Patent ID: 7764891

Claim:
A solid-concentration control system comprising: a toner tank that is configured to contain a liquid toner including color particles as solid content and silicone oil as a carrier liquid; a first supply unit that supplies a concentrated liquid toner to the toner tank; a second supply unit that supplies a dilute solution including silicone oil to the toner tank, the dilute solution further including a used liquid toner from which at least some of the color particles have been consumed; a supply controller that controls supply of at least one of the concentrated liquid toner and the dilute solution to the toner tank; a first solid-concentration measuring apparatus that measures a first solid concentration of the liquid toner contained in the toner tank; and a second solid-concentration measuring apparatus that measures a second solid concentration of the dilute solution supplied by the second supply unit, wherein each of the first and second solid-concentration measuring apparatuses includes an ultrasonic transmitter that transmits a single-pulse ultrasonic wave to the liquid toner or the dilute solution; an ultrasonic receiver that faces the ultrasonic transmitter through the liquid toner or the dilute solution, and that receives the single-pulse ultrasonic wave; an ultrasonic-transmission controller that controls the ultrasonic transmitter to transmit another single-pulse ultrasonic wave after a predetermined time has passed after the ultrasonic receiver receives the single-pulse ultrasonic wave; a response-time integrator that measures a response time from transmission to reception of each single-pulse ultrasonic wave, and integrates measured response time with respect to each set of N pulses, N being an integer larger than 1; and a concentration calculator that calculates the first or second solid concentration based on integrated response time, and the supply controller controls the supply to the toner tank based on a differential solid concentration, which is a difference between the first and second solid concentrations, to adjust the solid concentration to a predetermined concentration.