Patent Publication Number: US-6223130-B1

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

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood by reference to description, taken with the accompanying drawings, in which: 
     FIG. 1 is a schematic of a prior art flow control system; 
     FIG. 2 is a schematic of one embodiment of the invention for detecting holes in a fluid control system cassette; and 
     FIG. 3 is a block diagram illustrating a method of using one embodiment of the invention. 
     FIG. 4 is a block diagram illustrating a subset of the method of FIG.  3 . 
    
    
     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&#39;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 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 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 B  and L 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.