Abstract:
A method and apparatus are disclosed for using flow rate changes to extract additional information from an in-line flow sensor.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to fluid flow control systems, such as intravenous infusion pumps, and more particularly to feedback control infusion pumps with flow sensing, volume sensing, variable pressure control, and variable flow resistance. In particular, the present disclosure relates to a method and apparatus for extracting enhanced information from an in-line fluid flow sensor by imposing an abrupt flow rate change. 
         [0002]    A conventional large volume infusion pump is typically equipped with a motor that, in connection with a mechanical assembly and through the interface of a fluid barrier, pushes a small amount of fluid per motor “step.” The mechanism might be a cam, a leadscrew, or other such assembly. The fluid barrier might be a syringe, an extruded tube, a molded cassette, or other such device that separates the pumping mechanism from the fluid in question. In each case, the fluid movement is determined by a certain number of motor steps over time. 
         [0003]    At slow flow rates, the motor steps are relatively infrequent with long dwell periods. At high flow rates, the motor and mechanism are run at their maximal capacity until one element has reached its engineering limit. The flow rate is inherently pulsatile, yet this pulsatile nature is less significant at higher flow rates where the natural compliance of the outlet of the pumps serves to dampen the pulses into more or less a continuous stream of fluid. 
         [0004]    The motors used conventionally are inherently powerful enough to overcome significant forces and resistances, so they are capable of generating significant pumping forces. This forceful pumping is an artifact and has no desirable clinical effect. The sensing mechanisms commonly used are pressure based and are made with indirect contact with the fluid to be pumped. In most cases, the fluid barrier, such as an extruded tube, exerts far more force than the internal fluid pressures. The result is a lack of sensitivity to pressure changes and a lack of feedback as to the actual conditions of fluid flow. It is common for conventional pumps to operate indefinitely without recognizing that the actual fluid flow rate is far below the targeted level. 
         [0005]    Conventional motor driven pumps are notoriously inefficient with respect to external power consumption. For devices that have a high requirement for portability, this power inefficiency translates into unreliable operation. 
         [0006]    Prior to the use of pumps, most infusions were done by the adjustment of a gravity-based pressure (e.g., by adjusting the height of a liquid container) and the adjustment of inline resistance (e.g., by moving the position of a roller clamp), both in response to an inline flow sensing method (e.g., performed by a user counting drops into an air chamber). Although this prior art method was labor intensive and had a limited rate range, it offered some significant advantages over the subsequent “advances” in technology. First, the use of gravity head heights for a delivery pressure was energy efficient. No external power supply was required. Second, the pressure was low, so the dangers of high-pressure infusions were avoided. Third, the gravity infusions could be augmented with a low cost and readily available pressure cuff, supplementing the fluid flow to rates well above those possible by an instrumented “pump” line. Forth, a gravity administration was not capable of infusing large amounts of air into the output line, because the hydrostatic pressure goes to zero as the fluid source empties. 
         [0007]    An ideal infusion system will combine the meritorious aspects of a conventional “gravity” infusion with the benefits of a controlled intravenous infusion pump. In each aspect, this disclosure takes the desired principles of a gravity infusion and reduces the dependence upon skilled labor and extends the range and precision of fluid flow control and provides advanced information management capabilities. 
         [0008]    An ideal embodiment of an infusion device would be one with continuous flow, wide flow rate range, high energy efficiency, accuracy of volume delivered over time, minimal operating pressures, maximum sensitivity to external conditions, freedom from false alarms for air-in-line, simplicity, low cost, intuitive operation, automated information exchange, safety, and reliability. 
         [0009]    Certain infusions have historically been managed by air pressure delivery systems, most commonly found in the operating room and in emergency situations. Prior art attempts have been made to determine the flow rate via pressure monitoring and control. For example, U.S. Pat. No. 5,207,645 to Ross et al. discloses pressurizing an IV bag and monitoring pressure to infer flow rates. However, the prior art systems lack independent flow sensing, and, therefore, do not offer enough information to provide accurate and safe infusions. 
