Continuous ink jet printers have been widely used for industrial marking such things as bar codes and product dates. An electronically controlled drop to ensure accurate printing positioning carries out the printing. An ink concentration control system has been developed to improve stability of the drop formation and printing quality. The key features are a unique in-line viscometer and an ink concentration control system. The ink concentration is constantly monitored by measuring ink viscosity and temperature, and the ink concentration control system can always maintain the concentration accurately by adjusting the ratio of solvent in the ink. The viscometer consists of a plunger moving vertically in a cylinder, a solenoid to raise the plunger by electromagnetic force and a proximity sensor to detect the position of the plunger. The ink viscosity can be calculated by the fall time of the plunger from the top to the bottom in the cylinder. The viscometer can measure it rapidly (in a few seconds), precisely, and automatically. Also, the reliability of the viscometer can be improved by measuring the fall time of the plunger repeatedly in a short period.