A method of controlling the time at which a droplet is controllably charged as said droplet breaks off from a carrier fluid containing particles to be sorted by a flow cytometer, the flow cytometer having a particle fluid transport chamber with an output orifice that is coupled to a droplet generator that controls a point at which said droplet breaks off from said carrier fluid leaving said output orifice and is controllably charged, the flow cytometer further having a droplet charger to controllably charge said droplet and a droplet sorter that is operative to cause selected droplets to be deflected and thereby sorted along a sorted droplet deflection path, separate from an unsorted droplet path along which a stream of unsorted droplets travel, said method comprising the steps of: (a) monitoring a prescribed zone within said particle fluid transport chamber of the flow cytometer for the presence of a particle traveling therethrough, and producing an output pulse having a width that is representative of the length of time that said particle is within said prescribed zone as said particle travels through said particle fluid transport chamber , said particle fluid transport chamber having a fluid flow path with a cross-section that varies from a relatively wide upstream zone where the particles enter said particle fluid transport chamber to a constriction at said output orifice , (b) calculating a composite time delay for said particle in accordance with geometry parameters of successive zones of said particle fluid transport chamber through which said carrier fluid containing said particle passes, between the time that said particle is sensed in step (a) as being present in said prescribed zone and the time at which a droplet containing said particle will break off from said carrier fluid ; and (c) using said composite time delay calculated in step (b) to controllably charge said droplet as said droplet breaks off from said carrier fluid ; characterized in that calculating said composite time delay is further based on width of said output pulse , and in that step (b) comprises calculating a respective time delay component of said composite time delay for a zone of said particle fluid transport chamber in which said particle within the carrier fluid undergoes acceleration by performing integration of an acceleration function between opposite ends of said zone, said acceleration function being determined empirically or deterministically based upon geometry parameters of the fluid flow system. (a) monitoring a prescribed zone within said particle fluid transport chamber of the flow cytometer for the presence of a particle traveling therethrough, and producing an output pulse having a width that is representative of the length of time that said particle is within said prescribed zone as said particle travels through said particle fluid transport chamber , said particle fluid transport chamber having a fluid flow path with a cross-section that varies from a relatively wide upstream zone where the particles enter said particle fluid transport chamber to a constriction at said output orifice , (b) calculating a composite time delay for said particle in accordance with geometry parameters of successive zones of said particle fluid transport chamber through which said carrier fluid containing said particle passes, between the time that said particle is sensed in step (a) as being present in said prescribed zone and the time at which a droplet containing said particle will break off from said carrier fluid ; and (c) using said composite time delay calculated in step (b) to controllably charge said droplet as said droplet breaks off from said carrier fluid ; <EOS>