         [0010]    Under the best of circumstances, there is not enough information in the pressure signal alone to provide the accuracy needed for intravenous infusion therapy. Furthermore, there are a number of likely failure modes that would go undetected using the pressure signal alone. An infusion pump must be able to respond to events in a relevant time frame. International standards suggest that a maximum period of 20 seconds can lapse before fluid delivery is considered “non-continuous.” As an example, for an infusion of 10 ml/h, the system would want to resolve 20 seconds of flow, which corresponds to 0.056 mL. This volume represents one part in 18,000 of air volume of a 1,000 mL bladder. Temperature induced change in pressure brought about by a normal air conditioning cycle is far greater than this signal. The measurement of pressure alone is not adequate for an intravenous infusion device. No general purpose, full range, infusion devices using pressure-controlled delivery are known to be on the market. 
         [0011]    An entire class of “passive” infusion pumps exists whereby a constant pressure is created on a fluid filled container by way of a spring, elastomeric structure, gas producing chemical equilibrium, or other means. This constant pressure fluid is fed into a high resistance output line, providing relatively stable fluid flow. 
         [0012]    In typical pressure based flow control products, a relatively high pressure pushes fluid into a known, high, and fixed resistance, providing a constant flow rate with good immunity from changes in external conditions. It is a purpose of our prior commonly owned provisional application Ser. No. 60/777,193, filed on Feb. 27, 2006, and PCT Publication Nos. WO2007/098287, WO2007/098265, and WO2007/106232 to provide a highly flexible flow control system with a very broad flow rate range, operating under minimal pressures, with a relatively low and variable resistance. The entire contents of the aforementioned provisional and PCT applications are incorporated herein by reference. 
         [0013]    Embodiments of such devices control fluid flow based on a responsive fluid flow sensing means that forms a closed loop control by changing both the fluid driving pressure and the inline resistance. In contrast to the conventional approach to flow control wherein a user observes fluid flowing as it formed drops in an air chamber, then adjusts pressure by varying the head height of the fluid source, and then adjusts the inline resistance via a manual valve, our above-mentioned disclosures employ a flow sensing apparatus and method that automatically and accurately measures fluid flow rate, precisely adjusts the hydrostatic pressure of the fluid source, and precisely adjusts inline fluid flow resistance to achieve or maintain a target flow rate. 
       SUMMARY 
       [0014]    Certain embodiments of an in-line fluid flow sensor are based on the position of an object in the flow path in which the force of the fluid flow is balanced by an opposing force. The resultant equilibrium position is a function of the speed of fluid flow and of the fluid viscosity being measured. 
         [0015]    The present disclosure describes an apparatus and method of enhancing the information derived from such an in-line fluid flow sensor by examining its response to an abrupt change in flow rate. The response of the fluid flow sensor can enhance the sensitivity of the measurement, may provide diagnostic value, and can provide additional information, such as fluid viscosity. 
         [0016]    In certain embodiments of a fluid flow sensor, a signal can be analyzed to determine the position of an object in the flow path. This signal may have complex characteristics and the feature extraction that indicates the ball position may be challenged by the complexity of the signal. If a flow rate change is imposed upon the system, then the difference in the flow sensor signals taken at different flow rates will eliminate much of the underlying complexity and provide simplified feature extraction methods. In this way, the sensitivity of the flow sensor is enhanced, even with complex features or noisy environments. 
         [0017]    At any given point in time, the flow sensor signal represents a certain flow rate. When a flow rate change is imposed by a change in fluid driving pressure, then the resultant response is an indication of total systemic fluid flow resistance. In an infusion control device, this measurement of fluid flow resistance can have significant clinical and diagnostic value. 
