Abstract:
The present invention is directed toward a circuit that employs heated semiconductor elements to sense fluid flow speed and direction based on the cooling of the semiconductor element. The fluid flow speed and direction is determined by measuring the changes in the forward voltage drop across the semiconductor. The present invention improves on the previous art by enabling a single circuit to operate in either a constant-current or hybrid (constant-current/constant-temperature) mode where advantageous aspects of both modes are employed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit of U.S. Provisional Application No. 60/837,436, filed Aug. 11, 2006, the entire disclosure of which is hereby incorporated herein by reference. 
     
     FIELD 
       [0002]    The present invention is directed to sensing devices and more particularly to a flow sensor or an anemometer that can be used to measure wind speed and direction. 
       BACKGROUND 
       [0003]    Surface-based meteorological stations may be used in connection with monitoring a variety of environmental conditions. In connection with surface-based meteorological stations, it is often desirable to measure the speed and direction of the wind. Anemometers often take advantage of the relationship between heat dissipation and air speed. The principle of thermal anemometry relies on King&#39;s Law, which shows that the power required to maintain a fixed differential between the surface of a heated sensor and the ambient air temperature increases as the square root of air speed. 
         [0004]    Hot wire anemometers were first employed to exploit this phenomenon. One disadvantage of hot wire anemometers is the fact that they rely on metallic filaments that are fragile and unreliable. Because of this, anemometers have evolved that replace the fragile metallic filaments of hot wire anemometers with bipolar transistors and the like. By carefully controlling the amount of power supplied to a bipolar transistor in an air stream, it is possible to still exploit King&#39;s Law to determine wind speed. Two basic types of circuits have come about through this evolution. The first type of circuit is known as a constant temperature anemometer (CTA) and the second type of circuit is known as a constant power anemometer (CPA). CTAs generally employ a solid-state feedback control circuit to maintain a constant temperature difference between the heated sensor and the fluid temperature as measured by a second sensor. CTAs enable the measurement of fast-changing velocity fluctuations. However, one major drawback to classical CTAs is that they consume a significant amount of power, making their use in remote locations less desirable. 
         [0005]    CPAs, on the other hand, consume much less power than their CTA counterparts, thereby making them a promising circuit for remote anemometer applications. CPAs provide constant electrical power to a resistance element. A temperature sensor is attached to the heater element and is heated by conduction from the heater element. The difference between the temperature of the heated sensor and an ambient fluid temperature sensor is measured and the fluid velocity is determined. If the difference in temperature is small, then the fluid velocity is high. Conversely, if the temperature difference is large, then the fluid velocity is low. CPAs are not without their own drawbacks. For example, CPAs are slow to respond to changes in velocity and temperature due to the thermal inertia of the sensors and, unless specifically corrected, they have a limited range of temperature compensation. 
         [0006]    It would be desirable to have an anemometer circuit that offers the advantages of both the CPA and the CTA circuits. More specifically, it would be desirable to have an anemometer circuit that is capable of operating in a wide range of temperatures, responds to quick changes in fluid velocity, and is a relatively efficient user of power. 
       SUMMARY 
       [0007]    The present invention is directed to solving these and other problems and disadvantages of the prior art. Embodiments of the present invention provide a device for sensing flow rate of a fluid medium. In accordance with embodiments of the present invention, the device comprises a current source, a sensor device, a controller, and an analog to digital converter. The sensor device is exposed to a flow of the material, and is interconnected to the current source. The controller may control the current source to supply a selected amount of current to the sensor device. In accordance with at least one embodiment of the present invention, the selected amount of current is a constant and a temperature of the sensor device varies with at least a rate of the flow of material. The voltage drop across the sensor device varies with the temperature of the sensor device and a signal indicative of the voltage drop across the sensor device is provided to the controller. 
         [0008]    A device according to embodiments of the present invention comprises a circuit that can operate in a constant-current mode. In the constant-current mode, a constant current is set in the sensor device. The voltage drop across the sensor device changes slightly with temperature/fluid flow. From the relationship P=VI, it follows that if current is fixed, and voltage drop varies, the power will vary. 
