A control system is disclosed for an ink jet printer. The control system employs acoustic transducers to determine the velocity of sound in the ink and in a reference chamber containing only fresh ink. By comparing the two measurements it can be determined when to add additional solvent to the system...http://www.google.de/patents/US5424766?utm_source=gb-gplus-sharePatent US5424766 - Ink jet printer control system responsive to acoustical properties of ink