         [0018]    In certain embodiments of a fluid flow sensor, the viscosity of the fluid will represent an offset in the position of a flow object. Taken by itself, however, the flow sensor may not be able to distinguish between a change in fluid viscosity and changes in flow sensor response due to normal manufacturing tolerances. If a flow rate change is abruptly applied, then the difference in flow sensor response in the sensor can be used to infer fluid viscosity, because the underlying offset due to manufacturing tolerances is eliminated from the analysis. The absolute change in flow object position as a function of flow rate change is an indication of fluid viscosity. In a further analysis, the speed with which the flow object moves to its new equilibrium position is an additional function of fluid viscosity and may be based on either or both of an absolute position change and rate of position change of the sensor object. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
           [0020]      FIG. 1  is a functional block diagram of a fluid pumping system operable to embody an exemplary embodiment of the present invention. 
           [0021]      FIG. 2  is a functional block diagram flow sensor and control circuit for the system appearing in  FIG. 1 . 
           [0022]      FIG. 3  is an isometric sectional view illustrating an exemplary flow sensor with integral resistor element. 
           [0023]      FIGS. 4A and 4B  are isometric views of an exemplary flow sensor with integral resistor showing the emitter and receiver. 
           [0024]      FIG. 4C  is a side view of the flow sensor embodiment appearing in  FIGS. 4A and 4B . 
           [0025]      FIG. 5  is a graph of pixel voltage output for photosensor array for determining flow sensor element location. 
           [0026]      FIG. 6  is a graph of pixel voltage output for photosensor array with separate plots for flow sensor element location before and after a change in ball sensor element position. 
           [0027]      FIG. 7  is a graph of pixel voltage differences for the plots appearing in  FIG. 6 . 
           [0028]      FIG. 8  is a flow diagram outlining an exemplary method for determining sensor element location in accordance with the present invention. 
           [0029]      FIG. 9  is graph of flow sensor element position as a function of time during a period of an abrupt change in flow rate. 
           [0030]      FIG. 10  is a flow diagram outlining an exemplary embodiment for sensing fluid viscosity in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Referring to the drawings,  FIG. 1  depicts an exemplary flow control system  100  in accordance with an exemplary embodiment of the present invention. The system includes a pressure frame  10  that is of known total volume and contains within it an air bladder  20  and a flexible bag  30  that contains within it a liquid  40  to be delivered. 
         [0032]    The air bladder  20  is connected to a charging tank  60  of known volume via a conduit or line  22  extending between an outlet of the tank  60  and an inlet of the bladder  20 . A pneumatic pump  50  is pneumatically coupled to an inlet of the charging tank  60  via a line  52 . A bladder valve  24  in the line  22  may be selectively opened and closed to selectively couple and decouple the outlet of the tank  60  with the inlet of the bladder  20 . The charging tank may selectively be vented to atmosphere via a tank vent valve  62 . The air bladder  20  may be vented to atmosphere via an optional bladder vent valve (not shown). Alternatively, the bladder  20  may be vented to atmosphere by opening the valves  24  and  62 . 
         [0033]    The tank  60  is connected to a tank pressure sensor  66  and a tank temperature sensor  68 . The bladder  20  is connected to a bladder pressure sensor  26  and a bladder temperature sensor  28 . 
         [0034]    The liquid  40  is fluidically coupled to an output  70  via an inline flow sensor  80 , a fluid flow resistor  90 , and an output line  72 . The liquid  40  may be, for example, a medication fluid, intravenous solution, blood product, or the like, to be infused and the output  70  may be, for example, a patient or subject in need thereof. In the depicted embodiment of  FIG. 1 , the flow resistor  90  is shown downstream of the in-line flow sensor  80 . Alternatively, the flow resistor  90  may be positioned upstream of the flow sensor  80 . The flow resistor  90  and flow sensor  80  may be separate or may be integrally formed. 