         [0009]    Additionally, a device according to embodiments of the present invention may utilize the single circuit in either a constant-power mode, a constant-temperature mode, or a hybrid mode combining aspects of both of these modes. In the hybrid mode, a level of power may be applied to the sensor device such that its temperature lies within a certain predefined window. In such a case, the power is adjusted (as in a constant-temperature system) until the temperature of the sensor is within the desired window. Once the temperature of the sensor is within the desired window, the current supplied to the sensor device is held constant. The circuit, particularly in the hybrid mode, can keep the voltage drops of the sensor device in ranges such that lookup tables or comparatively simple math may be used to transform the voltage drop measurements into wind speed measurements. 
         [0010]    Another aspect of the present invention is the ability to control power consumption by the flow sensing device. More particularly, a control voltage can be applied in the circuit to control the power delivered to the sensor device. By reducing or zeroing this control voltage via the controller, it is possible to turn off the circuit, thereby reducing its power consumption. 
         [0011]    In accordance with still further embodiments of the present invention, a method for sensing a flow rate of a medium is provided. The method generally comprises exposing a flow sensor device to an ambient flow of a medium and supplying a constant current to the flow sensor device. A voltage drop across the flow sensor device is determined while continuing to supply the constant current to the flow sensor device. The amount of the voltage drop is a function of the temperature of the sensor device, and is indicative of the flow rate of the medium. As sensing continues, the constant current continues to be supplied to the sensor device, and the temperature of the sensor device is allowed to vary with at least a rate of ambient flow of the medium. 
         [0012]    Additional advantages of the present invention will become readily apparent from the following description, particularly when taken together with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  depicts an anemometer in accordance with embodiments of the present invention; 
           [0014]      FIG. 2A  depicts a cross-sectional top view of the anemometer components in a first configuration in accordance with embodiments of the present invention; 
           [0015]      FIG. 2B  depicts a cross-sectional top view of the anemometer components in a second configuration in accordance with embodiments of the present invention; 
           [0016]      FIG. 3  is a schematic diagram of a circuit used to sense fluid flow in accordance with embodiments of the present invention; 
           [0017]      FIG. 4  is a schematic diagram of a circuit used to sense fluid flow in accordance with other embodiments of the present invention; 
           [0018]      FIG. 5  is a schematic diagram of a circuit used to sense fluid flow in accordance with other embodiments of the present invention; 
           [0019]      FIG. 6  is a schematic diagram of a circuit used to sense fluid flow in accordance with other embodiments of the present invention; 
           [0020]      FIG. 7  is a flow diagram depicting a method of measuring fluid flow in accordance with embodiments of the present invention; and 
           [0021]      FIG. 8  is a flow diagram depicting a method of operating a fluid flow sensing system in a hybrid mode in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    With reference now to  FIG. 1 , an anemometer system  100  is depicted in accordance with at least some embodiments of the present invention. The anemometer system  100  may be deployed in weather stations, other meteorological sensor systems, and devices which can be used to measure the flow of various fluids such as water, air, and other gases. The anemometer system  100  may comprise a first or reference sensor device  104  and a plurality of second or flow sensor devices  108   a - d.  The reference sensor device  104  is depicted as being housed within a base plate  116  of the anemometer system  100  such that the reference sensor device  104  is protected from the fluid flow conditions. The base plate  116  serves as at least a portion of a flow shield that blocks all of the ambient fluid flow with respect to the reference sensor device  104 . By blocking all of the ambient fluid flow to the reference sensor device  104 , the reference sensor device  104  can be used to determine a zero-flow response at the ambient temperature of the fluid within which the anemometer system  100  is immersed. 
         [0023]    In accordance with at least one alternative embodiment of the present invention, the reference sensor device  104  may be located remotely with respect to the anemometer system  100 . More specifically, the reference sensor device  104  may not even be associated with a flow shield such as the base plate  116 . Rather, the reference sensor device  104  may be in a completely separate area that is not subject to the fluid flow. 
         [0024]    The flow sensor devices  108   a - d  are housed in support structures  112   a - d  respectively that serve as fluid flow shields for the flow sensor devices  108   a - d.  The support structures  112   a - d  are depicted as being oriented uniformly around the circumference of the base plate  116  such that the fluid flow from a certain direction in the plane of the base plate  116  is at least partially blocked from reaching a flow sensor device  108  by its support structure  112 . As can be appreciated by one skilled in the art, the configuration of the flow sensor devices  108  relative to one another may be adjusted both within their plane and out of plane with respect to one another without departing from the inventive aspects of the present invention. 
