Patent Publication Number: US-2021172775-A1

Title: Nanowires integration for real-time compensation

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Application No. 62/379,531 to Arwatz et al., filed Aug. 25, 2016, entitled “Nano-wires integration for real-time compensation,” which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to nanowire sensors for sensing fluid flow properties. 
     BACKGROUND 
     The resistance of a nanowire varies according to ambient conditions into which the nanowire is placed. A nanowire can therefore be used to measure ambient conditions surrounding the nanowire by driving a current through the nanowire and measuring the varying resistance. 
     The dissertation of Gilad Arwatz, Ph.D., entitled “Investigations of the fundamentals of passive scalar dynamics using nano-sensing devices,” Princeton University, 2015, which is incorporated herein by reference, describes turbulence as being the core of numerous investigations over several decades. Among the wide spectrum of turbulence aspects in the study described in the dissertation, the study focuses on temperature as a passive scalar advected in a turbulent velocity field. In this study, fundamental flow quantities are revisited by investigating statistically homogeneous and isotropic turbulence, with an imposed mean cross-stream linear temperature gradient. This is described as being made possible by developing a new fast response nano-sensor to minimize measurement errors inherent in conventional temperature probes (cold wires). As described in the study, it was observed that cold wire attenuation has widespread effects on most aspects of the measurements, resulting in the variance and the scalar rate of dissipation being significantly underestimated. 
     Newly acquired data allow for a theoretical study of the temperature spectra, the dissipation range, different scaling laws and intermittencies. By studying the evolution equations of the temperature spectra, conditions for self-preserving solutions were derived and described as being experimentally validated. Self-similarity of the dissipation subrange was explored, and described as revealing that the temperature field can be independently resolved without knowledge of the velocity field. The results are described as raising interesting questions about the underlying behavior of the scalar field, namely local equilibrium versus non-equilibrium. 
     Based on the proposed scaling and the significant departure of existing models from the expected power-law behavior in the inertial range, a model spectrum was developed based entirely on temperature-related variables, described as showing a convincing agreement with the experimental data in the dissipation range. 
     The underlying cause of scalar intermittencies, a well-established phenomenon reflected in the exponential tails of the scalar probability density function (PDF), is described as yet to be determined. The interplay between advection and diffusion was investigated through their timescales ratio, following the linear eddy model of Kerstein. The analysis is described as revealing a widening of the PDF as more of the low frequency content is excluded. The development of the new sensor, along with the fundamental study, is described as inspiring new ideas for measuring conductivity as a way to assess humidity in the atmospheric boundary layer or blood damage due to shear stresses. Overall, the study is described as shedding light on the importance of accurate and optimized measurement techniques in the pursuit of understanding turbulence. 
     An article entitled “Nanoscale sensing devices for turbulence measurements” by Y. Fan et al., which is incorporated herein by reference, describes a collection of nanoscale sensing devices developed specifically for high-frequency turbulence measurements. The sensors are all derived from the nanoscale thermal anemometry probe (NSTAP), which uses a free-standing platinum wire as an active sensing element. Each sensor is described as (a) being designed and fabricated to measure a specific quantity and (b) being customizable for special applications. In addition to the original NSTAP (for single-component velocity measurement), the sensors include the T-NSTAP (for temperature measurement), the x-NSTAP (for two-component velocity measurement), and the q-NSTAP (for humidity measurement). This article provides a summary of the NSTAP family including details of design and fabrication as well as presentation of flow measurements using these sensors. Also, a custom-made constant-temperature anemometer, described as allowing proper operation of the NSTAP sensors, is introduced. 
     SUMMARY OF THE INVENTION 
     As described herein, real-time accurate measurements of a flow property of a fluid are obtained with a nanowire sensor by compensating, in real-time, for errors in the measurement that are caused by other turbulent flow properties of the fluid affecting the nanowire. In accordance with some applications of the present invention, at least two nanowires are used to measure at least two different respective flow properties of a fluid, typically simultaneously in real-time. The resistance of each of the nanowires varies based on a different respective flow property, and each of the nanowires is operated in different respective modes of operation, such that the nanowires measure different respective flow properties. The measurements of one of the flow properties are used to correct the measurements of another flow property. For some applications, a first nanowire and a second nanowire are coupled to a Wheatstone bridge such that the varying resistance of the second nanowire affects the first nanowire, thereby correcting the measurement of the first nanowire. Output circuitry outputs the measurement obtained from the first nanowire that is corrected by the measurement obtained from the second nanowire. 
