Method for monitoring fluid flow from a volumetric pump

• US 5,180,287 A
• Filed: 08/12/1991
• Issued: 01/19/1993
• Est. Priority Date: 03/15/1990
• Status: Expired due to Term

• Current Assignee: Hospira Incorporated (Pfizer Inc.)
• Original Assignee: Abbott Laboratories Incorporated
• Inventors: Doll, Joseph E., Wu, Chung-You C., Lawless, Michael W., Natwick, Vernon R.
• Primary Examiner(s): Koczo, Michael
• Assistant Examiner(s): Scheuermann, David W.
• Application Number:
  US07/743,604
• Time in Patent Office: 526 Days
• Field of Search: 417/43, 417/53, 417/63, 137/486, 340/606
• US Class Current: 417/43
• CPC Class Codes: A61M 39/281 Automatic tube cut-off devices, e.g. squeezing tube on detection of air
• CPC Class Codes: A61M 5/142 Pressure infusion, e.g. using pumps
• CPC Class Codes: A61M 5/14228 with linear peristaltic action, i.e. comprising at least three pressurising members or a helical member
• CPC Class Codes: A61M 5/16854 by monitoring line pressure
• CPC Class Codes: F04B 43/082 the tubular flexible member being pressed against a wall by a number of elements, each having an alternating movement in a direction perpendicular to the axes of the tubular member and each having its own driving mechanism
• CPC Class Codes: F04B 49/065 and making use of computers
• CPC Class Codes: Y02A 90/10 Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation

Abstract

A method for determining if fluid is being displaced by a volumetric pump. The volumetric pump (30) includes an inlet cracking valve (46) and an outlet cracking valve (52) disposed on opposite sides of a plunger (48) used to displace fluid by compressing flexible tubing (34). During a pumping segment of a pumping cycle for the volumetric pump, the outlet cracking valve is closed with a cracking force that compresses the flexible tubing until the pressure of the fluid displaced by the plunger exceeds a cracking pressure, at which time the outlet cracking valve opens to enable fluid flow from the volumetric pump. A cracking flexure (182) provides the cracking force. As the outlet cracking valve opens in response to the fluid pressure exceeding the cracking pressure, a flow detector (54) mounted to the cracking flexure responds to the stress generated in the cracking flexure thereby, producing a signal indicative of fluid flow from the volumetric pump. Since a compressible gaseous fluid in the pumping portion of the flexible tubing does not develop the cracking pressure, the flow detector also provides an indication when a source of liquid (31) for the volumetric pump has run dry. The method comprises the steps of processing the signal to determine if it exceeds a predefined threshold that varies as a function of the rate at which the pump is set to deliver fluid.

Claims

1. A method for determining whether fluid is flowing from a positive displacement pump, which includes an outlet valve that opens to allow fluid to flow past it during a pumping cycle, when a fluid pressure within the pump reaches a predefined cracking pressure, comprising the steps of:

producing a signal indicative of a movement of the outlet valve from a closed position to at least a partially open position that allows fluid to flow from the pump in response to the fluid pressure within the pump exceeding the predefined cracking pressure;

establishing an electrical threshold for the signal;

comparing the signal to the threshold; and

determining that fluid is not flowing from the pump based upon the relative magnitudes of the signal and the threshold.

2. The method of claim 1, wherein the step of establishing the threshold comprises the steps of:

a. determining a pre-pressurization level of the signal during the pumping cycle at a first time interval that is substantially prior to pressurizing the fluid to the cracking pressure;

b. determining a post-pressurization level of the signal during the pumping cycle at a second time interval that is substantially after pressurizing the fluid to the cracking pressure; and

c. determining a difference between the post-pressurization level and the pre-pressurization level, the threshold being a function of said difference.

3. The method of claim 2, wherein the pre-pressurization level is an average of a plurality of pre-pressurization readings taken at a plurality of times during the first time interval.

4. The method of claim 2, wherein the post-pressurization level is an average of a plurality of post-pressurization readings taken at a plurality of times during the second time interval.

5. The method of claim 1, further comprising the step of compensating the threshold for variations in the signal as a function of a rate at which the pump delivers fluid.

6. The method of claim 1, further comprising the step of producing an alarm signal if the relative magnitude of the signal and the threshold indicates that fluid is not flowing from the pump.

7. The method of claim 6, wherein the step of producing the alarm signal occurs only if the relative magnitude of the signal and the threshold indicates that fluid is not flowing from the pump for a plurality of pumping cycles.

8. The method of claim 1, further comprising the step of determining whether fluid is not flowing from the pump due to an occlusion or due to the presence of a substantial volume of a gaseous fluid in the pump.

9. The method of claim 1, wherein pressurization of a substantially gaseous fluid by the pump can not develop a fluid pressure in excess of the cracking pressure, so that after the substantially gaseous fluid enters the pump, fluid flow from the pump is interrupted.

10. The method of claim 1, wherein the step of producing the signal comprises the step of monitoring a stress applied to a flexure that biases the outlet valve closed, to detect when the fluid pressure in the pump has exceeded the cracking pressure, since fluid flow from the pump occurs when the flexure is elastically deformed by the opening of the outlet valve.

11. A method for determining whether a liquid is flowing from an outlet of a positive displacement pump having an outlet valve that only opens to allow a fluid to flow from the pump when a pressure developed by the fluid exceeds a predefined cracking pressure, comprising the steps of:

a. applying a spring bias force to the outlet valve to develop a force that closes the outlet valve until the pressure of the fluid within the pump overcomes the force, causing the outlet valve to open sufficiently so that fluid flows from the pump;

b. sensing a movement of the outlet valve as fluid in the pump flows through the outlet valve after the pressure of the fluid exceeds the cracking pressure, forcing the outlet valve open, and by sensing the movement, producing a signal proportional to the opening of the outlet valve; and

c. in response to the signal, determining whether the liquid is flowing from the pump through the outlet valve.

12. The method of claim 11, further comprising the step of producing an alarm in the event that a cessation of liquid flow from the pump is detected.

13. The method of claim 11, wherein the step of sensing comprises the step of monitoring a stress in a spring that produces the spring bias force to determine whether the spring has been strained by the opening of the outlet valve.

14. The method of claim 13, wherein step of producing the signal comprises the step of mounting a strain gauge on the spring so that it is sensitive to stress of the spring and produces the signal in response thereto.

15. The method of claim 11, further comprising the step of detecting whether a substantial quantity of a gaseous fluid is within the pump, causing a cessation of fluid flow from the pump, as a function of an occlusion signal.

16. The method of claim 11, wherein the step of determining whether liquid is flowing from the pump comprises the steps of determining a baseline level, and comparing a magnitude of the signal to the baseline level, interruption of liquid flow from the pump causing the magnitude of the signal to be less than the baseline level.

17. The method of claim 16, wherein the step of determining the baseline level comprise determining an average level of the signal prior to pressurization of the fluid, determining an average level of the signal after pressurization of the fluid, and a difference between said average levels.

18. The method of claim 17, wherein the step of determining the baseline level further comprises the step of compensating the baseline level for variations in the signal due to a pumping rate of the pump.

19. The method of claim 17, wherein the step of determining the baseline level further comprises the step of determining a weighted average of the baseline level during successive pumping cycles.

20. The method of claim 19, wherein a weighting factor applied to determine the weighted average differs as a function of a selected pumping rate of the pump.