A printing fluid refill unit for a print engine includes a reservoir having a variable storage volume for holding an amount of printing fluid, the variable storage volume having a plurality of discrete volumetric increments, each of the discrete volumetric increments having equal volumes, the reservoir further having an actuator for reducing the variable storage volume by one or more of the discrete volumetric increments; a ratchet system forming part of the actuator, the ratchet system effecting variation of the storage volume by a minimum increment of one discrete volumetric increment; an information storage element for storing information on the amount of printing fluid contained in the refill unit and for connecting with an information reader incorporated in the print engine for reading the information stored by the storage element when the refill unit is mounted to the print engine; a selector for activating the actuator to dispense a volume of the printing fluid equal to a selected multiple of the discrete volumetric increments, wherein the information stored by the storage element enables the reader to control the selector so as to derive the selected multiple of the volumetric increments; and a sensor for sensing the position of the ratchet, whereby a remaining volume of ink in the variable storage volume is determined.