Abstract:
A system and method for measuring the flow rate of a liquid in a tube non-invasively has a heating element that generates energy that is applied to the liquid to produce a heat marker that is detected by a temperature sensor located at a known distance from the heating element and the flow rate is calculated from measuring the travel time of the heat marker from the heating element to the sensor. A second temperature sensor measures the ambient temperature of the liquid before the heat marker is produced and detection of the heat marker is made on the basis of the difference between the ambient temperatures and the temperature of the heat marker.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system and method for measuring the flow rate of a liquid in a tube non-invasively. 
     BACKGROUND OF THE INVENTION 
     Various devices have been used for measuring the rate of flow of the liquid in a tube or pipe. Hereafter the term “tube” is used to include any type of conduit in which a liquid can flow that can be used with the prior art and the invention. 
     For example, a variety of types of flow measuring devices used in chemical/pharmaceutical industries exist in which contact is made with the liquid as the flow rate measurement is being made. Such devices include a Coriolis flow meter which measures mass flow as a function of gyroscopic torque forces. Devices using this method are complex and expensive. They also have an accuracy of about ±0.4%. Another device is an ultrasonic flow meter which is suited for measuring gallons-per minute flow and whose accuracy is ±0.5%. There also is a continuous heat addition flow meter in which the liquid is heated by a probe immersed in the liquid and the downstream temperature is continuously measured, such as by a thermistor type sensor. In this type of device the accuracy of the measurement varies with the specific heat of the metered liquid and with ambient temperature fluctuations. Also available is a self-heating thermistor placed in contact with the liquid. The thermistor undergoes cooling proportional to the rate of flow of the liquid flowing past it. This type of device is nonlinear and the accuracy of the measurement result varies with the specific heat of the liquid and ambient temperature variations. U.S. Pat. Nos. 5,726,357 and 5,623,097 each disclose a semiconductor substrate on which is integrated a heating element and a heat sensing element. The fluid passes over the heating element and is detected as it passes over the sensing element. 
     In many applications it is desirable, and even necessary, to measure the liquid flow rate non-invasively, that is, without any part of the measuring device coming into contact with the liquid. This preserves the sterility of the liquid. Applications that require non-invasive measurement include medical devices such as infusion pumps for drug delivery, devices that feed nutrients to patients, and applications in which a disposable tube is used such as in a drug delivery system. 
     In many applications in which the flow rate is to be measured non-invasively the liquid flow rate is relatively low. Existing devices have difficulty in providing accurate measurement for low flow rate applications. Accurate measurement of low volumetric liquid flow rate is very important in analytical chemistry applications such as chromatography and capillary electrophoresis. 
     A number of systems exist for measuring liquid flow rate non-invasively. Typical of these is the system described in U.S. Pat. No. 5,764,539, in which a non-invasive temperature sensor is heated to a predetermined temperature which changes as the liquid passes by it. The temperature change is determined to detect the characteristics of the liquid and whether or not the liquid is flowing. In U.S. Pat. No. 4,938,079 a resonant microwave cavity provides heat markers in the flowing liquid which are detected by another resonant cavity based on the perturbations of the liquid by the heat markers. 
     In U.S. Pat. No. 6,582,393 an amount of liquid to be used as a medicinal dose is held in a chamber in an elastic tube formed by a pinch bar engaging the tube. The dose amount of liquid is heated by a heating block and is then released by releasing the pinch bar. The heated liquid dose is sensed by a heat sensor block and the travel time of the dose between the heating and sensing blocks is computed to give the dose flow rate. This information is used to maintain or correct the time of application of further doses of the liquid to achieve a predetermined dose rate. 
     In U.S. Pat. Nos. 6,932,796 and 7,268,859 and U.S. patent publication 2005/0005710 a tube a heating element heats liquid flowing in a tube to form a heat marker that is optically detected. The travel time of the heat maker between the heating element and the optical detector is used to compute the flow rate. The optical detectors used in the systems of these patents and patent publication do not actually determine the temperature of the heat marker and the configuration of the optical detector is relatively complex. 