         [0035]    In reference to  FIG. 2 , an embodiment of the fluid control system  100  includes the pump  50  including a pump motor  51 , the bladder valve  24  including a bladder valve motor  25 , the tank vent valve  62  including a tank vent valve motor  63 , the flow sensor  80  including an optical sensor  824  and an optical emitter  822 , the flow resistor  90  including a flow resistor motor  91 , the tank pressure sensor  66 , tank temperature sensor  68 , bladder pressure sensor  26 , bladder temperature sensor  28 , a sensor processor  210 , a controller processor  212 , a pump motor controller  214 , a tank vent valve motor controller  216 , a bladder valve motor controller  218  and a flow resistor motor controller  220 . 
         [0036]    The sensor processor  210 , controller processor  212 , pump motor controller  214 , tank vent valve motor controller  216 , bladder valve motor controller  218 , and flow resistor motor controller  220  may be implemented in a microprocessor, microcontroller, controller, embedded controller, or the like. Although the processors  210  and  212  and the controllers  214 - 220  are depicted in  FIG. 2  as discrete modules or processors for conceptual simplicity and ease of exposition, it is to be appreciated that modules  210 - 214  can share common hardware. Well-known internal components for processing and control modules, such as power supplies, analog-to-digital converters, clock circuitry, etc., are not shown in  FIG. 2  for simplicity and would be understood by persons skilled in the art. 
         [0037]    The controller processor  212  controls the pump  50  via the pump motor controller  214 , the tank vent valve  62  via the tank vent valve controller  216 , the bladder valve  24  via the bladder valve controller  218 , and the flow resistor  90  via the flow resistor motor controller  220 . Alternatively, the controller processor  212  may control one or more of the motors directly or via any other suitable known device. The controller  212  may also control the application of power to the optical emitter  822 . 
         [0038]    The sensor processor  210  receives a signal indicative of bladder temperature and pressure from the bladder temperature sensor  28  and bladder pressure sensor  26 , respectively. The sensor processor  210  receives a signal indicative of tank temperature and pressure from the tank temperature sensor  68  and tank pressure sensor  66 , respectively. The sensor processor  210  receives a signal from the optical sensor  824  indicative of the position of a flow sensor indicator element in the flow path as described below. 
         [0039]      FIG. 3  shows an exemplary flow sensor  80  with integral flow resistor  90 . The flow resistor  90  includes an inlet end  910  fluidly coupled to the fluid source  40  and an outlet  912  fluidly coupled to an inlet  810  of the flow sensor  80 . The flow sensor  80  includes an outlet end  812  fluidly coupled to the output  50  such as the vasculature of a patient, e.g., via an IV catheter or cannula as generally known in the art. Although the inline sensor  80  and flow restrictor  90  are depicted as an integral assembly in the embodiment of FIGS.  3  and  4 A- 4 C, it will be recognized that the flow resistor and the flow sensor units may be discrete assemblies fluidically coupled in serial fashion. 
         [0040]    In reference to FIGS.  3  and  4 A- 4 C, the flow resistor  90  includes a rotatable housing  914 , which may have a plurality of radially extending ribs or projections  916  forming a gear that may be selectively rotated by the motor  91 , which may be a stepper motor having an intermeshing member, or the like. The rotatable housing  914  is coupled to an axially movable needle resistor  917  wherein rotating the housing  914  in one direction causes the needle resistor  917  to move in one axial direction and rotating the housing  914  in the opposite direction causes the needle resistor  917  to move in the opposite axial direction, for example, via helical threads formed on an interior surface of the rotatable housing member  914 . As best seen in  FIG. 3 , the needle resistor axially moves between a first, closed position wherein the needle resistor engages a mating seat  918  and a fully open position. An annular gap  920  defined between the needle resistor  917  and the seat  918  increases as the valve moves from the closed position to the fully open position, thereby providing a variable flow resistance, which varies as a function of the degree of rotation of the housing  914 . 
         [0041]    The flow sensor  80  includes a housing portion  814  defining an axial channel or bore  816  receiving a ball member  818 . A spring member  820  urges the ball member  818  in a direction opposite to the direction of flow. The spring member  820  may be a coil spring (e.g., conical or cylindrical coil spring) or may be another resiliently compressible material such as a foam member, deflectable band or leaf spring, or the like. 
