A tank has inflow and outflow equal to 50% of maximum and its level is
at the setpoint, say 50%. A step change in outflow occurs to 60%
(+10%). Outflow now exceeds inflow so the level will fall. The output
from the level transmitter will also fall and, for our system, will match
the fall in level say 1% change in signal for a 1% change in level. The
LT signal will open the A/C valve more, by 1% in fact. The inflow is
now 51%, still less than the outflow. The level will continue to fall until
inflow equals outflow, i.e., (60%). This can only happen when the LT
signal has changed by 10%) and this change reflects a drop in level on
10%: i.e., 10% offset.