Abstract:
A flow measuring device, comprising a container for receiving a flow of liquid, a plurality of optical sensors associated with the container, each of the plurality of optical sensors producing an output signal dependent upon the presence of liquid reaching a level in the container associated with each optical sensor, means for modifying the output signal in accordance with at least one predetermined temperature compensation factor, and means for calculating a flow rate from the time at which the liquid traverses between two levels in the container.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to a flow measuring device and a method of operating the device. In particular, this disclosure relates to an optical flow measuring device for checking medical infusion devices and which compensates for the effects of temperature on the optical sensors. 
         [0002]    It is essential when treating patients that they receive the correct dosage of drugs or medication that is administered using a medical infusion device. Medical infusion devices have been known for many years, and are often utilized where it would otherwise be impractical, unreliable, or too expensive to administer the medication manually by medical practitioners. For example, medical infusion devices can administer medication at flow rates as low as 0.1 ml per hour. They can supply doses every minute or so or repeated boluses as requested by the patient up to maximum number per hour (e.g., in patient-controlled analgesia), or supply fluids whose volumes vary by the time of day. Small-volume pumps include motorized syringes and small electronic diaphragm pumps. Higher flow rates can be achieved using peristaltic pumps, gravity-fed infusion lines, and gravity-fed flow controllers that are also known in the prior art. 
         [0003]    While infusion devices differ in their mode of operation and range of delivery, what is common to all types of medical infusion devices is the need to accurately check that the device is delivering medication at the required dose. While it would be possible to have a testing device for each type of medical infusion device, it is desirable to have a common testing device for the many types of medical infusion devices that are available in the marketplace. 
         [0004]    U.S. Pat. No. 4,938,092 describes a known flow measuring device which can be used to test a variety of medical infusion devices. This flow measuring device has an optical measurement sensor that detects when a liquid in a vertical tube moves between two levels. Since the volume defined between the two levels is known, simply dividing the known volume by the time measured for the liquid to move between the two levels derives a flow rate. This technique has been further developed in U.S. Pat. No. 5,487,309 to utilize multiple pairs of optical sensors to improve the measurement discrimination and reduce error. 
         [0005]    This measurement technique, using optical measurement sensors, is susceptible to changes in temperature. Put simply, the voltage that is outputted from the sensor pair is temperature-dependent. In order to compensate for these temperature-dependent changes, it is often necessary to measure, or have knowledge of, the temperature, which necessitates a further transducer and other compensation circuitry to offset temperature-dependent effects. These additional components therefore increase the overall cost and complexity of the flow measuring device, and since these devices are often realized using a microprocessor, there is therefore a resultant computational cost as well. 
       SUMMARY 
       [0006]    It is an object of the present disclosure to provide a flow measuring device and a method of operating the device that overcomes, or at least reduces, the drawbacks associated with known optical medical infusion testing devices. The flow measuring device and a method of operating the device enable a reduction in the incidence of errors due to temperature-induced changes in the performance of the optical sensors. This is achieved without any measurement, or knowledge, of the actual temperature of the sensor pair. 
         [0007]    As disclosed herein, there is provided a flow measuring device, comprising: 
         [0008]    a container for receiving a flow of liquid; 
         [0009]    a plurality of optical sensors associated with the container, each of the plurality of optical sensors producing an output signal dependent upon the presence of liquid reaching a level in the container associated with each optical sensor; 
         [0010]    means for modifying the sensed output signal in accordance with at least one predetermined temperature compensation factor; and 
         [0011]    means for calculating a flow rate from the time at which the liquid traverses between two levels in the container. 
         [0012]    An advantage of using a flow measuring device according to the present disclosure for checking medical infusion devices is that a reduction in the incidence of errors due to temperature-induced changes in the performance of the optical sensors is achieved. This is achieved without any measurement, or knowledge, of the actual temperature of the optical sensor pair. 
         [0013]    Preferably, the flow measuring device further comprises means for displaying the calculated flow rate. 
         [0014]    Further preferably, the at least one predetermined temperature compensation factor compensates for the effects of temperature on the plurality of optical sensors. 
         [0015]    In use, the at least one predetermined temperature compensation factor may compensate for the effects of temperature on the plurality of optical sensors by minimizing the prospect of the sensed output signal exceeding a predetermined threshold voltage which produces a potentially false determination that liquid is present in the container. 
         [0016]    Preferably, the at least one predetermined temperature compensation factor compensates for the effects of temperature on the plurality of optical sensors by minimizing the prospect of the sensed output signal falling below the predetermined threshold voltage which produces a potentially false determination that liquid is not present in the container. 
         [0017]    Further preferably, the plurality of optical sensors are each configured as a light-emitting diode and corresponding photodetector positioned on opposing sides of the container. 
         [0018]    In use, the plurality of optical sensors may be positioned to define at least two levels in the container having at least one known volume therebetween. 
         [0019]    Preferably, the means for modifying, the means for calculating, and the means for displaying are implemented in a microprocessor or digital signal processor. 
         [0020]    Further preferably, the microprocessor or digital signal processor also includes additional programmable functionality, which is selected from the group comprising, but not limited to, any one of the following: the capability to determine and display the delivered liquid volume, flow rate, and/or back pressure. 
         [0021]    Also according to the present disclosure, there is provided a method for controlling a flow measuring device, the flow measuring device comprising a container for receiving a flow of liquid and a plurality of optical sensors associated with the container and positioned to define at least two levels in the container. The method comprises the steps of: 
         [0022]    sensing at least one output signal from each of the plurality of optical sensors (V Out ) that is dependent upon the presence of liquid reaching a level in the container associated with each optical sensor; 
         [0023]    modifying the sensed output signal in accordance with at least one predetermined temperature compensation factor; and 
         [0024]    calculating a flow rate from the time at which the liquid traverses between two levels in the container. 
         [0025]    In use, the step of modifying the sensed output signal in accordance with at least one predetermined temperature compensation factor may further comprise the steps of: 
         [0026]    priming the container with liquid and modifying the output signal from each of the plurality of optical sensors to a first voltage output level (V WetSet ); 
         [0027]    removing liquid from the container and sensing a second voltage output level (V DrySet ) from each of the plurality of optical sensors; and 
         [0028]    calculating a threshold voltage (V T ) from the first voltage output level (V WetSet ) and the second voltage output level (V DrySet ). 