     For some applications, one nanowire can be used to obtain measurements of different flow properties by alternating the nanowire between different modes of operation. In this case, the resistance of the nanowire varies in response to different flow properties when the nanowire is operated in the different modes of operation respectively. The measurements of one of the flow properties is used to correct the measurements of the other flow property. 
     There is therefore provided, in accordance with some applications of the present invention, a method for measuring at least two flow properties of a fluid, the method including: 
     providing at least two nanowires, the resistance of each nanowire varying based on a value of a different respective flow property such that each nanowire is configured to measure the different respective flow property; and 
     operating each of the nanowires in a different respective mode of operation, to measure the at least two flow properties simultaneously in real-time. 
     For some applications, operating includes operating at least one of the nanowires with constant current anemometry (CCA) and operating at least another of the nanowires with constant temperature anemometry (CTA). 
     For some applications, providing includes providing at least two nanowires, a first one of the nanowires being configured to measure a flow property of the fluid selected from the group consisting of: temperature, humidity, and velocity. 
     For some applications, providing includes providing at least two nanowires, a second one of the nanowires being configured to measure another flow property of the fluid selected from the group consisting of: temperature, humidity, and velocity. 
     For some applications, the method further includes using a measurement obtained from at least one of the nanowires to correct a measurement obtained from at least another of the nanowires. 
     For some applications: 
     providing the two nanowires includes providing a temperature-measuring nanowire and a velocity-measuring nanowire, and 
     using a measurement comprises using a temperature measurement obtained from the temperature-measuring nanowire to correct a velocity measurement obtained from the velocity-measuring nanowire. 
     For some applications, operating includes using an output measurement from a second one of the two nanowires as an offset for a first one of the two nanowires, prior to measurement by the first nanowire. 
     For some applications, the method further includes driving the first nanowire using first nanowire-driving circuitry and driving the second nanowire using second nanowire-driving circuitry. 
     For some applications, operating includes using an output measurement from a second one of the two nanowires to process measurement data output by a first one of the two nanowires. 
     For some applications, the method further includes driving the first nanowire using first nanowire-driving circuitry and driving the second nanowire using second nanowire-driving circuitry. 
     There is further provided, in accordance with some applications of the present invention, a method for measuring different flow properties of a fluid, the method including: 
     providing a nanowire, the resistance of the nanowire varying based on values of the different flow properties when operated in different modes of operation respectively, such that the nanowire is configured to measure different flow properties when operated in different modes of operation respectively; 
     alternating the nanowire between different modes of operation; 
     measuring different flow properties of the fluid during the different modes of operation, respectively; and 
     using the measurements of one of the flow properties to correct the measurements of another flow property. 
     For some applications, alternating the nanowire between different modes of operation includes driving the nanowire with different respective levels of current. 
     For some applications: 
     alternating the nanowire between different modes of operation includes alternating the nanowire between two different modes of operation, 
     measuring different flow properties includes measuring two different flow properties during the two different modes of operation, respectively, and 
     using the measurements includes using the measurements of one of the flow properties to correct the measurements of the other flow property. 
     For some applications, alternating the nanowire between the two different modes of operation includes driving the nanowire with an alternating current. 
     For some applications, measuring two different flow properties of the fluid includes measuring the two different flow properties at a measurement frequency of less than half of a frequency of the alternating current. 
     For some applications, driving the nanowire with the alternating current includes driving the nanowire with an alternating current having a frequency of 1-2000 Hz. 
     For some applications, driving the nanowire with the alternating current includes driving the nanowire with an alternating current having a frequency of 1-100 Hz. 
     For some applications, driving the nanowire with the alternating current includes driving the nanowire with an alternating current having a frequency of 100-2000 Hz. 
     For some applications, driving the nanowire with an alternating current includes driving the nanowire with an alternating current in a form selected from the group consisting of: a square wave, a triangle wave, or a sinusoidal wave. 
     For some applications: 
     alternating the nanowire between the two different modes of operation includes using a switching circuit to switch between the two modes of operation, and 
     using the measurements of one of the flow properties to correct the measurements of the other flow property includes processing data output by the nanowire after two different flow properties have been measured. 
     For some applications, using a switching circuit to switch between the two modes of operation includes using a switching circuit to switch between operating the nanowire with constant current anemometry (CCA) and operating the nanowire with constant temperature anemometry (CTA). 
     For some applications, using a switching circuit to switch between the two modes of operation includes using a switching circuit to switch between operating the nanowire using two different constant current anemometry (CCA) circuits. 
