An apparatus for storing fluid containing bags comprising a shelf having a bag-supporting surface, and a pair of laterally extending projections. The optical signal and the modulating signal are propagated at an angle greater than zero degrees with respect to one another to adjust a path length of at least one of the signals relative to the other of the signals. The method also includes the steps of storing a database of appropriate initial conditions that resulted in satisfactory transient response in a rule-base and using the rule-base at start-up of the paint system or if the paint system fails to be satisfactorily controlled.