         [0025]    By partially blocking the flow in different directions to a flow sensor device  108 , information regarding a direction of the fluid flow relative to the anemometer system  100  can be obtained from taking differences between the temperatures of the various flow sensor devices  108 . A known algorithm may be employed to determine fluid flow direction. 
         [0026]    Although the anemometer system  100  is depicted as having four flow sensor devices  108 , one skilled in the art will appreciate that a greater or lesser number of flow sensor devices  108  may be employed to determine fluid flow speed and direction. In the most degenerate case, a single flow sensor device  108  may be used to determine fluid flow speed. In another embodiment, two flow sensor devices  108  may be used to determine fluid flow speed and fluid flow direction relative to a single axis. In still another embodiment, three flow sensor devices  108  may be employed to determine fluid flow speed and fluid flow direction relative to two axes. A greater number of flow sensor devices  108  may be employed for redundancy and to increase the accuracy with which fluid flow direction and speed can be determined. In addition, shielded and unshielded flow sensor devices  108  can be used in combination. 
         [0027]      FIG. 2A  depicts a first possible orientation of four flow sensor devices  108   a - d  as seen from above the anemometer system  100 . The four flow sensor devices  108   a - d  may be located along a common circular circumference in 90 degree increments from one another. The reference sensor device  104  may be located remotely from the flow sensor devices  108  in order to obtain ambient temperature measurements. Each flow sensor device  108  is blocked by a support structure  112 . The support structure  112  associated with each flow sensor device  108  outwardly blocks fluid flow from reaching the flow sensor device  108 . Accordingly, each flow sensor device  108  will be exposed to a fluid flow from a different direction. For example, the upper most flow sensor device  108 a would be fully exposed to a fluid flowing upward in the diagram of  FIG. 2A  while the lower most flow sensor device  108   c  would be completely shielded from the same flow. The other two flow sensor devices  108   b  and  108   d  would be partially exposed to the same fluid flow in the upward direction. The differences in exposure to the same fluid flow can be exploited to determine a fluid flow speed and direction. 
         [0028]      FIG. 2B  depicts a second possible orientation of four flow sensor devices  108   a - d  as seen from above the anemometer system  100 . In this particular configuration, the flow sensor devices  108   a - d  are still located along a common circular circumference in 90 degree increments from one another and the reference sensor device  104  may still be remotely located from the anemometer system  100 . However, in this configuration of flow sensor devices  108   a - d,  each support structure  112  may inwardly block fluid flow from reaching its associated flow sensor device  108 . Thus, each flow sensor device  108  will be exposed to a fluid flow from a different direction. For example, the upper most flow sensor device  108   a  would be fully exposed to a fluid flowing downward in the diagram of  FIG. 2B  while the lower most flow sensor device  108   c  would be completely shielded from the same flow. The other two flow sensor devices  108   b  and  108   d  would be partially exposed to the same fluid flow in the upward direction. Again, the differences in exposure to the same fluid flow can be exploited to determine the speed and direction of the fluid flow. 
         [0029]    The flow sensor devices  108   a - d  can also be spaced apart in a non-uniform fashion around a common circumference. Such a configuration of flow sensor devices  108   a - d  may be useful in applications where fluid flow direction is considered less important to determine and fluid flow distribution (e.g., fluid flow dynamics in a pipe or the like) is of more interest. 
         [0030]    In addition, an unshielded flow sensor device  108  may be provided for sensing fluid flow speed only. An embodiment of the anemometer system  100  may provide an unshielded flow sensor device  108  in addition to partially shielded flow sensor devices  108 . The combination of unshielded and partially shielded flow sensor devices  108  can be used to provide information regarding the fluid velocity (i.e., speed and direction). 
