An apparatus for use with a liquid delivery system, the liquid delivery system including a transparent cylinder for housing the liquid and a plunger movable along an axis of the cylinder for expelling the liquid through an outlet, the apparatus comprising: (a) a sliding cover configured for sliding engagement with the cylinder so as to be slidable along the cylinder parallel to the axis from a first position to a second position, wherein said sliding cover is implemented as a cap with a central bore for receiving an end portion of a pen injector having a projecting needle; (b) a set of sensors (110, 120) housed in said sliding cover so as to move together with said sliding cover, said set of sensors comprising at least a first sensor and a second sensor, said first sensor being a plunger sensor generating a signal, said plunger sensor being configured for non-contact sensing of at least part of the plunger as said sliding cover slides in engagement with the transparent cylinder such that variations in said first signal are indicative of the plunger passing a defined location along said sliding cover; (c) a cradle slidingly mounted within said central bore, said cradle configured for receiving the end portion of the pen injector, said cradle being spring biased towards an end position for engaging the end portion of the pen injector when said sliding cover is in said first position, and being retractable to move together with the end portion of the pen injector as said sliding cover slides to said second position; and (d) a processing system associated with said set of sensors so as to receive outputs of said sensors, said processing system being configured to identify a variation in an output of said plunger sensor when said plunger passes the plunger, said processing system being further configured to process said outputs to derive a location of the plunger along the cylinder. (a) a sliding cover configured for sliding engagement with the cylinder so as to be slidable along the cylinder parallel to the axis from a first position to a second position, wherein said sliding cover is implemented as a cap with a central bore for receiving an end portion of a pen injector having a projecting needle; (b) a set of sensors (110, 120) housed in said sliding cover so as to move together with said sliding cover, said set of sensors comprising at least a first sensor and a second sensor, said first sensor being a plunger sensor generating a signal, said plunger sensor being configured for non-contact sensing of at least part of the plunger as said sliding cover slides in engagement with the transparent cylinder such that variations in said first signal are indicative of the plunger passing a defined location along said sliding cover; (c) a cradle slidingly mounted within said central bore, said cradle configured for receiving the end portion of the pen injector, said cradle being spring biased towards an end position for engaging the end portion of the pen injector when said sliding cover is in said first position, and being retractable to move together with the end portion of the pen injector as said sliding cover slides to said second position; and (d) a processing system associated with said set of sensors so as to receive outputs of said sensors, said processing system being configured to identify a variation in an output of said plunger sensor when said plunger passes the plunger, said processing system being further configured to process said outputs to derive a location of the plunger along the cylinder.