         [0029]    Preferably, the step of calculating a threshold voltage (V T ) from the first voltage output level (V WetSet ) and the second voltage output level (V DrySet ) is obtained from V T =V WetSet +K(V Dryset −V Wetset ), where K=0.5, or another value. 
         [0030]    Further preferably, the method comprises the steps of: 
         [0031]    priming the container with liquid and sensing a third voltage output level (V wet ) from each of the plurality of optical sensors; 
         [0032]    removing liquid from the container and sensing a fourth voltage output level (V Dry ) from each of the plurality of optical sensors; 
         [0033]    repeating the priming and removing steps over a range of temperatures; and 
         [0034]    calculating a compensation factor α that is a ratio of the change of V Dry  to the change of V Wet  with temperature. 
         [0035]    In use, the method further comprises the step of: 
         [0036]    storing the first voltage output level (V WetSet ), the second voltage output level (V DrySet ), the threshold voltage (V T ) and the compensation factor α in a non-volatile memory. 
         [0037]    Preferably, the method further comprises the steps of: 
         [0038]    calculating a fifth voltage output level (V Wetshift ) from V Wetshift =V Wet −V WetSet ; and 
         [0039]    compensating the fifth voltage output level (V Wetshift ) by the compensation factor α stored in the non-volatile memory. 
         [0040]    Further preferably, the method further comprises the step of: 
         [0041]    calculating a temperature-compensated signal (V OutCorrected ) from V OutCorrected =V Out −V Wetshift . 
         [0042]    In use, the method further comprises the step of: 
         [0043]    displaying the calculated flow rate. 
         [0044]    Further, according to the present disclosure, there is provided a non-transitory computer program product having computer-executable instructions stored thereon for controlling a flow measuring device, the flow measuring device comprising a container for receiving a flow of liquid, and a plurality of optical sensors associated with the container and positioned to define at least two levels in the container, the computer program product comprising: 
         [0045]    computer-executable instructions for sensing at least one output signal from each of the plurality of optical sensors (V Out ) that is dependent upon the presence of liquid reaching a level in the container associated with each optical sensor; 
         [0046]    computer-executable instructions for modifying the sensed output signal in accordance with at least one predetermined temperature compensation factor; and computer-executable instructions for calculating a flow rate from the time at which the liquid traverses between two levels in the container. 
         [0047]    It is believed that a flow measuring device and a method of operating the device in accordance with the present disclosure at least addresses the problems outlined above. Those skilled in the art will recognize variations of the present disclosure are possible and it is intended that the present disclosure may be used other than as specifically described herein. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0048]    Specific non-limiting embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings, in which: 
           [0049]      FIG. 1  illustrates an optical level sensor pair that is used in the flow measuring device according to the present disclosure; 
           [0050]      FIG. 2  illustrates how the optical sensor pair produces an output voltage V out  that is significantly different when liquid is present in the vessel of the flow measuring device and also illustrates five variables that are used when compensating for temperature-induced changes in the performance of the optical sensor pair; 
           [0051]      FIG. 3  is a graph illustrating how the voltage obtained from the optical sensor pair varies as a function of temperature when liquid is present V Wet , and not present V Dry , in the vessel of the flow measuring device; 
           [0052]      FIGS. 4 and 5  are respectively graphs illustrating how the present disclosure compensates for the effects of varying temperature on the sensor pair by minimizing the prospect of either of the compensated signals V DryCorrected  or V WetCorrected  reaching the threshold voltage V T  which could give a potentially false determination that liquid is present, or not, in the vessel of the flow measuring device; 
           [0053]      FIG. 6  illustrates calibration steps that are taken during initial factory setup of the flow measuring device to obtain the parameters V WetSet , V DrySet  and V T . V WetSet  and V T  are then stored in the memory of the device; and 
           [0054]      FIG. 7  illustrates a method of operating the flow measuring device according to the present disclosure which compensates for the effects of temperature on the optical sensors. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]    Referring now to the drawings, a level sensor pair which is used in a flow measuring device for checking medical infusion devices is shown in  FIG. 1 .  FIG. 1  shows that a light-emitting diode  10  (the abbreviation “LED” will be used throughout in place of “light emitting diode”) and a photodetector  12  are placed on opposing sides of a container or vessel  14 . The LED  10  and photodetector  12  are configured as a level sensor pair. 
         [0056]    At least one further level sensor pair is positioned on the generally vertical vessel  14  to detect when a liquid  16  in the vessel  14  moves between the two distinct levels defined by the sensor pairs. When checking a medical infusion device, the test liquid  16  that is generally used is water because of its availability and inherent chemical and optical properties. Since the volume defined between the two levels is known, simply dividing the known volume by the time measured for the liquid to move between the two levels derives a flow rate. 
         [0057]      FIG. 1  illustrates that when there is no liquid  16  between the LED  10  and photodetector  12 , the sensor pair produces a known voltage V out  that is significantly different from when a liquid is present. The output of the sensor pair is monitored by a processing unit (not shown), and as a test fluid meniscus  18  travels up the vessel  14 , the rate of flow and volume can be determined based on the pre-calibrated volume between each sensor pair. The skilled person will appreciate that the processing unit will also include a microprocessor with instructions written in software for controlling the flow measuring device and displaying information such as flow rate, delivered volume, and back pressure. 
         [0058]      FIG. 2  illustrates how the optical sensor pair produces an output voltage V out  that is significantly different when liquid  16  is present in the vessel  14  of the flow measuring device and also illustrates five variables that are considered when determining whether the vessel  14  is wet or dry. Infrared transmission is higher when water is present in the vessel  14 , and in the circuit configuration shown in  FIG. 1 , the voltage V out  rises in the presence of air between the LED  10  and photodetector  12 . 
         [0059]    As can be seen from  FIG. 2 , as the meniscus  18  travels up the vessel  14 , V Out  produces a “hump.” As the meniscus  18  moves up the vessel  14  with only air between the LED  10  and photodetector  12  sensor pair, V out  is high (less light transmission). As the meniscus  18  reaches the sensor pair it casts a shadow which causes V out  to go even higher. When liquid is positioned between the LED  10  and photodetector  12  sensor pair, the light increases rapidly as the infrared travels through only liquid, and V out  rapidly drops off, as shown in  FIG. 2 . 
         [0060]      FIG. 2  also shows the relationship between the output voltage of the sensor pair V out  and five system variables that are used in the determination of whether there is liquid in the vessel  14 , i.e., it is wet, or whether there is air present in the vessel  14 , i.e., it is dry. These include:
       V Dry  which is the measured voltage when there is air present in the vessel  14 ;   V wet  which is the measured voltage when there is liquid present in the vessel  14 ;   V WetSet  which is the voltage level that is set initially with liquid present by adjustment manually or automatically of resistors R 1  and R 2  in  FIG. 1 ;   V DrySet  is the voltage that is observed when the vessel  14  was dry during initial setup; and   a threshold voltage V T  between both V WetSet  and V DrySet .       
 
