An ink level detector for use with an ink jet printer includes two electrodes for sensing changes in ink level inside an ink reservoir due to consumption. Fresh ink is supplied to the reservoir from an ink cartridge which is mounted in an upper wall of the reservoir. At least one of the two electrodes is positioned adjacent to a location where the cartridge is mounted to the upper wall of the reservoir.

The ink jet printing system contains at least one ink jet printing head operatively connected to printing medium reservoirs by a conduit system. A partial ink flow is passed through a regulating apparatus for viscosity-dependent regulation. On exceeding the ink viscosity or ink density, a spindle body constructed as a float contacts a switch constructed as a proximity switch. The switch is operated such that a corresponding signal is supplied to a control valve and as a result the supply of solvent to a main ink reservoir of the printing medium reservoirs is initiated. The regulating apparatus may constitute an independent assembly associated with the main ink reservoir or can be integrated into the main ink reservoir.

There is described a process and apparatus which compares the pressure at the interface between the upper surface of a granular bed and the liquid in which it is submerged with the pressure above the liquid to maintain a predetermined head of liquid above the bed irrespective of expansion and contraction of the bed.

In an ink-jet printer or the like, in order to maintain a predetermined concentration of ink, a diluent equal in quantity to the solvent of ink, which has been evaporated, is supplemented to the ink recirculation system.

A continuous ink jet printer employing conductive ink and having a main ink supply, a supplemental ink supply, and a replenishment carrier fluid supply includes a fluid system that replenishes ink or carrier fluid based on the resistivity of ink in the main ink supply.