Patent ID: 8121504

Claim:
An apparatus for measuring a concentration of a liquid developer, the apparatus comprising: a developer container having a sidewall and an opening, and adapted to contain the liquid developer for measurement; a stirring mechanism adapted to stir the liquid developer contained in the developer container; a load detector adapted to detect a load due to stirring of the liquid developer when the liquid developer is stirred by the stirring mechanism; and a controller configured to control a liquid surface of the liquid developer by controlling a speed at which the liquid developer is stirred to a selected speed so as to force the liquid surface upwardly along the sidewall of the container to a predetermined level, the predetermined level defined by a position of the opening of the developer container through which opening an excessive amount of the liquid developer is overflowing or has overflowed by stirring at the selected speed; and the controller configured to calculate the concentration of the liquid developer based on the load detected by the load detector when the liquid surface of the liquid developer is controlled to the predetermined level by stirring at the selected speed.