         [0031]      FIG. 3  is a circuit schematic depicting a circuit  300  that may be employed in connection with the anemometer system  100  in accordance with embodiments of the present invention. The circuit  300  comprises an operational amplifier  316  having an inverting  328  and non-inverting  332  inputs and an output  336 . The operational amplifier  316  acts as a current source for the circuit  300 . The output of the operational amplifier  316  is connected to an anode side of a first diode  320  as well as the input of an analog-to-digital converter  304  via a first node N 1 . The first diode  320  is an example of a sensor device  104 ,  108  that may be employed in accordance with embodiments of the present invention. The first diode  320  may comprise a typical pnjunction diode that conducts positive current (i.e., current flowing from the anode of the first diode  320  to the cathode of the first diode  320 ). In accordance with at least one embodiment of the present invention, the first diode  320  may comprise a silicon diode that has an average 0.6 V drop across it while conducting. The voltage across the first diode  320  (e.g., the potential difference between the anode and cathode side of the first diode  320 ) may change with temperature. For example, the voltage drop across the first diode  320  may change at a rate of approximately 0.2 mV per degree Celsius change in temperature about the first diode  320 . Knowing the characteristics of the first diode  320  and its responsiveness to changes in temperature, it is possible to determine a temperature change and therefore a fluid flow speed relative to the diode  320  upon observation of a voltage change across the first diode  320 . 
         [0032]    Although the sensor device  104 ,  108  is depicted and described as a diode, other semiconductor devices may be employed as sensor devices  104 ,  108 . One example of a semiconductor device that may be employed in lieu of the first diode  320  is a transistor, such as a bipolar junction transistor. Another example of a semiconductor device that may be employed is a Zener diode that conducts current when a reverse voltage is applied thereto. In addition, non-semiconductor devices may be used as sensor devices  104 ,  108 . For example, the sensor devices  104 ,  108  may comprise common resistors or thermistors. 
         [0033]    The cathode side of the first diode  320  is connected to the inverting input  328  of the operational amplifier  316  at a second node N 2 . Also connected to the second node N 2  is a first resistor  324 . The opposite side of the first resistor  324  is connected to a common voltage point, such as ground. Generally speaking, however, the common voltage point may comprise any potential that is lower than can be output from the operational amplifier&#39;s  316  output. 
         [0034]    The output of the analog-to-digital converter  304  is connected to the input of a microcontroller  308 . The output of the microcontroller  308  is connected to a digital-to-analog converter  312  that converts the digital output of the microcontroller  308  back into an analog wave form or signal suitable as an input to the non-inverting input  332  of the operational amplifier  316 . As can be appreciated by one skilled in the art, the analog-to-digital converter  304  and/or the digital-to-analog converter  312  may be internally located within the microcontroller  308 . 
         [0035]    The microcontroller  308  is adapted to read the output of the operational amplifier  316  via the analog-to-digital converter  304  and determine the voltage drop across the first diode  320 . The voltage drop across the first diode  320  is output at a data output  340 , which provides a signal that can be converted into a temperature of the sensor and thus a flow rate of an ambient fluid. Alternatively, the microcontroller  308  may convert the voltage readings into temperature values and output the temperature values and provide signals indicative of the measured temperature values at the data output  340 . As still another example, the controller  308  can calculate the rate of fluid flow and provide that information at the data output  340 . 
         [0036]    The voltage at the output  336  of the operational amplifier  316  (i. e., the voltage at the first node N 1 ) is equal to the summation of the voltage at the non-inverting input  332  V REF  and the voltage drop across the first diode  320 . Based on this relationship, the microcontroller  308  is able to determine the voltage drop across the first diode  320  by determining the difference between the output  336  voltage of the operational amplifier  316  and the known voltage at the non-inverting input  332  V REF . In a constant-current mode of operation, the variable voltage drop across the first diode  320  is used to determine fluid flow speed, whereas in a constant-temperature mode of operation, the difference between the output voltage  336  and V REF  is held constant and the variations of V REF  are used to determine fluid flow speed. 
         [0037]    The microcontroller  308  also acts to control the voltage applied to the non-inverting input  332  of the operational amplifier  316  (i.e., V REF ). In accordance with embodiments of the present invention, in a constant-current mode of operation, the microcontroller  308  maintains the voltage V REF  at a constant value to maintain a constant current at the output  336  of the operational amplifier  316 . By maintaining a constant voltage at the non-inverting input  332 , the voltage at the inverting input  328  (i.e., V N2 ) also remains constant. Because the voltage at the non-inverting input  332  is held constant and because the value of the first resistor  324  is a fixed value, the current through the first resistor  324  is maintained at a constant value. This causes a constant current to be provided to the first diode  320 . This constant-current mode of operation differs from the operation of previous CPAs in that current through the first resistor  324  is maintained at a constant value, instead of maintaining the power dissipated by the circuit elements at a constant value. 
