You are in luck, gone are the days of your only choice being a 19mm Optical Density (OD) probe. Today, you have no fewer than three (3) from which to choose. The Optek comes in either a 5 or 10mm optical path length (OPL) while the Sartorius offering comes in a 10mm OPL. The Mettler Toledo solution uses backscattering in place of absorbance so no OPL. The one best for you will really depend on the specifics of your application.