     For some applications, using the switching circuit to switch between operating the nanowire using two different CCA circuits includes using a switching circuit to switch between operating the nanowire using a first CCA circuit that drives the nanowire with a current of 5-50 mA and a second CCA circuit that drives the nanowire with a current of 0.1-1 mA. 
     There is further provided, in accordance with some applications of the present invention, apparatus for measuring flow properties of a fluid, the apparatus including: 
     a Wheatstone bridge; 
     a first nanowire coupled along a first leg of the Wheatstone bridge, the first nanowire being configured to measure a first flow property; 
     a power source configured to supply power to the Wheatstone bridge; 
     a second nanowire coupled to the Wheatstone bridge or to the power source; and 
     output circuitry, configured to output a measurement obtained from the first nanowire that is corrected by a measurement obtained from the second nanowire. 
     For some applications, the first nanowire is configured to measure a flow property selected from the group consisting of: 
     temperature, humidity, and velocity. 
     For some applications: 
     the selected flow property is temperature, 
     a longitudinal length of the first nanowire is 60-250 microns, 
     a width of a cross-section of the first nanowire, the cross-section being taken perpendicular to a direction of current flow in the nanowire, is 1-2 microns, and 
     a height of the cross-section of the first nanowire is 0.1-0.2 microns. 
     For some applications: 
     the selected flow property is velocity, 
     a longitudinal length of the first nanowire is 30-100 microns, 
     a width of a cross-section of the first nanowire, the cross-section being taken perpendicular to a direction of current flow in the nanowire, is 1-2 microns, and 
     a height of the cross-section of the first nanowire is 0.1-0.2 microns. 
     For some applications: 
     the selected flow property is humidity, 
     a longitudinal length of the first nanowire is 10-60 microns, 
     a width of a cross-section of the first nanowire, the cross-section being taken perpendicular to a direction of current flow in the nanowire, is 0.1-1 microns, and 
     a height of the cross-section of the first nanowire is 0.05-0.1 microns. 
     For some applications, the second nanowire is configured to measure another flow property selected from the group consisting of: temperature, humidity, and velocity. 
     For some applications: 
     the other flow property is temperature, 
     a longitudinal length of the second nanowire is 60-250 microns, 
     a width of a cross-section of the second nanowire, the cross-section being taken perpendicular to a direction of current flow in the nanowire, is 1-2 microns, and 
     a height of the cross-section of the second nanowire is 0.1-0.2 microns. 
     For some applications: 
     the other flow property is velocity, 
     a longitudinal length of the second nanowire is 30-100 microns, 
     a width of a cross-section of the second nanowire, the cross-section being taken perpendicular to a direction of current flow in the nanowire, is 1-2 microns, and 
     a height of the cross-section of the second nanowire is 0.1-0.2 microns. 
     For some applications: 
     the other flow property is humidity, 
     a longitudinal length of the second nanowire is 10-60 microns, 
     a width of a cross-section of the second nanowire, the cross-section being taken perpendicular to a direction of current flow in the nanowire, is 0.1-1 microns, and 
     a height of the cross-section of the second nanowire is 0.05-0.1 microns. 
     For some applications, a longitudinal length of the second nanowire is 0.6-9 times a longitudinal length of the first nanowire. 
     For some applications, the longitudinal length of the second nanowire is 1-3 times the longitudinal length of the first nanowire. 
     For some applications, a ratio of a width to a height of a cross-section of the first nanowire is 0.8-1.2 times a ratio of a width to a height of a cross-section of the second nanowire, the cross-sections being taken perpendicular to a direction of current flow in the first and second nanowires. 
     For some applications, a volume of the first nanowire is 0.8-1.2 times a volume of the second nanowire. 
     For some applications, a volume of the second nanowire is 4-1000 times a volume of the first nanowire. 
     For some applications, the volume of the second nanowire is 15-180 times the volume of the first nanowire. 
     For some applications, the second nanowire is coupled in series with the Wheatstone bridge, the second nanowire being disposed between the power source and the Wheatstone bridge, such that a voltage supplied to the Wheatstone bridge is altered according to a resistance of the second nanowire that varies based on a value of the second flow property. 
     For some applications, the second nanowire is coupled in parallel to the Wheatstone bridge such that a current supplied to the Wheatstone bridge is altered according to a resistance of the second nanowire that varies based on a value of the second flow property. 
     For some applications, the second nanowire is coupled along a second leg of the Wheatstone bridge. 
