Apparatus and method for detection of a leak in a membrane of a fluid flow control system

An apparatus, method, and computer program product for detection of fluid leakage through a membrane in a fluid flow control system. The fluid flow control system has a first chamber and a second chamber. A membrane is disposed between the first chamber and the second chamber. The second chamber has a connection to a pressure tank, the pressure tank has a fluid with a pressure, and the connection defines a fluid path. The method indudes in a first step, blocking the fluid path. The pressure of the fluid in the pressure tank is then adjusted. The pressure is measured in the pressure tank which creates a pressure measurement at each of a first set of multiple timed intervals while the fluid path is blocked and after the pressure is adjusted. A blocked pressure rate is calculated based on the pressure measurements in the pressure tank at the first set of multiple timed intervals. Next, the fluid path is unblocked. The pressure is measured within the pressure tank creating a pressure measurement at each of a second set of multiple timed intervals after the fluid path is unblocked. Then, an unblocked pressure rate is calculated based on the pressure measurements in the pressure tank at the second set of multiple timed intervals. Finally a leakage rate is calculated based on the blocked pressure rate and the unblocked pressure rate. An alarm is caused when the leakage rate becomes greater than a predetermined threshold value.

TECHNICAL FIELD
 The present invention relates to fluid flow control systems and more
 specifically to the detection of fluid leakage in a fluid control system.
 BACKGROUND
 Numerous devices exist in the prior art for controlling the flow of fluid.
 A subclass of such devices includes fluid flow control systems. Fluid flow
 control systems regulate the rate of distribution of transport fluid
 through a line. Some examples of fluid control systems are kidney dialysis
 machines and intravenous blood transfusion devices. Fluid flow control
 system may include a cassette holder in which a disposable cassette is
 placed and wherein transport fluid is pumped by a membrane which is part
 of the cassette.
 FIG. 1 shows a portion of a prior art flow control system 14 which includes
 a cassette 10 mounted on a cassette holder 12. A flexible membrane 11
 covers the face of the flow control system cassette 10 and is permanently
 attached to the cassette 10.
 The flow control system 14 has a valving chamber 17 located in the cassette
 and a valve control volume 19 located in the cassette holder 12 which
 defines a valve 50. A portion of the flexible membrane 11 separates the
 valving chamber 17 and the valve control volume 19 and acts as a barrier
 to keep control fluid in the valve control volume 19 from mixing and
 contaminating transport fluid in the valving chamber 17. The control fluid
 is delivered to the valve control chamber 19 through a valve control fluid
 line 15.
 The flow control system 14 has a pump chamber 18 located in the flow
 control system cassette 10 and a pump control volume 100 located in the
 cassette housing 12 which defines a pump 52. A portion of the flexible
 membrane 11 separates the pump chamber 18 and the pump control volume 100
 and acts as a barrier to keep the control fluid in the pump control
 chamber 100 from mixing and contaminating the transport fluid in the pump
 chamber 18 while transport fluid is being pumped into or out of the pump
 chamber 18. The control fluid is delivered to the pump control chamber 100
 through a pump control fluid line 16.
 One problem with such a system is the cassette membrane may become
 punctured during transportation and handling of the cassette. If pinholes
 develop in the cassette membrane, the transport fluid may leak into the
 cassette holder requiring the cassette holder to be cleaned and replaced.
 Additionally, the control fluid may contaminate the transport fluid. The
 prior art system described above did not determine if there is a leak in
 the cassette after it is mounted in the cassette holder and prior to any
 transport fluid being pumped through the cassette.
 SUMMARY OF THE INVENTION
 In accordance with one embodiment of the invention, a method for detecting
 a leakage rate of fluid through a membrane in a fluid flow control system
 is provided. The fluid flow control system has a first chamber and a
 second chamber, the membrane is disposed between the first chamber and the
 second chamber, the second chamber has a connection to a pressure tank,
 the pressure tank has a fluid with a pressure, and the connection defines
 a fluid path. The method includes in a first step, blocking the fluid
 path. The pressure of the fluid in the pressure tank is then adjusted. The
 pressure is measured in the pressure tank which creates a pressure
 measurement at each of a first set of multiple timed intervals while the
 fluid path is blocked and after the pressure is adjusted. A blocked
 pressure rate is calculated based on the pressure measurements in the
 pressure tank at the first set of multiple timed intervals.
 Next, the fluid path is unblocked. The pressure is measured within the
 pressure tank creating a pressure measurement at each of a second set of
 multiple timed intervals after the fluid path is unblocked. Then, an
 unblocked pressure rate is calculated based on the pressure measurements
 in the pressure tank at the second set of multiple timed intervals.