         [0038]    The forward voltage drop of the first diode  320  varies as the temperature about and/or flow of fluid past the first diode  320  changes. The voltage at the output  336  of the operational amplifier  316  varies in response to these resistance changes of the first diode  320  since the value of both voltage inputs and thus the current output by the operational amplifier  316  remain the same. The microcontroller  308  receives at its input the value of the output voltage  336  of the operational amplifier  316 , knows the digital-to-analog converter  312  output voltage V REF , and from these known values determines the voltage drop across the first diode  320 . The voltage drop across the first diode  320  can then be correlated to a fluid temperature or rate of fluid flow. 
         [0039]    In accordance with other embodiments of the present invention, the microcontroller  308  may operate the circuit  300  in a constant-temperature mode. In a constant-temperature mode of operation, the microcontroller  308  adjusts the current output by the operational amplifier  316  to keep the first diode  320  at a desired temperature. By keeping the first diode  320  at a constant temperature, the forward voltage drop of the first diode  320  remains constant. The fluctuations in power required to maintain this temperature, measured via changes in V REF , are used to determine the fluid flow speed across the first diode  320 . 
         [0040]    In accordance with still other embodiments of the present invention, the microcontroller  308  may operate the circuit  300  in a hybrid mode. In the hybrid mode of operation, the microcontroller  308  causes the first diode  320  to be operated within a predetermined temperature window and maintains a constant current through the first diode  320 . The microcontroller  308  may selectively and dynamically change the operational mode of the circuit  300  depending upon the changes in conditions about the circuit such as temperature and fluid flow speed without having to change any configuration of circuit  300  elements. 
         [0041]      FIG. 4  depicts a circuit  400  that may be employed in connection with the anemometer system  100  in accordance with embodiments of the present invention. The circuit  400  comprises a plurality of operational amplifiers  416   a  to  416 N and a plurality of instrumentation amplifiers  428   a  to  428 M. A first set of the operational amplifiers  416   a  to  416 N may be used as current sources to the circuit  400 , while a second set of the instrumentation amplifiers  428   a  to  428 M provide an output based on a voltage difference between their inputs. 
         [0042]    The number of source operational amplifiers  416  in the first set (ie., N) may be greater than or equal to two, while the number of instrumentation operational amplifiers  428  in the second set (i.e. M) is equal to N−1, where N and M are both integers. A first set of resistors  424   a  to  424 N are provided, each corresponding to a source operational amplifier in the first set of operational amplifiers  416   a  to  416 N. These resistors  424  are used to control the current flowing through the diodes  420   a  to  420 N. The voltage applied across each resistor  424   a  to  424 N is substantially the same. The values of the resistors  424   a  to  424 N are substantially the same, thereby allowing the same constant current to be provided therethrough. Therefore, in a constant-current mode of operation, the same current is provided to each of the diodes  420   a  to  420 N. By providing the same amount of current to the reference sensor  104  as the flow sensors  108 , the reference sensor  104  can be used to provide a baseline voltage drop for the ambient fluid temperature while voltage drop across the flow sensors  108  will reflect the fluid flow in addition to fluid temperature. Accordingly, the difference between the voltage drop across a flow sensor  108  and the voltage drop across the reference sensor  104  will be indicative of the fluid flow. 
         [0043]    Additionally, a second set of resistors  432   a  to  432 M are provided, each corresponding to an instrumentation amplifier in the second set of operational amplifiers  428   a  to  428 M. The resistors  432   a  to  432 M are used to set the gain of each instrumentation amplifier  428   a  to  428 M. 
