Abstract:
A Kelvin sensed hot-wire anemometer includes four electrically conductive pins and a filament welded to all four pins, preferably using a single filament. A current source is coupled to the two innermost pins so as to provide current flow in the segment of filament between the two innermost pins. The two outermost pins are coupled to a high impedance voltage sense amplifier that senses the voltage drop across the energized segment of filament between the two innermost pins. The resistance of the filament is determined based on the current provided to the filament and the measured voltage. The Kelvin sensed hot-wire anemometer can be used in a number of applications, including, but not limited to medical devices that measure gas flow rates during inhalation and exhalation.

Description:
FIELD OF THE INVENTION 
   The field of the invention relates to hot-wire anemometers. Specifically, the invention relates to hot-wire anemometers for use in measuring the combined temperature and flow rate of a fluid. 
   BACKGROUND OF THE INVENTION 
   Hot-wire anemometers are most often used to measure fluid velocity based on the amount of heat connected away by a fluid passing over a heated wire. In typical hot-wire anemometers, a hot wire or filament is heated by either a constant current (constant-current anemometers) or, alternatively, heated to a constant temperature (constant-temperature anemometers). In either case, the amount of heat lost due to convection is a function of the fluid velocity passing over the filament. 
   The amount of heat that is dissipated by a heated filament located in a fluid stream depends on a number of factors including the filament&#39;s temperature, the geometry of the filament, the temperature of the fluid, and the fluid velocity. The filament&#39;s temperature is determined by measuring its electrical resistance. Empirical data and/or mathematical algorithms are used to calculate the temperature and the flow rate based on the measured resistance. Because metals used to fabricate suitable filaments have resistivity coefficients on the order of 0.1%/° C., a high degree of accuracy is needed for measuring the actual resistance of the filament. 
   One medically-related application for hot-wire anemometers is their use in measuring the inspiration and expiration flow rates of a patient. Many lung function tests require knowing details on the rate at which air is entering and exiting a patient&#39;s lungs. Because the maximum realistic flow rate range encountered during inspiration and expiration is relatively low (e.g., between 0 and about 20 L/s), the resistance change in the filament is also small. For example, a filament having a resistance of 2.2 ohms at room temperature may only see a 0.03 ohm change in resistance over the entire realistic flow rate range. Because there is such a small change in the resistance in the filament, it is imperative that this change be measured with great accuracy and precision. 
   In prior art hot-wire anemometers, for example, as shown in FIGS.  1 ( a ) and  1 ( b ), a single wire anemometer probe  2  is used that is disposed inside a tubular housing. A filament  6  (shown in FIG.  1 ( b )) is welded between two pins  8  (one is obstructed from view in FIG.  1 ( a )) that extend from the middle of the probe  2  to outside the housing  4 . The probe  2  is detachably attached to a cable  10  which has mating receptacles (not shown) for receiving the two pins  8 . The cable  10  communicates with circuitry for calculation of the gas flow rate passing over the filament  6 . There are several problems, however, with the prior art probe  2  that prevents the acquisition of accurate and precise resistance measurements. 
   In the prior art probe  2 , there is no way to differentiate between resistivity of the filament  6  and resistivity caused by the cable  10  and the connection between the cable  10  and the two pins  8 . Any resistance change caused by the cable  10  and/or the connection will be seen by the circuitry as a change in the resistance of the filament  6 , thereby resulting in an erroneous temperature and gas flow calculation. There are several mechanisms by which resistance errors can be introduced in the prior art probe  2 . These include, for example, (1) changes in ambient temperature, (2) time variations, and (3) physical disturbance/movement of the cable  10 . Some of these errors cannot be eliminated nor reversed without a complete recalibration of the probe  2 , which can take a considerable amount of time and effort. 
   Practical considerations require that the probe be designed in such a manner that allows a user to attach and remove the probe from a cable connecting the probe to the unit housing the electronics. This is particularly true when the probe is disposable or requires frequent replacement, maintenance, or cleaning. Consequently, cables and connectors are virtually required in all probe designs, thereby insuring the existence of the aforementioned error mechanisms. 
   Thus, there is a need for a detachable hot-wire anemometer probe that can precisely and accurately measure the resistance of a filament without the introduction of resistance errors caused by various environmental artifacts. The probe unit would be modular in that it could be attached/disconnected to a separate device containing the circuitry using a conventional cable. 
   SUMMARY OF THE INVENTION 
   In a first aspect of the invention, a hot-wire anemometer includes four electrically conductive pins comprising a pair of outer pins and a pair of inner pins. A filament is welded to each of the four electrically conductive pins, preferably using a single filament on all four pins. A current source is coupled to the pair of inner pins and adapted to provide current flow in the filament between the pair of inner pins. A voltage sense amplifier is coupled to the pair of outer pins. 