 Finally a leakage rate is calculated based on the blocked pressure rate
 and the unblocked pressure rate.
 In another embodiment of the method a further step is added. An alarm is
 caused when the leakage rate becomes greater than a predetermined
 threshold value. The alarm may originate in the processor. The alarm may
 also be either a visual alarm or an auditory alarm.
 In a further related embodiment, in the step of measuring a pressure at a
 first set of multiple timed intervals and in the step of measuring a
 pressure at a second set of multiple timed intervals the pressure is
 measured with a transducer. In yet another related embodiment, in the step
 of calculating a blocked pressure rate and in the step of calculating an
 unblocked pressure rate, the rates are calculated in a processor.
 In yet another related embodiment, additional steps are added. After the
 step of measuring the pressure at a first set of multiple timed intervals,
 each of the pressure measurements is stored in a memory unit and the
 pressure measurements are then provided to the processor. Additionally,
 after the step of measuring the pressure at a second set of multiple timed
 intervals, each of the pressure measurements may be stored in the memory
 unit and then provided to the processor.
 In another embodiment of the invention, the embodiment is directed toward a
 flow control system. The system may include a first chamber and a second
 chamber with a membrane disposed between the first and second chambers.
 The system further includes a pressure tank containing a fluid having a
 pressure connected to the second chamber. A transducer is disposed within
 the pressure tank which creates a pressure signal. A valve is disposed
 between the chamber and the pressure tank. The system also includes a
 valve controller connected to the valve, a pump connected to the pressure
 tank and a processor connected to the transducer, to the pump and to the
 valve controller.
 The processor performs the following. The processor signals the valve
 controller to shut the valve. The processor adjusts the pressure of the
 fluid in the pressure tank with the pump. The pressure signal is read from
 the transducer at a first set of predetermined timed intervals and a
 baseline leak rate is calculated based on the first set of pressure
 signals while the valve is shut by the processor. The processor then sends
 a signal to the valve controller to open the valve. The processor reads
 the pressure signal from the transducer at a second set of predetermined
 timed intervals while the valve is open and calculates a membrane leak
 rate based on the second set of pressure signals. A leakage rate is
 calculated based on the baseline leak rate and the membrane leak rate and
 an alarm signal is created if the leakage rate exceeds a predefined value.
 The alarm signal may be an auditory or a visual alarm. In a preferred
 embodiment the fluid may be air.
 The system may further include a memory unit for storing the pressure
 signals at the first set of predetermined timed intervals and storing the
 pressure signals at the second set of predetermined timed intervals.
 A computer program product is provided, in yet another embodiment of the
 invention. The computer program product is a computer usable medium having
 computer readable program code thereon. The computer readable program code
 includes:
 program code for activating a valve controller for blocking the fluid path.
 program code for adjusting the pressure of the fluid in the pressure tank;
 program code for reading the pressure in the pressure tank;
 program code for creating a pressure measurement at each of a first set of
 multiple timed intervals while the fluid path is blocked and after the
 pressure is adjusted;
 program code for calculating a blocked pressure rate based on the pressure
 measurements in the pressure tank at the first set of multiple timed
 intervals;
 program code for activating the valve controller unblocking the fluid path;
 program code for reading the pressure within the pressure tank;
 program code for creating a pressure measurement at each of a second set of
 multiple timed intervals after the fluid path is unblocked;
 program code for calculating an unblocked pressure rate based on the
 pressure measurements in the pressure tank at the second set of multiple
 timed intervals; and
 program code for calculating a leakage rate based on the blocked pressure
 rate and the unblocked pressure rate.
 The computer program product may further include program code for causing
 an alarm when the leakage rate becomes greater than a predetermined
 threshold value.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS
 An embodiment of the apparatus for the detection of a leak in a membrane of
 a fluid flow control system cassette is shown in FIG. 2. The detection
 apparatus may be used in a fluid flow control system similar to the fluid
 flow control systems described in U.S. Pat. No. 4,778,451 to Kamen and in
 related patents U.S. Pat. Nos. 4,976,162, 5,088,515, and 5,178,182 all to
 Kamen, which are incorporated by reference herein in their entirety.
 In an embodiment of the apparatus, the fluid flow control system includes a
 cassette holder 212 in which a cassette 200 is placed. The cassette holder
 212 may be a housing in which the cassette is enclosed or it may be a
 shelf on which the cassette is mounted. In one embodiment of the apparatus
 where the fluid control system is used for kidney dialysis, multiple
 patients may use the same cassette holder where each patient has their own
 disposable cassette.
