Flow rate balanced, dynamically adjustable sheath delivery system for flow cytometry

A method of adding sheath fluid to a flow cytometer having a positively pressurized reservoir holding both a volume of sheath fluid and a volume of positively pressurized air, wherein the flow cytometer is configured to flow sheath fluid from the positively pressurized reservoir through a nozzle at an out-flow rate, the method comprising:

• pumping, using a pump interposed between an external container containing sheath fluid and the positively pressurized reservoir, the sheath fluid from the external container into the positively pressurized reservoir at an in-flow rate;
• determining the out-flow rate of fluid flowing out of the nozzle based on data from a visual sensor configured to detect droplet locations of the sheath fluid flowing out of the nozzle;
• reducing the in-flow rate of the sheath fluid flowing into the positively pressurized reservoir to zero;
• and
• increasing, after the reducing and based upon the determination, the pressure of the volume of positively pressurized air so that the out-flow rate of fluid flowing out of the nozzle, as determined by the visual sensor, remains substantially constant.