A sensor system that provides an adjustable threshold level for the sensed quantity is described. The flow and conductivity measuring integrated sensor is of a small size that enables it to be inserted into the permeate tube of connected membrane elements during RO unit operation. It is possible to realize the cup holder having high holding performance for a tall container without requiring a large storing space. The control information from the secondary control element, however, may be discarded either at the forwarding element or at a blocking agent between the secondary control element and the forwarding element. Based on the determined reducing agent concentration, the plating bath solution is modified. An optical fiber comprises at least one photonic crystal means coupled to a first end thereof, and a magnetic material coupled to the at least one photonic crystal means.