 A transport fluid may be pumped through the cassette 200 once the cassette
 200 is connected to the cassette holder 212. In this embodiment of the
 apparatus, the cassette 200 includes at least two chambers: a pump chamber
 218 and a valving chamber 217, however it is possible that the apparatus
 has a single chamber or multiple chambers. In a preferred embodiment, the
 cassette has a flexible exterior membrane 211 which will deform in
 response to pressure from a control fluid. This deformation of the
 membrane causes the transport fluid to be pumped.
 When the cassette 200 is properly positioned with respect to the cassette
 holder 212 the cassette membrane 211 is exposed to two chambers defined by
 the cassette holder 212: a valve control chamber 219 and a pump control
 chamber 300. In other embodiments of the apparatus, the cassette holder
 212 may have a single chamber or multiple chambers. The valve control
 chamber 219 and the pump control chamber 300 of the cassette holder 212
 align with the pump chamber 218 and the valving chamber 217 of the
 cassette, respectively. Pressure in the valve control chamber 219 and the
 pump control chamber 300 is regulated by a valve control valve 221 and by
 a pump control valve 222. The valve control valve 221 is controlled by a
 valve controller 223 and the pump control valve 222 is controlled by a
 pump valve controller 229. A control fluid line 220 supplies a control
 fluid from a pressure reservoir volume 224. The pressure reservoir volume
 may also be referred to as a pressure tank. The pressure of the control
 fluid within the pressure tank may be increased through pump 240 or
 relieved by opening a vent valve 242. Additional valves, pumps, chambers
 and pressure reservoir tanks may be incorporated into the apparatus
 without changing the overall function of the fluid control system.
 By alternating the opening and closing of the pump control valve 222 and
 the valve control valve 221, the control fluid can be dispersed from the
 pressure reservoir volume 224 to change the pressure placed on the
 membrane 211 at the pump control chamber 300 and at the valve control
 chamber 219. Through alternating pressure change, the transport fluid is
 directed through the cassette 200.
 The system may precisely and accurately measure the volume of fluid being
 transported using known methods, such as Boyle's law, as disclosed in U.S.
 Pat. No. 4,808,161 or acoustic spectral analysis as disclosed in U.S. Pat.
 No. 5,349,852 herein incorporated by reference in their entirety. The
 pressure in the pressure reservoir volume 224, is measured by a pressure
 transducer 225. (Any instrument for converting a fluid pressure to an
 electrical, hydraulic, optical or digital signal will be referred to as a
 "transducer".) The output signal from the pressure transducer 225 is
 relayed to a data processing unit 226, such as, a microprocessor.
 The data processing unit 226 has a memory unit 227 capable of storing and
 retrieving data from the data processing unit 226. The data processing
 unit 226 has the ability to control the operation of the valve control
 valve 221 by a valve controller 223 and the pump control valve 222 by the
 pump valve controller 229 and the vent valve 242 by the vent valve
 controller 244. The data processing unit 226 also controls an alarm unit
 228. The alarm unit 228 may be, but is not limited to, an auditory alarm
 or a visual alarm. The alarm unit 228 may also contain shutdown mechanisms
 that, when activated, prevents the use of a damaged flow control system
 cassette 220.
 FIG. 3 is a block diagram showing a method of using one embodiment of the
 invention. FIG. 4 is a block diagram illustrating a subset on the method
 of FIG. 3. The steps of the following described method are performed on
 the flow control system prior to transport fluid being pumped through
 lines 250 and 252. The cassette is in a "dry" state, such that no
 transport fluid has entered the cassette and the control fluid is not
 pressurized by the pump 240.
 During the first step (Step 30), the data processing unit 226 will verify
 that a flow control system cassette 200 is mounted on the cassette holder
 212. The flow control system has either a contact switch, or a sensor
 which sends a signal to the data processing unit 226 indicating that the
 cassette 200 is in the proper position for operation of the control flow
 system and pumping of the transport fluid.
 If a flow control system cassette 200 is properly mounted on the cassette
 holder 212, the data processing unit 226 proceeds to close valves 221, 222
 and 242 (Step 32) wherein the data processing unit 226 sends a signal to
 the valve controller 223 to close the valve control valve 221 and sends a
 signal to the pump valve controller 229 to dose the pump control valve 222
 thereby isolating the pressure reservoir volume 224 from the valve control
 chamber 219 and the pump control chamber 300. By isolating the cassette
 holder from the cassette, a baseline leak rate may be calculated for the
 cassette holder.