         [0042]    The spring  820  bears against the ball  818  and applies a force to the ball in the direction opposite to the direction of fluid flow. An adjustment mechanism, such as a threaded member engaging the fixed end of the spring  820  may be provided to axially advance or retract the fixed spring end to adjust the force preload of the spring  820  on the ball  818 . In operation, fluid flow will exert a force on the sensor ball  818  against the urging of the spring  820 , which force increases as the flow rate increases. The ball  818  thus moves until an equilibrium position is reached such that the force of the compression spring  820  on the ball  818  is balanced by the force of the fluid flow against the ball  818 . 
         [0043]    In reference to  FIGS. 4A-4C , the optical emitter  822 , which may be, for example, an LED array, is provided on a first side of the housing  814  and the optical receiver  824 , which may be, a photosensitive array, charge-coupled device (CCD) array, photodiode array, complimentary metal oxide semiconductor (CMOS) digital detector array, or the like, is provided on a second side of the housing  814  opposite the first side. The optical emitter  822  transmits light through the housing  814  and into the cavity  816 . The light incident upon the ball  818  is transmitted through the ball  818  and opposite wall of the housing  814  to form a light intensity pattern on the optical sensor  824 . Where the fluid flowing through the channel  816  is a generally opaque fluid or otherwise has a high absorbance of the light emitted by the emitter  822 , the ball  818  may be a clear ball, e.g., formed of acrylic or other transparent polymeric material, which serves to dramatically reduce the optical path length of the fluid in the optical path between the emitter  822  and the sensor  824  in the vicinity of the ball  818 , thereby reducing the absorption of light by the fluid surrounding the ball in the flow passageway. Also, the use of a clear ball sensor element  818  allows the ball to function as a lens to transmit and focus the light. 
         [0044]    The optical transmitter  822  may include one or more light source elements having a wavelength, for example, in the infrared (IR), visible, or ultraviolet (UV) region and the housing and ball member may be formed of a material that optically transmits light of the light source wavelength. The light source  822  may be an array of light elements, such as LEDs, or laser, etc. The light source may be segmented along the axis or may be a continuous, e.g., scanned or otherwise optically formed beam. The light source may illuminate the detector array along its length simultaneously or by sequentially scanning along its length. The refractive effect of a transparent ball member may have a focusing effect on the light passing therethrough that may be detected by the photosensor array. Alternatively, a nontransmissive ball  818  may be employed and the ball position may be determined by detecting the position of a shadow cast by the ball on the photosensor array. In still further embodiments, the ball member may have reflective surface and the optical sensor array may be positioned to detect light reflected from the surface of said ball. 
         [0045]    The output from the photosensitive array is a set of pixel voltage values which vary in accordance with the amount of light impinging on the each pixel of the photosensitive array. The pixel voltage values may be sampled and digitized using an analog-to-digital converter and stored as digital data in an electronic storage medium as a numerical representation of the pixel output voltage levels, and thus, light intensity levels, along the detector array. 
         [0046]    The output of the optical sensor  824  may be passed to the sensor processor  210 , which may include a position-detection module or circuitry wherein the axial position of the ball  818  within the channel  816  is determined. The axial position of the ball  816  may in turn be used to determine a flow rate and/or calibrate or correlate ball positions with known flow rates calculated by other means such as plural volume measurements made using the method outlined in the aforementioned U.S. provisional application Ser. No. 60 and PCT Publication Nos. WO2007/098287, WO2007/098265, or WO2007/106232. 
         [0047]    Referring now to  FIG. 5 , there appears a graph of pixel voltage signal  230  of the photosensor array  824  as a function of pixel position. In the depicted example, the pixel voltage measurements were made using half-&amp;-half as the fluid  40  and the flow sensor  80  was specifically detuned to represent a worst case scenario for the flow sensor and provide maximum challenge to the fluid control system. The graph of  FIG. 5  shows that the flow sensor signal is complex and difficult to analyze for the position  231  of the flow object, which is somewhat ambiguous. 