         [0066]    In operation, the processing unit determines the vessel  14  to be wet if V Out ≦V T . The threshold voltage V T  is determined during an initial setup of the device as: 
         [0000]        V   T   =V   WetSet   +K ( V   Dryset   −V   Wetset )  (Eq.1)
 
         [0000]    where V T  can be halfway between V WetSet  and V DrySet  (K=0.5 or another value). 
         [0067]    A problem, however, arises using this method due to V Wet  and V Dry  changing with the temperature of the sensor pair, which can lead to faulty determination of wet and dry values if either V Dry  or V Wet  reaches the threshold V T . This is shown schematically in  FIG. 3  which shows how the voltage obtained from the optical sensor pair varies as a function of temperature when liquid is present V Wet  and not present V Dry , in the vessel  14  of the flow measuring device. 
         [0068]    The present disclosure details a method of compensating for the effects of varying temperature of the sensor pair by minimizing the prospect of either V Dry  or V Wet  reaching the threshold voltage V T . This is achieved without knowing the actual temperature of the sensor pair. Instead, a corrected figure for V Out  can be calculated by knowing the current value of V Wet , and subtracting from the original V WetSet . V Wet  is available whenever the tube is known to be full of liquid. 
         [0069]    So: 
         [0000]        V   Wetshift   =V   Wet   −V   WetSet   (Eq.2)
 