     All of the existing non-invasive liquid flow rate measure devices and systems are relatively complex and relatively expensive. Accordingly, a need exists to provide a system and method that can measure flow rates non-invasively, with such system being easy to operate, providing accurate results even for low volume flow rates and being of a relatively low cost. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with the invention a system and method is provided for measuring liquid flow rates non-invasively. In the invention a housing is provided in which is mounted a heat producing element and a temperature sensor that is downstream of the heat producing element in the direction of the liquid flow. The heat producing element applies a heat pulse marker to the liquid flowing in a tube placed in the housing. In a typical medical application the tube would be of an elastic plastic material and would be disposable. 
     The heating element can be, for example, a focused pulsed beam of ultrasonic energy of an intensity sufficient to produce the desired quantity of heat for the heat marker or by a laser diode or any semiconductor heating element. The liquid absorbs the energy from the heating element and is heated in a small area that serves as a heat marker. The temperature sensor, such as an infrared (IR) type heat sensor, is spaced at a known fixed distance from the heating element. The temperature sensor senses the heat marker in the flowing liquid. An electronic circuit is coupled to both the heating element and the temperature sensor. The electronic circuit controls the time of production of the pulse of energy supplied to the flowing liquid to form the heat marker. It also determines the time at which the heat marker in the flowing liquid passes by the temperature sensor. Since the distance between the heating element and temperature sensor is known, the flow rate can be computed from the measured transit time of the heat marker traveling over the known fixed distance. 
     In a preferred embodiment of the invention a second temperature sensor is placed upstream of the heating element. The second temperature sensor measures the temperature of the liquid to provide a baseline value against which the temperature of the heat marker sensed by the first temperature sensor is compared. In this manner, the system is self-regulating since detection of the heat marker can be set to be recognized at a predetermined temperature difference between the liquid before heating and the temperature of the heat marker. It also preserves the accuracy of the system when the ambient temperature of the environment in which the tube is located changes or a liquid of a different temperature is provided to flow through the tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings in which: 
         FIG. 1  is a perspective view of a housing for the flow rate detection system and method of the invention; 
         FIG. 2  is a top plan view of a part of the housing of  FIG. 1  and also a schematic block diagram of the electronic circuit portion of the system; 
         FIGS. 3A and 3B  show partly in cross section different types of heating elements; and 
         FIG. 4  is a timing diagram showing the heat marker traveling in the liquid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a housing  10  that can be made of any suitable material, such as plastic. The housing  10  is illustratively shown as being of a generally rectangular shape although it can have any desired shape. The housing  10  includes a base  12  having a slot  14  that is generally semi-circular in shape, and that extends across the base width. The slot  14  holds a tube  20  in which a liquid flows. The tube  20  can be of any material including plastic, glass, ceramic, or metal. A tube of plastic material works best with the invention since the energy that is applied to the tube to produce the heat marker in the flowing liquid will be dissipated in the tube wall material. If the tube  20  is of plastic, it can be of either a hard material or a compressible material. The tube wall can have any thickness, which will be accommodated for by the magnitude of the energy that is generated to be applied through the tube wall to the liquid to produce the heat marker. 
     Housing  10  as shown has a hinged cover  16  that has a slot  18  across its width to overlie the tube. A cable  17  extends from the base  12 . The cable has the necessary wires to connect the temperature sensors and heating elements that are located in the base to external circuitry that is used in providing power to the components and for measuring the flow rate. The cover  16  has a latching mechanism  19  that holds the cover to the base  12 . When the cover  16  is closed the tube  20  is held between the slots  14  and  18 . The housing  10  and slots  14  and  18  can of any desired size and shape to accommodate the type and size of the tube in which the liquid flow rate is being measured. 
       FIG. 2  shows details of the base part  12  of the housing  10  in which a tube  20  is placed in the slot  14  with liquid flowing in the tube from left (upstream) to right (downstream), as shown in the drawing. Going from the upstream (source) direction of the liquid flow in the tube there are a first temperature sensor  30 , a heating element  40  and a second temperature sensor  50 . These components are described in detail below. The components  30 ,  40 ,  50  are in one wall of the part of the base  12  in which the slot  14  is formed. The components can be molded into the wall if the base is molded or inserted into cavities made in the base to hold the components with the faces component faces that oppose the tube  12  being sealed by a suitable plastic such as an epoxy. The thickness of the seal is typically 0.015 inches (0.38 mm) to 0.030 inches (0.76 mm). This has the advantage of not subjecting the components to dirt or moisture, thereby minimizing component failure and eliminating cleaning problems. 