         [0044]    The circuit  400  operates in a similar fashion to the circuit  300  depicted in  FIG. 3 , whereby the voltage drop across the diodes  420   a  to  420 N are used to determine fluid temperature and subsequently fluid flow speed. The difference in the second circuit  400  is that output differentials between various diodes  420   b  to  420 N are compared to an output of a reference diode  420   a.  The reference diode  420   a  may correspond to a reference sensor device  104  that is shielded from the fluid flow and provides a reading of zero-flow rate at the ambient fluid temperature. The other diodes  420   b  to  420 N may correspond to one of the flow sensor devices  108  that are open to the flow of the fluid. The output  336  of the first operational amplifier  416   a  (i.e., V OUT1 ) is supplied to an inverting input of each instrumentation operational amplifier  428   a  to  428 M. The outputs  336  of the other operational amplifiers  416   b  to  416 N (i.e., V OUT2 , V OUT3 , V OUTN ) are supplied to the non-inverting input of each instrumentation amplifier  428   a  to  428 M. 
         [0045]    The instrumentation amplifiers  428   a  to  428 M provide, as an output, the difference between the voltage across the reference diode  420   a  and the voltage across each corresponding diode  420   b  to  420 N; this difference is multiplied by a gain factor related to the value of resistor  432 . For example, the second instrumentation amplifier  428   b  provides, as an output, an amplified difference between V OUT3  and V OUT1 . As such, the ambient temperature is accounted for in the output of the instrumentation amplifiers  428   a  to  428 M, which are then provided to the input of a corresponding analog-to-digital converter  404   a  to  404 M and subsequently to the microcontroller  408  as input. 
         [0046]    In accordance with embodiments of the present invention, the outputs of each analog-to-digital converter  404   a  to  404 M are supplied to the microcontroller  408  in separate data paths as depicted. However, in accordance with alternative embodiments of the present invention, the outputs of the each analog-to-digital converter  404   a  to  404 M may be provided to the microcontroller  408  via a common data bus where the outputs of each converter  404   a  to  404 M maintain a logical separation on the bus but are otherwise provided to the microcontroller  408  over the same input port or channel. As can be appreciated by one skilled in the art, analog-to-digital converters with multiple input channels could replace the single channel analog-to-digital converters shown. 
         [0047]    The microcontroller  408  outputs the data received from each instrumentation amplifier  428   a  to  428 M. The microcontroller  408  also ensures that a substantially constant voltage is applied to each non-inverting input of the source operational amplifiers  416   a -N in the constant-current and hybrid mode of operation. Since the microcontroller  408  knows and controls the input to each operational amplifier  416   a  to  416 N and knows the output voltage at each instrumentation operational amplifier  428   a  to  428 M, the microcontroller  408  can determine the voltage drop across each diode  420   a  to  420 N due to fluid flow, all while maintaining a constant current through each resistor  424   a  to  424 N. 
         [0048]      FIG. 5  is a schematic diagram of another circuit  500  that may be employed in connection with the anemometer system  100  in accordance with embodiments of the present invention. The circuit  500  is similar to circuits  300  or  400  except that it includes a transistor  532  and a second resistor  528  that amplify the output of the operational amplifier  516 . The transistor  532  may include any type of transistor such as a pnp or npn bipolar junction transistor, Darlington transistor, or field-effect transistor. In accordance with a preferred embodiment, the transistor  532  comprises an npn bipolar junction transistor. 
         [0049]    The operational amplifier  516  produces the same output as before, namely the reference voltage output by the digital-to-analog converter  512  plus the voltage drop across the first diode  520 . The second resistor  528  is connected to the output of the operational amplifier  516 , which in turn is connected to the base region of the transistor  532 . The collector region of the transistor  532  may be connected to a voltage source +V that helps boost the signal output at the emitter region. The output at the transistor  532  emitter region is supplied to an analog-to-digital converter as in  FIG. 3  or to an instrumentation amplifier  428   a  to  428 M as in  FIG. 4 . The rest of circuit  500  is essentially identical to either the first circuit  300  or second circuit  400 . Namely, the non-inverting input of the operational amplifier  516  is connected to the output of the first diode or other sensor device  520  and an input of the first resistor  524 . 
         [0050]      FIG. 6  is a schematic diagram of yet another circuit  600  that may be employed in connection with the anemometer system  100  in accordance with embodiments of the present invention. The circuit  600  is almost identical to circuit  500  except that it includes a second transistor  636  that further amplifies the output of the first transistor  632 . The second transistor  636  may include any type of known semiconductor transistor such as a pnp or npn bipolar junction transistor, Darlington transistor, or field-effect transistor. In accordance with a preferred embodiment, both the first transistor  632  and second transistor  636  comprise an npn bipolar junction transistor. 