         [0048]    Referring now to  FIG. 6 , the ball  820  was moved by the imposition of a modified flow rate and a subsequent measurement of the pixel voltage values of the photosensor array  824  was made (see signal  232 ). The new ball position  233 , based on the pixel voltage values, is likewise somewhat ambiguous. 
         [0049]      FIG. 7  is a graph  234  of the pixel voltage differences between the first signal  230  and the second signal  232 . Subtracting the second signal from the first signal cancels or reduces common mode complexity and/or noise of the two signals and the first ball position  231   a  and second ball position  233   a  appear as clearly identifiable peaks, even though positions  231  and  233  based on the individual signals  230  and  232 , respectively, were ambiguous. Alternatively, the first signal can be subtracted from the second signal, in which case the ball position can be similarly determined, but wherein the resultant function will be the negative function relative to the function  234  appearing in  FIG. 7 , i.e., reflected about the x-axis. 
         [0050]    A method for detecting the flow sensor indicator element is outlined in the flowchart of  FIG. 8 . At step  240 , a first signal from the photodetector array is provided to the sensor processor  210 . At step  244 , the flow rate is changed. The flow rate may be changed by introducing air into the charging tank  60  with the pump  50  to increase the pressure in the tank to a pressure greater than the pressure in the bladder  20  and opening the bladder valve  24 . The pressure increase in the bladder  20  is preferably an abrupt pressure increase, e.g., to provide a step function change in fluid driving pressure, e.g., by popping or otherwise rapidly opening the valve  24 . Alternatively, the change in flow rate may be a decrease in pressure. For example, if the pressure in the charging tank  60  is lower than the pressure in the bladder  20 , then the rapid opening of the valve  24  will abruptly reduce the driving pressure. In alternative embodiments, an optional bladder vent valve (not shown) may be provided for venting the bladder to reduce the pressure in the bladder  20 . 
         [0051]    At step  248 , a second signal from the photodetector array is provided to the sensor processor  210  representative of fluid flow rate at the new driving pressure. At step  252 , one the first and second signals is subtracted from the other to provide clearly identifiable peaks representative of the ball axial position as described above. 
         [0052]    It will be recognized that in a flow control system employing a pressurized bladder as the fluid driving force, it may be necessary to periodically increase the pressure in the bladder, for example, to achieve a desired flow rate. Also, once a desired flow rate has been achieved, periodic increases in the bladder  20  pressure will be necessary to maintain a desired flow rate since the bladder  20  will expand and the pressure in the bladder  20 , and thus flow rate will thereby decay, as the fluid  40  exits the bag  30  and is delivered to the subject  70 . Thus, even where the primary purpose of the pressure increase in the bladder  20  is to establish or maintain a desired flow rate, the observation of the ball position using the sensor  824  before and after the pressure increase in accordance with the present disclosure provides an additional benefit in that ball position can be determined with enhanced accuracy. 
         [0053]    As discussed above, comparing ball position before and after an abrupt change in flow rate can advantageously be used to provide a clear indication of sensor ball position. In a further aspect, observation of ball position during the abrupt change in flow rate provides the ability to measure viscosity of the fluid  40 . It has been found that viscosity of the fluid  40  can be determined by one or both of (1) the distance the ball moves in response to a change in flow rate (driving pressure); as well as (2) the rate at which the ball moves to the new position. The higher the viscosity, the further the ball moves in response to a change in flow rate. In addition, the higher the viscosity of the fluid, the longer it takes for the ball to assume its new equilibrium position in response to an abrupt change in flow rate. 
