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

1. A sheath fluid system for controlling pressure of a sheath fluid in a flow cytometer, the sheath fluid system comprising:

a reservoir configured to contain both a volume of sheath fluid and a volume of positively pressurized air and fluidically connected to a nozzle;

an external container configured to contain sheath fluid;

a pump fluidically interposed between the external container and the reservoir and configured to pump the sheath fluid from the external container to the reservoir;

an air regulator configured to regulate the volume of the positively pressurized air in the reservoir;

a compressor configured to supply compressed air to the air regulator;

an optical sensor configured to detect droplet locations of the sheath fluid flowing out of the nozzle;

a level controller configured to:

continuously pump, using the pump, the sheath fluid from the external container into the reservoir to maintain a substantially constant sheath fluid level in the reservoir so that an in-flow rate of sheath fluid flowing into the reservoir is substantially equal to an out-flow rate of the sheath fluid flowing out of the nozzle, and

adjust the in-flow rate of the sheath fluid flowing into the reservoir by adjusting a pump speed of the pump whenever the substantially constant sheath fluid level changes; and

an air pressure controller configured to:

determine the out-flow rate of fluid flowing out of the nozzle based on data from the optical sensor, and

control the pressure of the volume of positively pressurized air in the reservoir based upon the determination, so that the out-flow rate of fluid flowing out of the nozzle, as determined by the optical sensor, remains substantially constant.

2. The sheath fluid system of claim 1 , wherein the air pressure controller is further configured to:

detect a vertical location of a breakoff point at which droplets separate from a stream of fluid exiting the nozzle of the flow cytometer;

compare the vertical location with a desired vertical location; and

adjust the pressure of the volume of positively pressurized air in the reservoir to cause the vertical location of the breakoff point to substantially match the desired vertical location.

3. The sheath fluid system of claim 2 , wherein:

the optical sensor is a droplet camera configured to record images of the stream, and

the process of detecting a vertical location of a breakoff point comprises using the images of the stream recorded by the droplet camera.

4. The sheath fluid system of claim 1 , wherein the level controller is further configured to set a default in-flow rate by estimating the out-flow rate at which the sheath fluid is flowing out of the reservoir.

5. The sheath fluid system of claim 1 , wherein:

the level controller is further configured to determine a level of the sheath fluid in the reservoir, and

adjusting the in-flow rate of the sheath fluid flowing into the reservoir is based, at least in part, on the determination of the level of the sheath fluid in the reservoir.

6. The sheath fluid system of claim 5 , further comprising a level sensor configured to determine a level of a fluid in the reservoir, wherein determining the level of the sheath fluid in the reservoir is based on a level sensor signal generated by the level sensor.

7. The sheath fluid system of claim 6 , wherein the level sensor is an ultrasonic detector.

8. The sheath fluid system of claim 1 , wherein the level controller is further configured to:

take a plurality of measurements of a height of the sheath fluid in the reservoir, and

determine whether a trend in the height of the sheath fluid has been observed.

9. The sheath fluid system of claim 8 , wherein adjusting the in-flow rate of the sheath fluid flowing into the reservoir is performed in response to determining that a trend in the height of the sheath fluid has been observed.

10. The sheath fluid system of claim 1 , further comprising:

a three-way valve fluidically interposed between the pump and the reservoir; and

a waste container fluidically connected to the three-way valve, wherein the level controller is further configured to:

determine that an airlock has occurred in the pump, and

switch the three-way valve to cause the pump to flow the sheath fluid from the external container into the waste container.

11. The sheath fluid system of claim 10 , wherein determining that the airlock has occurred in the pump comprises determining that the out-flow rate of the sheath fluid from the reservoir is greater than the in-flow rate of the sheath fluid to the reservoir.

12. The sheath fluid system of claim 1 , wherein the level controller is further configured to adjust the sheath in-flow rate in response to a dampened feedback control loop.

13. The sheath fluid system of claim 12 , wherein:

the level controller is further configured to perform the dampened feedback control loop, and

the dampened feedback control loop comprises:

(a) measuring a first level of the sheath fluid in the reservoir;

(b) measuring, after measuring the first level of the sheath fluid in the reservoir by a first time period, a second level of the sheath fluid in the reservoir; and

(c) calculating a pump speed based on the first level and the second level of the sheath fluid in the reservoir.

14. The sheath fluid system of claim 13 , wherein the level controller is further configured to repeat (a) through (c).

15. The sheath fluid system of claim 13 , wherein calculating the pump speed comprises determining an out-flow rate of the sheath fluid from the reservoir based on the pump speed and a change in the level of the sheath fluid in the reservoir between the first level and the second level of the sheath fluid in the reservoir.

16. The sheath fluid system of claim 1 , wherein:

the level controller includes a proportional integral derivative controller, and

the process of adjusting the in-flow rate is performed in response to the proportional integral derivative controller.

17. The sheath fluid system of claim 1 , wherein the external container is configured to be refilled with sheath fluid.

18. The sheath fluid system of claim 1 , wherein:

the external container is configured to be fluidically disconnected from the pump,

the level controller is further configured to reduce, at least when the external container has been fluidically disconnected from the pump, the in-flow rate of the sheath fluid flowing into the reservoir to zero, and

the air pressure controller is further configured to increase, at least when the external container has been fluidically disconnected from the pump and at least when the in-flow rate of the sheath fluid flowing into the reservoir is zero, the pressure of the volume of positively pressurized air based upon the determination, so that the out-flow rate of fluid flowing out of the nozzle, as determined by the optical sensor, remains substantially constant.

19. The sheath fluid system of claim 18 , wherein:

the external container is configured to be fluidically connected to the pump after being fluidically disconnected from the pump,

the level controller is further configured to increase, at least when the external container has been fluidically connected to the pump, the in-flow rate of the sheath fluid flowing into the reservoir to an in-flow rate that is greater than the out-flow rate of fluid flowing out of the nozzle, and

the air pressure controller is further configured to reduce, at least when the external container has been replaced and when the in-flow rate of the sheath fluid flowing into the reservoir is greater than the out-flow rate of fluid flowing out of the nozzle, the pressure of the volume of positively pressurized air based upon the determination, so that the out-flow rate of fluid flowing out of the nozzle, as determined by the optical sensor, remains substantially constant.