         [0051]    The operational amplifier  616  produces the same output as before, namely the reference voltage output by the digital-to-analog converter  612  plus the voltage drop across the first diode  620 . The second resistor  628  is connected to the output of the operational amplifier  616 , which in turn is connected to the base region of the first transistor  632 . The collector region of the first transistor  632  may be connected to a voltage source +V that helps boost the signal output at the emitter region. The output at the first transistor  632  emitter region is supplied to the base region of the second transistor  636 . The collector region of the second transistor  636  may be connected to another voltage source +V that further boosts the signal output at the emitter region. The source voltage +V supplied to the second transistor  636  may be the same as the source voltage +V supplied to the first transistor  632 , although such a configuration is not required. For example, the first transistor  632  may be supplied with a source voltage +V of a first amount while the second transistor  636  may be supplied with a source voltage +V of a second different amount. The output of the second transistor  636  is supplied to an analog-to-digital converter as in  FIG. 3  or to an instrumentation amplifier  428   a  to  428 M as in  FIG. 4 . The use of transistors  632  and  636  to boost the operational amplifier  616  output is advantageous since the operational amplifier alone may deliver insufficient current to adequately heat the sensor device or maintain equality between the voltages present at the non-inverting and inverting inputs. 
         [0052]      FIG. 7  is a flow chart depicting a method of measuring fluid flow in accordance with at least some embodiments of the present invention. The method begins when a sensor device  108  is exposed to a fluid flow (step  704 ). Of course, more than one sensor device  108  may be exposed to the same fluid flow but in a different position and/or orientation. As can be appreciated by one skilled in the art, another sensor device  104  may be immersed within the same fluid but protected from the flow to provide a base reading of the fluid&#39;s ambient temperature. 
         [0053]    While the sensor device  108  is exposed to the fluid flow, a selected amount of current is supplied to the sensor device  108  (step  708 ). The amount of current supplied to the sensor device  108  may be selected depending upon the type of fluid, the type of sensor device  108  employed, the ambient temperature of the fluid, and the optimal temperature operating range of the sensor device  108 . The selected amount of current is supplied to the sensor device  108  via a current source such as an operational amplifier. The amount of current provided to the sensor device  108  may be selected based on a trial-and-error basis where a first current amount is supplied to the sensor device  108  and the reaction of the sensor device  108  (e.g., the voltage drop across the sensor device  108 ) to the first amount of current is determined, then a second current amount is supplied to the sensor device  108  and the reaction of the sensor device  108  to the second amount of current is determined and based on the comparison of the reactions one of the two current amounts, or some third current amount, is selected as the current that will be provided to the sensor device  108  during the constant-current mode of operation. In another embodiment, the amount of current provided to the sensor device  108  may be determined a priori based on temperature of the fluid about the sensor device  108 . 
         [0054]    The current supplied by the current source is controlled by the amount of voltage applied to the current source (e.g., V REF ). In accordance with embodiments of the present invention, a controller such as the microcontroller  308 ,  408  may maintain the voltage applied to the current source at a constant value, thereby ensuring the amount of voltage supplied at the current source&#39;s inverting input is also constant. 
         [0055]    The voltage drop across the sensor device  108  is measured while the selected amount of current is supplied to the sensor device  108  (step  712 ). The voltage drop across the sensor device  108  is determined by monitoring the voltage at the output of the current source. In particular, the voltage drop across the sensor device  108  is monitored to determine the changes in voltage and subsequently the changes in temperature of the sensor. Based on the changes in temperature of the sensor device  108  the fluid flow may be determined by the controller  308 ,  408 . As the voltage across the sensor device  108  changes in response to temperature and flow changes, the voltage output of the current source changes. 
