Although many circuits use the varying-capacitance method for checking liquid levels, this simple resistive circuit is much easier to construct. Even a tank of a liquid, such as water, has sufficient conductive salts in solution for this method to work. The probe uses a metal rod that supports 10 insulated wires, which have stripped ends pointing down. As the level of liquid rises, resistors Rl through Rl0 are successively brought into circuit, each drawing an extra 10 p.A through the meter. Shunt resistors Rll and R13 calibrate the meter for a full-scale reading when the tank is full.
Liquid-level-checker - schematic

Resistor R12 limits the current through the meter. If tank isn`t rectangular-ie, if the volume of the liquid it contains isn`t directly proportional to the liquid`s depth-space the resistors accordingly or use a nonlinear progression of resistor values and retain constant resistor spacing.