 In the pressurize volume step (Step 34), the pressure reservoir volume 224
 is pressurized with a control fluid. The data processing unit sends a
 signal to the pump 240 to pressurize the control fluid. In a preferred
 embodiment, the control fluid is air. The pressure of the control fluid of
 the pressure reservoir volume 224 may also be decreased by creating a
 partial vacuum with pump 240 on the control fluid. In other embodiments, a
 second pressure reservoir tank and a control fluid valve may be
 incorporated into the system to provide a partial vacuum reservoir for the
 system. The control fluid valve may be placed at a position along the
 control fluid line 220 with the second tank attached to the control fluid
 valve. The pressure of the control fluid within the second tank may be
 decreased to below atmospheric by the vacuum pump. The control fluid valve
 may then be opened, decreasing the overall pressure of the control fluid.
 As in other embodiments, the data processing unit 226 controls operation
 of the vacuum pump and the control fluid valve.
 In the step of recording and measuring (step 36), the signal from the
 pressure transducer 225 is sent to the data processing unit 226, then
 converted into data by an analog to digital conversion. In other
 embodiments, the transducer 225 may produce a digital signal where the
 data processing unit 226 would not perform an analog to digital
 conversion. A plurality of measurements at predetermined times are saved
 over a sampling period and finally stored in the memory unit 227 in
 digital form. In one embodiment, a first pressure measurement is made and
 stored at the beginning of the sampling period and at the end of the
 sampling period, a second pressure measurement is made. The selection of
 the sampling period length is determined, in part, by such factors as the
 size of the pressure reservoir and the resolution of the pressure
 transducer. The larger the pressure reservoir and the higher the
 resolution of the transducer the shorter the sampling period needs to be.
 In the step of determining a baseline leak rate of the system(L.sub.B)
 (step 38), the data processing unit 226 first retrieves the measurement
 data from the memory unit 227 and calculates a baseline leak rate by first
 taking the difference between the pressure measurement at the beginning of
 the sampling period and the measurement at the end of the sampling period
 and dividing by the sampling period. Other methods for determing a rate
 may also be implemented, where more than two measurement values are used,
 such as, determining a least-squares-fit line prior to calculating the
 baseline leakrate. In the step of opening the valve (step 40), the data
 processing unit 226 sends a signal to the valve controller 223 and the
 pump valve controller 229 to open the valve control valve 221 and the pump
 control valve 222, respectively.
 In the next step (step 42), the pressure transducer 225 produces a pressure
 signal in the pressure reservoir volume 224 and sends the signal back to
 the data processing unit 226 where the signal is converted from analog to
 digital. The digital data is sampled at least twice during the sampling
 period and the data is then stored in the memory unit 227. In one
 embodiment, a first pressure measurement is made and stored at the
 beginning of the sampling period and at the end of the sampling period, a
 second pressure measurement is made.
 The data processing unit 226 then calculates the leak rate of the membrane
 (L.sub.M) (Step 44) by first taking the difference between the pressure
 measurement at the beginning of the sampling period and the measurement at
 the end of the sampling period and then dividing by the sampling period.
 All of the data measurements that are used for calculating LM are obtained
 while the valve control valve 221 and the pump control valve 222 are open.
 In other embodiments, alternative techniques for calculating the membrane
 leakrate may be used when there are more than two pressure measurements.
 Such techniques are known to those skilled in the art and include
 calculating a least-squares-fit line prior to calculating the membrane
 leakrate.
 In comparing L.sub.B and L.sub.M (step 46), the data processing unit 226
 compares the two leak rates and determines if the difference between the
 leak rates is greater than a critical leak rate. The critical leak rate is
 an empirically determined value found by measuring the leak rate of the
 cassette with known defects in the membrane.
 If the data processing unit 226 determines that the difference between the
 two leak rates is greater than the critical leak rate, the data processing
 unit 226 will initiate an alarm sequence (Step 48). The alarm sequence may
 include activating an auditory or visual indicator and may also include a
 shutdown procedure to prevent the use of a faulty flow control system
 cassette 200. Comparing the baseline leak rate for the system and the leak
 rate of the membrane, allows the data processing unit to determine if the
 membrane has been punctured or is defective before it is used for pumping
 the transport fluid. This provides a higher level of safety by eliminating
 the possibility of contaminating the transport fluid through exposure to
 the control fluid. Additionally, this system aids in the accuracy of the
 volumetric measurement of transport fluid that is delivered by stopping
 the fluid flow control system from operating when a puncture occurs which
 would bleed off transport fluid from its intended destination and produce
 erroneous results. Additionally the system prevents transport fluid from
 flowing into the cassette holder. If transport fluid flows into the
 cassette holder, the cassette holder must be cleaned.
 Although the invention has been described with reference to several
 preferred embodiments, it will be understood by one of ordinary skill in
 the art that various modifications can be made without departing from the
 spirit and the scope of the invention, as set forth in the claims below.