         [0054]    Referring now to  FIG. 9 , there appears a graph in which there is plotted a curve  260  representative of sensor ball position as a function of time during an abrupt increase in fluid driving pressure for a low viscosity fluid. The ball moved from position  2  to position  11 , for a span of nine units of difference. The slope  262  represents the speed at which the sensor ball moved from its initial position to its final position for the low viscosity fluid. A curve  264  is representative of sensor ball position as a function of time for the same change in flow rate for a relatively high viscosity fluid. The ball moved from position 0 to position 13, for a span of 13 units of difference with the high viscosity fluid. The slope  266  represents the speed at which the sensor ball moved from its initial position to its final position for the high viscosity fluid. 
         [0055]    The slope  266  for the high viscosity fluid is lower than the slope  262  for the low viscosity fluid, and the distance moved for the higher viscosity was greater than the distance moved for the lower viscosity fluid, thus indicating that, for higher viscosities, the fluid will push the ball further, yet will do so at a lower speed taking significantly longer to reach its equilibrium position. The graph also shows how the nominal starting positions  268  and  270  for the low and high viscosity fluids, respectively, for the same flow rates may vary due to the difference in viscosity. 
         [0056]    Referring now to  FIG. 10 , a method for determining the viscosity of a fluid being delivered in a flow control system is illustrated. At step  280 , an abrupt change in flow rate is effected, e.g., by increasing the pressure in the tank  60  and popping the valve  24  to introduce a step change in fluid driving pressure. At step  284 , the axial position of the sensor ball is monitored as a function of time during the flow rate change until the ball assumes a new equilibrium position. Alternatively, the change in flow rate may be effected by reducing the pressure in the bladder  20 , e.g., by reducing the pressure in the tank  60  and popping the valve  24 , or, by using an optional bladder vent valve (not shown). 
         [0057]    At step  288 , one or both of absolute position change and the rate of position change of the flow sensor element is calculated, e.g., by comparing ball pixel position along the sensor array and/or by determining the average slope of position as a function of time for the period of time in which it took the ball to move from its initial equilibrium position at the initial flow rate to its new equilibrium at the new flow rate. At step  292 , the viscosity of the fluid being delivered is determined from the change in ball position and/or rate of sensor element response, for example, by comparing calculated ball position change and/or rate thereof to prestored values for fluids of known viscosity, which may be stored in database, look up table, data file, etc. 
         [0058]    In operation, the type of fluid  40  to be infused may be input into the flow control system, e.g., by the operator using a user interface of the processor  210  and/or  212 . Alternatively, the type of fluid  40  may be identified by reading a bar code (or other optically readable indicia) or radio frequency identification (RFID) tag on or in the fluid container, e.g., by a bar code (optical) scanner or RFID scanner. The viscosity as determined in step  292  may then be checked to determine whether it is consistent with an expected fluid viscosity based on prestored viscosity characteristics associated with the fluid type input by the operator (e.g., stored in a database, lookup table, data file, memory, etc.). For example, in the case of IV infusion fluids, many fluids or at least categories of fluids, such as blood products (e.g., whole blood, platelets, plasma, immunoglobulins, packed red cells etc.), saline, dextrose, albumin, lactated ringers solution, amino acids, lipid emulsions, parenteral nutritional solutions, etc., will have different viscosity characteristics. If the viscosity determined at step  292  is different from the expected viscosity, the operator may be alerted to this potential error condition, thereby providing an additional safeguard. 
         [0059]    In further aspects, the observation of ball movement during an abrupt change fluid driving pressure may also be used to detect other error conditions. The change in flow rate in response to a change in fluid driving pressure is indicative of the total systemic resistance. For example, if the ball position does not change after the fluid driving pressure is increased, the line may be occluded, and the operator may be alerted to this potential error condition. Additionally, in the face of a potential occlusion, the pressure in the bladder may be reduced to a lower level, e.g., using a bladder vent valve, if provided, or by reducing the pressure in the charging tank  60  (e.g., via tank vent valve  62 ) and opening the bladder valve  24 , e.g., to reduce the chance of an unwanted release bolus. 
         [0060]    The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.