         [0070]    A correction is then applied to V Out  to give V OutCorrected , follows: 
         [0000]        V   OutCorrected   =V   Out   −V   Wetshift   (Eq.3)
 
         [0071]    This results in a temperature compensation response, as shown schematically in  FIG. 4 . 
         [0072]    With many proprietary optical sensor pairs available in the marketplace, the dry value rises less with increases in temperature, and therefore may be overcorrected by this method. A factor α can be introduced to cause the V OutCorrected  values for wet and dry conditions to remain approximately equidistant from V T , as shown in  FIG. 5 . 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     OutCorrected 
                   
                   = 
                   
                     
                       V 
                       Out 
                     
                     - 
                     
                       
                         ( 
                         
                           
                             1 
                             - 
                             α 
                           
                           2 
                         
                         ) 
                       
                        
                       
                         V 
                         Wetshift 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where α is the ratio of the change in the V Dry  to the change in V Wet  with temperature, i.e., the ratio of the gradients of V Dry  and V Wet  in  FIG. 3 . The factor α is determined by experiment and depends on the particular proprietary optical sensor pair used. Typically, α is found to be around 0.6. As can be seen from Eq.4 and  FIGS. 4 and 5 , the addition of the factor α modifies the V OutCorrected  for both wet (V WetCorrected ) and dry (V DryCorrected ) conditions, respectively, around V T . 
         [0073]    It is the obtained value of V OutCorrected  that is used to determine if the vessel  14  is wet if V OutCorrected ≦V T . 
         [0074]      FIG. 6  illustrates calibration steps that are taken during initial factory setup of the flow measuring device to obtain the parameters V WetSet , V DrySet  and V T  that are subsequently stored in the memory of the flow measuring device. 
         [0075]    As shown in  FIG. 6 , during factory setup, the vessel  14  is initially filled with liquid  20 . At step  22 , the resistors R 1  and R 2  are adjusted, manually or automatically, to obtain V WetSet . This voltage is chosen to maximize the difference between V Wet  and V Dry . V WetSet  is typically 20% of the maximum possible V out . V WetSet  is the voltage that is set initially with liquid present in the vessel  14 . At step  24 , the liquid is emptied from the measuring vessel  14  and a measurement of V DrySet  performed  26 . 
         [0076]    As the determination of V WetSet  and V DrySet  has been made, the processing unit then calculates at step  28  the threshold voltage V T  using Eq. 1. Typically, the constant K is 0.5. At step  30 , V WetSet  and V T  are stored in a non-volatile memory of the processing unit. 
         [0077]    The factor α is then obtained and stored 32 in the non-volatile memory of the processing unit. The factor α is the ratio of the change of V Dry  to the change of V Wet  with temperature, i.e., the ratio of the gradients of V Dry  and V Wet  in  FIG. 3 . The factor α is determined by experiment and depends on the particular proprietary optical sensor pair used. Typically, α is found to be around 0.6. 
         [0078]      FIG. 7  shows how the flow measuring device of the present disclosure is used in operation and compensates for the effects of varying temperature of the sensor pair by determining the corrected output V OutCorrected . 
         [0079]    At step  40 , a flow rate test is initiated and the vessel  14  is then filled with liquid  42 . The voltage V Wet  is then measured  44  when there is liquid present in the vessel  14 . A calculation of V WetShift  can then be performed  46  using Eq. 2. At step  48 , the calculated V WetShift  is compensated by factor α using Eq. 4 to shift the responses to remain approximately equidistant from V T . V WetShift  is then stored in the non-volatile memory at step  50 . 
         [0080]    Using such an approach, a flow measuring device including such a compensation algorithm is able to compensate for temperature-induced changes in the performance of the optical sensors and accurately check that the medical infusion device under test is delivering medication at the required dose. In this way, with each subsequent measurement performed at step  52 , the output voltage of the sensor pair V Out  is corrected  54  to give V OutCorrected  using Eq. 3. 
         [0081]    It is the obtained value of V OutCorrected  that is used to determine at step  56  if the vessel  14  is wet, i.e., if V OutCorrected ≦V T . As the time taken for the liquid meniscus to traverse two or more levels with a known volume between is measured, it is possible to accurately measure the flow rate. 
         [0082]    Various alterations and modifications may be made to the present disclosure will without departing from the scope of the invention.