Abstract: Ink concentration in a system is maintained regardless of the duty cycle under which the system is operating. In an ink jet system, a print head receives ink from the main ink supply and forms continuous drops. The drops needed for printing to form the desired image are selected from the continuously formed drops. Based on the selection, a count signal is produced indicative of the number of drops printed. An ink level sensor in the main ink reservoir generates a low ink level signal when ink in the reservoir reaches a predetermined low level, the difference between a normal level and the low level corresponding to a predetermined cycle volume. A fluid connection selectively allows flow into the main ink reservoir from either the external supply of ink or the external supply of ink replenisher.