An alternative method of illustrating proportional control is by means of
a simple float system (Figure 6). Assume the inflow and outflow are
equal and the level is at the setpoint. If an increase in outflow occurs the
level in the tank must fall. The float will also fall as the level falls. This
drop in float position will cause the valve on the inflow to open more
thus increasing the inflow. Eventually the fall in level will result in a
valve opening, which will restore the mass balance between the inflow
and the outflow.