Replacement Stopper with Secchi Disc for Eisco Turbidity Tubes with a 2″ diameter. A plastic secchi disc with alternating black/white quadrants is attached to a heavy rubber stopper. A screw post threads through both the secchi disc and stopper to secure them together. Rubber stopper top diameter: 48mm; secchi disc diameter: 42mm; overall length including knob: 80mm (3.1″).