I elements are systems with time delay of the 1st order and thus contain only one energy storage element. A typical example found in everday use is a tank or a bunker, whose input variable is the mass flow (mass per unit of time) and its output variable is the fill level. When there is a inlet flow which is constant in time, the fill level increases linearly until the maximum fill level is reached. An example for an electrical I element is a capacitor, in which the input variable selected is the charging current and the output variable is the capacitor's voltage: when the charge current is constant the capacitor voltage increases linearly (theoretically ad infinitum).