   In a second aspect of the invention, the hot-wire anemometer of the first aspect includes a second set of four electrically conductive pins including a pair of outer pins and a pair of inner pins. A second filament is welded to each of the four electrically conductive pins (second set). A current source is coupled to the second pair of inner pins and adapted to provide current flow in the second filament between the second pair of inner pins. A voltage sense amplifier is also provided and coupled to the second pair of outer pins. 
   In this second embodiment, one of the filaments (either the first or second) is used to measure the temperature of the fluid. The other filament is heated to a pre-determined temperature above the measured temperature of the fluid. The power supplied to the heated filament is then used to calculate the flow rate of the fluid across the filament. 
   It is thus an object of the invention to provide a hot-wire anemometer that precisely measures the actual or true resistance of the filament using a Kelvin sensing scheme. 
   It is a further object of the invention to provide a hot-wire anemometer that can be used to obtain real-time temperature and flow-rate measurements of a moving fluid. While the device has a preferred use in the medical field, the device and method of use is not so limited and can be used to measure fluid velocity and temperature in any number of settings. Similarly, while the device is primarily contemplated for use in measuring the flow rate and temperature of gases, the device and method can also be applied to liquids. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a side view of a prior art probe. 
     FIG.  1 ( b ) is an end view of the prior art probe of FIG.  1 ( a ). FIG.  1 ( b ) also shows a detachable cable. 
     FIG.  2 ( a ) is a side view of a probe according to one embodiment. 
     FIG.  2 ( b ) is an end view of the constricted region of the probe of FIG.  2 ( a ) taken along the line A—A. Also shown is the cable and the unit housing the circuitry. 
       FIG. 3  is a schematic view of a hot-wire anemometer with its associated circuitry. 
     FIG.  4 ( a ) is a side view of a probe according to another embodiment. 
     FIG.  4 ( b ) is an end view of the constructed region of the probe of FIG.  4 ( a ) taken along the line A—A. The detachable cable is also shown. 
     FIG.  4 ( c ) is a perspective view of a terminal end of a cable. 
       FIG. 5  is a schematic view of a hot-wire anemometer with its associated circuitry according to the embodiment shown in FIGS.  4 ( a ),  4  ( b ), and  4 ( c ). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   FIGS.  2 ( a ) and  2 ( b ) illustrate a hot-wire anemometer  18  according to a preferred embodiment of the invention. The hot-wire anemometer  18  is in the form of a probe  20  and includes a body  22  having a lumen  24  therethrough for the passage of a fluid  26 . The fluid  26  may be a gas or a liquid depending on how the hot-wire anemometer  18  is used. For medical applications, including this preferred embodiment, the fluid  26  is a gas. The body  22  includes a mouthpiece section  28  over which a patient places his or her mouth. The body  22  also includes a constricted region  30  where gas measurements are preferably made. It should be understood, however, that gas measurements may be taken at other locations within the probe  20  that are outside the constricted region  30 . The body  22  is preferably formed of a material such as plastic that can be cleaned using a liquid disinfectant. 
   Referring now to FIG.  2 ( b ), four pins  32 ( a-d ) project through the body  22  and are held fixedly in place. The four pins include two outer pins ( 32 ( a ) and  32 ( d )) and two inner pins ( 32 ( b ) and  32 ( c )). The four pins  32 ( a-d ) are preferably made of 304 stainless steel and plated with gold. One end of the pins  32 ( a-d ) projects inside the body  22  into the lumen  24  while the opposing end of the pins  32 ( a-d ) projects outside the body  22 . A filament  34  is secured to the ends of the four pins  32 ( a-d ) forming four nodes (two inner and two outer) at the connection point. Preferably, the filament  34  is secured to the pins  32 ( a-d ) by spot welding.  FIG. 3  shows the spot welds  36  on the top of each pin  32 ( a-d ). The filament  34  is preferably made of  304  stainless steel. The filament  34  preferably has a circular cross-sectional profile with a diameter of approximately 0.001 inch. 
   A cable  38  is provided for engaging with the pins  32 ( a-d ). The cable  38  preferably has mating receptacles  40 ( a-d ) (shown in FIGS.  2 ( b ) and  3 ) for engaging with the pins  32 ( a-d ). The cable  38  allows the probe  20  to be attached and removed from the cable  38 . The other end of the cable  38  is attached to a unit  42  housing the circuitry (disclosed in detail below) used to calculate the temperature and flow rate of the fluid  26  passing over the filament  34 . The cable  38  includes a number of wires (a-d) corresponding to each receptacle  40 ( a-d ). The cable  38  preferably allows the probe  20  to be used some distance away from the unit  42 . For example, for many lung function tests, the patient is standing or engaging in some sort of physical activity (i.e., treadmill or bicycle). The cable  38  thus permits the probe  20  to be used in a variety of test conditions with the unit  42  located at a remote location that does not interfere with the particular lung function test. 