     I the placement of the components  30 ,  40   50  the second temperature sensor  50  is spaced from the heating element  40  by a known fixed distance designated as “L” which is a factor used in computing the liquid flow rate. The spacing between the first temperature sensor  30  and the heating element  40  is not critical but, in a preferred embodiment of the invention, it is also made the distance L for convenience in computation. 
     In the preferred embodiment of the invention, the temperature sensors  30  and  50  are infrared (IR) IR heat detectors. Suitable IR heat detectors for use are Melexis—series MLX90614 obtained from Melexis, Inc. of Concord N.H. These IR detectors have a programmable response time, small size (miniature package To −39) and are of relatively low cost. 
     A heat pulse generator  60  that is external to the sensor base  12  supplies the required power to the heating element  40  to generate a pulse of energy to be transmitted through the wall of tube  20  to be applied to the liquid to heat it and form a heat marker. The timing of the application and the duration of the heat pulses is controlled by a microprocessor  70 . The heat pulse generator and microprocessor, as well as all other electronic components can be within or external of the housing  10  as desired. 
       FIGS. 3A and 3B  show different types of heating elements  40 . In  FIG. 3A  the heating element is an ultrasonic transducer  40 A that receives voltage from the heat pulse generator  60  and converts the voltage into electro-mechanical (ultrasonic) energy. The transducer  40 A preferably is of the type whose output energy can be focused to concentrate the energy at a fixed point in the liquid flowing in the tube. The energy pulse from the transducer  40 A passes though the tube wall, as indicated by the curved lines  42 , and is absorbed by the liquid to produce a heat bolus, or mass, that serves as the heat marker. The transducer  40 A would normally engage the wall of the tube  20  and would have sufficient power supplied by the generator  60  with the power requirements being determined by the type of tube material and the tube wall thickness. Different types of liquids have different heat absorption factors to different ultrasonic energy frequencies. Therefore, the frequency of the ultrasonic energy is selected so that the maximum amount of heat will be absorbed by the liquid in the tube. 
     Aa a typical example, the tube  20  being of an elastic plastic material such as: 
     a) PVC inside diameter 0.20″ (5.0 mm) with wall thickness 0.065″ (1.65 mm) 
     b) PVC inside diameter 0.5″ (12.7 mm) with wall thickness of 0.1″ (0.25 mm) 
     c) PVC inside diameter 0.125″ (3.18 mm) with wall thickness of 0.030 (0.76 mm) 
     d) Same as above but Teflon material. 
     The PVC and TEFLON can be either flexible or rigid tubing. 
     Using the above types of tubing an ultrasonic transducer that would produce about one watt of energy at a frequency of about 1 MHZ for about 10 microseconds would produce a heat marker H of about 7° C. in excess of a liquid at an ambient temperature of 22° C. That is, the heat marker would be at 29° C. 
     The above examples are not to be considered a limiting since the invention can be used with tubing of other sizes and materials with suitable selection, placement and operation of the components  30 ,  40 ,  50 . For example, a higher wattage and/or longer duration and/or different frequency ultrasonic pulse would be used with plastic tubing having thicker walls than those shown above. 
     In  FIG. 3B  the heating element  40 B is a laser diode that is suitably powered by the heat pulse generator  60 . When a laser diode is used, the laser wavelength output can be selected to maximize the heat absorption by the liquid. In a preferred embodiment of the invention, a laser diode is used having an output near about 1550 nm wave length. At this wave length the heat absorption coefficient of water and many other liquids is relatively high. Such a laser diode is relatively inexpensive and is commercially available. See, for example, Newport Corporation Spectra Physics Division (Santa Clara, Calif.) Model ML 925B45F. The light output energy  42  from the laser diode can be focused directly from the diode or through an optical system (not shown) to be concentrated for application into a selected point of the flowing liquid. 
     Using either the ultrasonic transducer  40 A of  FIG. 3A , or the laser diode  40 B of  FIG. 3B , or any other suitable type of heating element, the size of the heating element and the output of the pulse generator  60  are selected to produce the desired size of heat marker bolus that flows in the liquid. Other forms of heating elements also can be used in such as ???? 
