The Ethanol Blend Optimizer is a unique patented system that uses precision engineered piston cylinder assemblies, rigidly mounted together and mechanically linked, to mix a combined BOB/ethanol sample flow to the analyzers in the required ratio at very high precision better than± 0.05%. The complete system is mounted in a temperature-controlled cabinet. A concurrent heat exchanger and pressure control is provided on the inlet BOB sample and ethanol flows, to ensure they are the same temperature and pressure before mixing to maintain the highest possible accuracy. Safety features include leak detection in the cabinet that automatically isolates the BOB sample and ethanol inlet lines. The piston cylinders provide a very wide flow rangeability, without impacting precision, that is simply not possible with other mixing techniques.