     FIG. 3  illustrates one preferred example of the circuitry  44  used in calculating the flow rate and/or temperature of a fluid  26  passing over a filament  34  of a hot-wire anemometer  18  according to a preferred embodiment of the invention. Also shown in  FIG. 3  is a schematic representation of the pins  32 ( a-d ), filament  34 , and mating receptacles  40 ( a-d ). In a preferred embodiment, the circuitry  44  includes a driver circuit  46  that provides current (shown by arrow I) to the hot-wire anemometer  18  so as to heat the filament  34 . The driver circuit  46  is connected at connection points  48  to wires  39 ( b ) and  39 ( c ) of the cable  38 . The wires  39 ( b ) and  39 ( c ) are, in turn, coupled to receptacles  40 ( b ) and  40 ( c ) of the cable  38 .  FIG. 3  shows the four pins  32 ( a-d ) engaged with their respective receptacles  40 ( a-d ). With respect to receptacles  40 ( b ) and  40 ( c ), current I is supplied by the driver circuit  46  through the receptacle  40 ( c ) and through the corresponding pin  32 ( c ) to the filament  34 . The filament  34  is attached to the pins  32 ( a-d ) through spot welds  36 ( a-d ). Current I flows from pin  32 ( c ) through the filament  34  toward pin  32 ( b ). Current I returns to the driver circuit  46  via pin  32 ( b ), the corresponding receptacle  40 ( b ), and wire  39 ( b ). 
   As seen in  FIG. 3 , current I passes only in the portion of filament  34  between the two inner pins  32 ( b ) and  32 ( c ). Current does not flow through outer pins  32 ( a ) and  32 ( d ) because these pins are coupled to a voltage measuring sense amplifier  50  that has a high input impedance. Consequently no current flows through pins  32 ( a ) and  32 ( d ). The two outer pins  32 ( a ) and  32 ( d ) are connected to the filament  34  via welds  36 ( a ) and  36 ( d ). The outer pins  32 ( a ) and  32 ( d ) engage with receptacles  40 ( a ) and  40 ( d ) that are electrically connected to wires  39 ( a ) and  39 ( d ), respectively. The wires  39 ( a ) and  39 ( d ) are coupled at connection points  52  to the inputs  54  to the voltage sense amplifier  50 . 
   The voltage sense amplifier  50  detects the voltage drop across the segment of filament  34  between the two inner pins  32 ( b ) and  32 ( c ). Specifically, the voltage across welds  36 ( b ) and  36 ( c ) is identical to the voltage across the two inputs  54  to the voltage sense amplifier  50 . Because no current can flow through the outer two filament segments (between pins  32 ( a )- 32 ( b ) and between pins  32 ( c ) and  32 ( d )) as well as the pins  32 ( a ),  32 ( d ), receptacles  40 ( a ),  40 ( d ), and connection points  52 , there is no voltage drop across these components. The voltage sense amplifier  50  thus receives the precise “true” voltage across the energized portion of the filament  34  (between pins  32 ( b ) and  32 ( c )). 
   The voltage sense amplifier  50  amplifies the voltage to a level required by the circuitry  44  for determining the resistance of the energized portion of the filament  34 . As shown in  FIG. 3 , in a preferred embodiment, the amplified voltage is supplied to the driver circuit  46 . The resistance in the filament  34  is calculated using Ohm&#39;s law based on the current provided via the drive circuit  46  and the voltage seen by the voltage sense amplifier  50 . In a preferred embodiment, the driver circuit  46  calculates the resistance based on the current and measured voltage drop. However, a separate circuit or microprocessor may be used to calculate the resistance of the filament  34 . 
     FIG. 3  shows a fluid  26  passing over the filament  34 . When the filament  34  is heated (above the temperature of the fluid  26 ) by application of an electrical current I and the filament  34  is exposed to fluid flow, convective heat transfer occurs from the filament  34  to the fluid  26 . This heat transfer, being a function of the fluid velocity, causes small changes in the filament  34  temperature, and therefore its resistance. Because the hot-wire anemometer  18  precisely measures the resistance of the filament  34  (without resistance artifacts caused by cables, pins, receptacles, and welds), the temperature of the filament  34  can be obtained with great accuracy and precision. For any given fluid temperature and filament  34  geometry, the convective heat transfer value (h c ) can be determined from the filament  34  temperature change from its “zero fluid velocity” value. h c  can then be directly correlated to fluid velocity using King&#39;s law. Advance knowledge of the fluid temperature is required in this embodiment. 
