electrical instrument, to indicate the level or volume in the tank (Fig. 1b).
Float with magnetic switches:
In a closed tank a float pivoting about a fixed point can move a magnet close to the wall of the tank down or up as the level rises or falls about a selected level. A similar magnet just outside the tank is flipped by magnetic repulsion to operate electrical contacts to give a corresponding on-off signal. Magnetic repulsion rather than magnetic attraction is employed to create a definite toggle effect (Fig. 1c).
Float with buoyancy effect:
A float constrained in its vertical movement will exert a varying force on the restraining mechanism. This force in turn can be measured or converted into an analog electrical signal which can be calibrated to indicate the liquid level. In such an application, level can be measured only over the height of the float, so such floats usually have slender dimensions (Fig. 1d).