     In the operation of the system the ambient, or normal, temperature of the liquid is measured by the first temperature sensor  30 . The heat marker in the liquid is sensed as it flows past the second temperature sensor  50 .  FIG. 4  shows a diagram of temperature versus time in which the liquid shown in line A flows past the first temperature sensor  30  at the time t 0 . At time t 1  a heat pulse marker, or bolus, H is applied to the liquid in the tube as explained above. The heat marker H then flows past and is detected by the second temperature sensor  50  at time t 2 . The second temperature sensor  50  is located at the fixed distance L from the heating element  40 . 
     Measurement of the time of transit of the heat marker H over the fixed distance L gives the liquid flow rate in accordance with the following:
 
 Q=A×L/t   d  where
         Q=Flow rate   A=Cross sectional area of the tube   L=Distance between heating element  40  and temperature sensor  50     t d =average transit time less the time lost due to the response of tube material in the heat detector. That is:
 
 td=t   m   −t   t   −t   l , where
   t m =multiple time measurements   t t =Calculated delay in tubing due to thermal time constant associated with plastic tubing   t l =Response time of heat detector.
 
The time t d  is known in advance and is programmed into the microprocessor  70 . Since all of tm, tt and tl are known the value td is calculated. Since A and L also are known, the flow rate Q is calculated by the microprocessor.
       

     In the components of the electronic circuit, as shown in  FIG. 2 , the microprocessor  70  is programmed with the values A, D, t t  and t l . The outputs of the temperature sensors  30  and  50  are connected to an analog to digital (A/D) converter  64  that converts the measured temperature into digital format. Some temperature sensors include this function so that the A/D converter might not be needed. The microprocessor  70  produces a timing signal on line  62  to cause the heat pulse energy generator  60  to produce an output that is applied to the heating element  40 . The timing signal also starts a transit time period, compensated by the various delay factors discussed above, that is ended by the detection of the heat pulse by the second temperature sensor  50 . The microprocessor calculates the flow rate Q from the measured transit time period using the formulas discussed above. The measured flow rate calculated by the microprocessor can be of any required dimensional quantity, e.g. cc/min, cc/hr or any other unit. This is the microprocessor output which can be displayed by a suitable display device located on the housing  10  or output to a display remote from the housing. The calculated flow rate data can be supplied from the microprocessor output to another device to be used for flow rate control or any other purpose. 
     The microprocessor  60  is preferably programmed to make multiple measurements of the transit time td of the heat pulse from the heating element  40  to the second temperature sensor  50  and from these multiple measurements calculate the value tm. The microprocessor also can be programmed to perform as many calculations of Q over a predetermined period of time as desired, to average the calculations of Q, to take a maximum or some other value of Q from a group of measurements, etc. 
     Using the two temperature sensors  30  and  50  has an advantage in that common mode temperature changes can be eliminated. That is, the ambient (before heat pulse is applied) temperature of the liquid is measured by the first temperature sensor  30  and is used as a base line value by the microprocessor. The microprocessor  70  is programmed to respond to detection of a heat pulse marker H at a predetermined temperature, for example ?? degrees above the base line value. Therefore, if the ambient temperature of the liquid varies either up or down it will have no effect on the accuracy of the flow rate measurement since the base line value varies in this manner. The same advantageous effect is obtained if a different liquid having a different ambient temperature is substituted. 
     The system of the invention has numerous advantages. It is completely non-invasive so that it can be used in applications where sterility of the liquid is required. It has high measurement accuracy with a fast response time. Further, different sizes of the tube can be accommodated by the housing  10  such as tube diameters of from 1 mm to 15 mm diameter. The tubes can be of the disposable type. Also, the system can accommodate tubes of different types of plastic material. The system also can be used for measuring liquid flow rate in glass/metal tubing. 
     Specific features of the invention are shown in one or more of the drawings for convenience only, as each feature may be combined with other features in accordance with the invention. Alternative embodiments will be recognized by those skilled in the art and are intended to be included within the scope of the claims. Accordingly, the above description should be construed as illustrating and not limiting the scope of the invention. All such obvious changes and modifications are within the patented scope of the appended claims.