The ink jet printer for the labelling of goods has a write head, which comprises at least one outlet opening for ink droplets, a printing mechanism connected to said outlet opening, a pneumatic pump and an electrical control. It has at least one supply tank for fluid flowing through the outlet opening. The supply tank is releasably connected to the write head in a coupling area. In the write head, a first pressure sensor is arranged at the compressed-air line, which detects the air pressure in the compressed-air line and outputs an air pressure signal to the controller. A second pressure sensor is arranged at the ink line, which detects the pressure in the ink line and outputs an ink pressure signal to the controller. Said controller outputs a signal “ink empty”, if the ink pressure signal declines by more than 10% in relation to the air pressure signal.