Abstract: In a continuous ink jet printing system a viscosity determining means (12) has a measuring tank (29). An inlet (32) to the tank is connected to the system ink circuit to receive ink under pressure and the inlet has a flow restriction. A vent/return pipe (34) vents the tank and enables ink to be returned to the reservoir. An outlet (33) drains ink from the tank and returns it to the pump, the outlet including a valve (13) to open and close the outlet. A level detector (30,31) in the tank is connected to a timer (38) to determine the time taken for the tank to fill from a first level to a second level detected by the level detector and hence to provide a representation of viscosity of the ink.