   FIGS.  4 ( a ),  4 ( b ),  4 ( c ), and  5  show an alternative embodiment that uses two separate filament wires  34 ,  60 . In this embodiment, the probe  20  is identical to that disclosed in the previous embodiment with the exception that hot-wire anemometer  18  includes a second set of four pins  62 ( a-d ) and a second filament  60  welded via spot welds  63 ( a-d ) to the second set of pins  62 ( a-d ). Four nodes (two outer and two inner) are created at the locations where the pins  62 ( a-d ) contact the filament  60 . In this embodiment, the first set of pins  32 ( a-d ) and the second set of pins  62 ( a-d ) are used for different purposes. The first set of pins  32 ( a-d ) is used just as the embodiment described above, namely, current I passes from a driver circuit  46  through pin  32 ( c ) and to the region of the filament  34  located between the two inner pins  32 ( b ) and  32 ( c ). This current I heats the filament  34  as in the previously described embodiment. Current I returns to the driver circuit  46  via pin  32 ( b ). The voltage drop on the filament  34  is measured using a voltage sense amplifier  50  connected to the two outer pins  32 ( a ) and  32 ( d ). In this regard, the first set of pins  32 ( a-d ) and filament  34  are called “hot”. 
   The second set of pins  62 ( a-d ) and filament  60  are referred to as “cold”. The second set of pins  62 ( a-d ) and filament  60  are used to measure the temperature of the fluid  26  (i.e., gas) passing through the probe  20 . The temperature of the fluid  26  is determined using the same Kelvin sensing techniques as the hot filament  34 . In this regard, the temperature of the fluid  26  is determined from the temperature of the filament  60  which is obtained by its measured resistance. The filament  60  is preferably made of  304  stainless steel. The filament  60  preferably has a circular cross-sectional profile with a diameter of approximately 0.001 inch. In a preferred embodiment, filament  60  should match filament  34  in geometry and scale in order to match their thermal behavior. 
   In the cold filament  60 , a very small current I′ is passed through the filament  60  so as not to cause significant heating of the filament  60 . The current I′ is passed through the segment of filament  60  located between the two inner pins  62 ( b ) and  62 ( c ). The voltage drop on the filament  60  is measured with a separate voltage sense amplifier  62  via inputs  65 . Preferably, the same driver circuit  46  receives the amplified voltage from the voltage sense amplifier  62 . As with the hot wire, the resistance in the cold filament  60  is calculated based on the current I′ and measured voltage using Ohm&#39;s law. 
   In this second embodiment, instead of the cable  38  having just four receptacles  40 ( a-d ) and four wires  39 ( a-d ), the cable  38  has a total of eight receptacles  40 ( a-d ),  68 ( a-d ) and eight wires  39 ( a-d ),  70 ( a-d ). FIG.  4 ( c ) shows the full compliment of receptacles  40 ( a-d ),  68 ( a-d ) and wires  39 ( a-d ),  70 ( a-d ). 
   In the embodiment with the two filaments  34  and  60 , the cold filament  60  is used to measure the temperature of the fluid  26 . Preferably, the hot filament  34  is heated with a current I so as to maintain a constant temperature differential above the temperature of the fluid  26  measured by the cold filament  60 . The second filament  34  is used to measure the flow rate of the fluid  26  based on its power consumption, which is a direct function of h c . For example, the cold filament  60  might measure a gas temperature of 25° C. during patient inhalation. Consequently, the driver circuit  46  for the hot filament  34  would deliver a current I so as to heat the filament  34  to a predetermined temperature above the temperature of the gas. In the current example, the temperature differential might be set to 85° C. Accordingly, the hot filament  34  would be heated to a temperature of 110° C. While a 85° C. temperature differential has been disclosed, it should be understood that other temperature differentials may also be used. It is preferable that this embodiment is dynamic in that when the temperature of the fluid  26  changes the temperature of the filament  34  changes in a corresponding manner. The changes in the current I applied to the filament  34  can be done on a real-time or rear real-time basis. This embodiment is particularly advantageous because it cancels out the impact of temperature changes in the measured fluid  26  and surrounding environment. 
   In the preferred embodiments, the hot-wire anemometer  18  is used in conjunction with a probe  20  to measure the temperature and flow rate of a gas. It should be understood, however, that the hot-wire anemometer  18  may function without a body  22  and may just comprise four pins  32 ( a-d ) and an electrically coupled filament  34 . Similarly, while the hot-wire anemometer  18  is used to measure the temperature and flow rate of gases, the hot-wire anemometer  18  can be used in other applications where the fluid may be a liquid. 
   While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.