         [0056]    The changes in voltage across the sensor device  108  are monitored by the controller  308 ,  408 , and in response to such changes, the voltage supplied at the output of the current source adjusts to compensate (step  716 ). The current source compensates for the changes in voltage across the sensor device  108  to maintain a constant current through the fixed resistance and therefore a constant current through the sensor device  108  (step  720 ). If the current through the sensor device  108  is maintained at a constant level, then any changes in resistance due to temperature changes will result directly in voltage changes. The voltage changes across the sensor device  108  can be used to determine temperature changes and thus fluid flow rate. The method then returns to step  712  to continue measurement of the voltage drop across the sensor device  108 . 
         [0057]      FIG. 8  is a flow chart depicting a method of operating an anemometer system  100  in a hybrid mode (e.g., a combination of constant-temperature and constant-current mode) without changing the physical circuitry of the anemometer system  100  in accordance with at least some embodiments of the present invention. The method begins by selecting a desired operating temperature window for the sensor device  108  (step  802 ). The operating temperature window may be selected based on the expected average temperature of the fluid, the expected average speed of the fluid flow, and the type of the sensor device  108 . The operating temperature window is selected to maintain the sensor device  108  within a predetermined resistance and voltage drop. 
         [0058]    A sensor device  108  is then exposed to a fluid flow (step  804 ). As noted above, multiple sensor devices  108  may be exposed to the fluid flow. Furthermore, another sensor device  104  may be immersed in the fluid but completely protected from the fluid flow. 
         [0059]    Next, the amount of current required to maintain the sensor device  108  within the temperature window is determined (step  812 ). The operating current required to maintain the sensor device  108  within the predetermined temperature window will also vary based upon the type of sensor device  108  employed. Knowing the value of the fixed resistance  324 ,  424  used to maintain a constant current and the desired power, the desired amount of current can be determined. 
         [0060]    The amount of current provided to the sensor device  108  will also vary based on the environment in which the anemometer system  100  is employed. Advantageously, the use of different temperature windows can be employed such that a single circuit can be utilized in a number of different situations without requiring the reconfiguration of the circuit or replacement of circuit elements. 
         [0061]    Once the proper operating current has been selected, the determined amount of current is supplied to the sensor device  108  via a current source (step  816 ). A controller  308 ,  408  sets the amount of voltage provided to the non-inverting input of the current source, which in turn sets the amount of power and current provided to the sensor device  108 . As fluid flows past the sensor device  108  heat is removed from the sensor device  108 , thereby changing the internal resistance or forward voltage across the sensor device  108  and dissipating power in the form of heat. As long as the sensor device  108  is maintained in the temperature window, the current supplied to the sensor device  108  does not vary. If the temperature of the sensor device  108  falls outside of the predetermined temperature window, then a new current will have to be selected. 
         [0062]    While controlling the current source, the controller  308 ,  408  also measures the voltage drop across the sensor device  108  (step  820 ). The voltage drop across the sensor device  108  may be determined with a direct measurement or by measuring the output voltage of the current source and subtracting the voltage supplied to the current source. As the controller  308 ,  408  continues to cause this determined current to be provided to the sensor device  108 , the circuit treats measurements of voltage as that from a constant-current type system. The voltage drop across the sensor device  108  can then be used to determine fluid speed (step  824 ). The use of a predefined operating window in this hybrid mode is advantageous because a lookup table or simple math can be employed to determine the fluid speed. The lookup table can be used since the sensor device  108  is being maintained within the predetermined temperature window and is being supplied a substantially constant current. 
         [0063]    The controller  308 ,  408  monitors the changes in voltage across the sensor device  108 . Additionally, the voltage supplied to the current source is maintained at a substantially constant value to ensure that the predetermined current is provided to the sensor device  108  (step  828 ). The method then returns to step  808  where the controller  308 ,  408  continues to cause the determined amount of current to be provided to the sensor device  108 . 
         [0064]    If at any point it is determined that measurement data is no longer required, the control voltage supplied to the current source can be reduced or zeroed to effectively let the anemometer rest. This rest period can be used to conserve power consumption and maximize the period of time that the anemometer system  100  can be used without having to replace batteries or the like. This is particularly useful in remote applications where access to the anemometer system  100  is limited or difficult. Moreover, the controller  308 ,  408  may control the timing of rest periods and sensing periods to further manage power consumption. 
         [0065]    The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with various modifications required by their particular application or use of the invention. It is intended that the appended claims be construed to include the alternative embodiments to the extent permitted by the prior art.