Flow rate balanced, dynamically adjustable sheath delivery system for flow cytometry
Disclosed is a sheath delivery system that uses a continuous flow of sheath fluid into a pressurized internal reservoir that substantially matches the outflow of sheath fluid through the nozzle of a flow cytometer. A substantially constant level of the sheath fluid is maintained. If the sheath fluid level falls below a desired level, or goes above a desired level, a dampened control system is used to reach the desired level. In addition, air pressure in the pressurized internal container is controlled so that an external sheath container can be removed and refilled with additional sheath fluid without stopping the sheath delivery system 100 . Differences in pressure are detected by a droplet camera, which measures the droplet breakoff point to determine the pressure of the sheath fluid in the nozzle.
1. A method of controlling pressure of a sheath fluid in a positively pressurized reservoir holding a volume of sheath fluid and a volume of positively pressurized air in a flow cytometer configured to flow sheath fluid from the positively pressurized reservoir through a nozzle at an out-flow rate, the method comprising:
continuously pumping, using a pump interposed between an external container and the positively pressurized reservoir, the sheath fluid from the external container into the positively pressurized reservoir to maintain a substantially constant sheath fluid level in the positively pressurized reservoir so that an in-flow rate of sheath fluid flowing into the positively pressurized reservoir is substantially equal to the out-flow rate of the sheath fluid flowing out of the nozzle;
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;
controlling 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 visual sensor, remains substantially constant; and
adjusting the in-flow rate of the sheath fluid flowing into the positively pressurized reservoir by adjusting a pump speed of the pump whenever the substantially constant sheath fluid level changes.
2. The method of claim 1 wherein the process of adjusting the sheath in-flow rate is performed in response to a dampened feedback control loop.
3. The method of claim 2 , wherein the dampened feedback control loop comprises:
(a) measuring a first level of the sheath fluid in the positively pressurized reservoir;
(b) measuring, after measuring the first level of the sheath fluid in the positively pressurized reservoir by a first time period, a second level of the sheath fluid in the positively pressurized reservoir; and
(c) calculating a pump speed based on the first level and the second level of the sheath fluid in the positively pressurized reservoir.
4. The method of claim 3 , further comprising repeating (a) through (c).
5. The method of claim 3 , wherein calculating a pump speed comprises determining an out-flow rate of the sheath fluid from the positively pressurized reservoir based on the pump speed and a change in the level of the sheath fluid in the positively pressurized reservoir between the first level and the second level of the sheath fluid in the positively pressurized reservoir.
6. The method of claim 1 wherein the process of adjusting the in-flow rate is performed in response to a proportional integral derivative controller.
7. The method of claim 1 , further comprising:
setting a default in-flow rate by estimating the out-flow rate at which the sheath fluid is flowing out of the positively pressurized reservoir.
8. The method of claim 1 further comprising: detecting a vertical location of a breakoff point at which droplets separate from a stream of fluid exiting the nozzle of the flow cytometer;
comparing the vertical location with a desired vertical location; and
adjusting the pressure of the volume of positively pressurized air in the positively pressurized reservoir to cause the vertical location of the breakoff point to substantially match the desired vertical location.
9. The method of claim 8 wherein the process of detecting a vertical location of a breakoff point comprises:
using a droplet camera to record images of the stream.
10. The method of claim 1 , further comprising:
taking a plurality of measurements of a height of the sheath fluid;
determining whether a trend in the height of the sheath fluid has been observed.
11. The method of claim 10 , wherein the step of adjusting the in-flow rate of the sheath fluid flowing into the positively pressurized reservoir is performed in response to determining that a trend in the height of the sheath fluid has been observed.
12. The method of claim 1 , further comprising determining a level of the sheath fluid in the positively pressurized reservoir, wherein adjusting the in-flow rate of the sheath fluid flowing into the positively pressurized reservoir is based, at least in part, on the determination of the level of the sheath fluid in the positively pressurized reservoir.
13. The method of claim 12 , wherein determining the level of the sheath fluid in the positively pressurized reservoir is based on a level sensor signal generated by a level sensor configured to determine a level of a fluid in the positively pressurized reservoir.
14. The method of claim 1 , further comprising:
determining that an airlock has occurred in the pump; and
switching a three-way valve fluidically interposed between the pump and the positively pressurized reservoir to cause the pump to flow the sheath fluid from the external container into a waste container that is not positively pressurized.
15. The method of claim 14 , wherein determining that the airlock has occurred in the pump comprises determining that the out-flow rate of the sheath fluid from the positively pressurized reservoir is greater than the in-flow rate of the sheath fluid to the positively pressurized reservoir.