     For some applications, the apparatus further includes matching circuitry coupled to the second nanowire and configured to regulate a current flowing through the second nanowire. 
     For some applications, the first nanowire is configured to measure humidity and is sensitive to velocity as well, and the second nanowire is configured to measure velocity, the apparatus further comprising matching circuitry configured to tune the second nanowire such that a velocity sensitivity of the second nanowire matches the velocity sensitivity of the first nanowire. 
     For some applications: 
     (a) the first and second nanowires are configured to measure velocity of the fluid and are sensitive to temperature as well, and 
     (b) the first nanowire is disposed such that it is exposed to the velocity of the fluid and the second nanowire is disposed such that it is shielded from the velocity of the fluid. 
     For some applications, the second nanowire is disposed such that an angle between the second nanowire and a flow field of the fluid is less than 20 degrees. 
     For some applications, the second nanowire is disposed such that it is parallel to the flow field of the fluid. 
     For some applications, apparatus further includes a porous structure positioned to limit the velocity of the fluid within the porous structure, the second nanowire being disposed inside the porous structure. 
     For some applications, the apparatus further includes a porous structure positioned to limit the velocity of the fluid at the second nanowire. 
     For some applications, the apparatus further includes a chamber in which the second nanowire is disposed, the chamber being shaped to define exactly one opening which allows ambient fluid temperature conditions to affect the second nanowire without facilitating a fluid-motion-inducing pressure differential within the chamber. 
     For some applications, a diameter of the exactly one opening is 0.1-1 mm. 
     The present invention will be more fully understood from the following detailed description of applications thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-4  are schematic illustrations of sensing apparatus in accordance with some applications of the present invention; 
         FIGS. 5A-D  are schematic illustrations of a fluid flow field with two nanowires in the flow field, in accordance with respective applications of the present invention; 
         FIG. 6-8  are schematic illustrations of the sensing apparatus in accordance with some applications of the present invention; 
         FIG. 9A  is a schematic illustration of the sensing apparatus coupled to a multicopter, in accordance with some applications of the present invention; 
         FIG. 9B  is a schematic illustration of the sensing apparatus coupled to a fixed-wing drone, in accordance with some applications of the present invention; 
         FIG. 10  is a schematic illustration of a gas meter with the sensing apparatus disposed within the gas meter, in accordance with some applications of the present invention; and 
         FIG. 11  is a schematic illustration of a turbine with the sensing apparatus disposed within the turbine, in accordance with some applications of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Flow properties of a fluid include temperature of the fluid, humidity, and velocity of the fluid. Nanowires of different dimensions exhibit different sensitivities to each of the abovementioned flow properties respectively, the resistance of each nanowire varying based on the respective flow property that the nanowire is sensitive to. Different nanowires can therefore be configured to measure different flow properties respectively. 
     For some applications, a nanowire that is configured to measure temperature has the following dimensions:
         a longitudinal length of the nanowire is 60-250 microns,   a width of a typically but not necessarily rectangular cross-section of the nanowire is 1-2 microns, the cross-section being taken perpendicular to a direction of current flow in the nanowire, and   a height of the cross-section of the nanowire is 0.1-0.2 microns.       

     For some applications, a nanowire that is configured to measure velocity typically has the following dimensions:
         a longitudinal length of the nanowire is 30-100 microns,   a width of a typically but not necessarily rectangular cross-section of the nanowire is 1-2 microns, the cross-section being taken perpendicular to a direction of current flow in the nanowire, and   a height of the cross-section of the nanowire is 0.1-0.2 microns.       

     For some applications, a nanowire that is configured to measure humidity typically has the following dimensions:
         a longitudinal length of the nanowire is 10-60 microns,   a width of a typically but not necessary rectangular cross-section of the nanowire is 0.1-1 microns, the cross-section being taken perpendicular to a direction of current flow in the nanowire, and   a height of the cross-section of the nanowire is 0.05-0.1 microns.       

     The sensitivity of a nanowire to the different abovementioned fluid flow properties is based on the general principle that resistance of a nanowire changes with temperature. When the nanowire is cool, its resistance varies with changes in the ambient fluid temperature, thus the nanowire can be used to measure the ambient fluid temperature. When the nanowire is heated, it becomes sensitive to heat transfer from the ambient environment. Depending on the Peclet number, a dimensionless parameter that is a function of both the nanowire dimensions and flow parameters of the fluid, the heated nanowire is more sensitive to either (a) heat transfer by thermal conduction, which translates to a humidity measurement, or (b) heat transfer by convection, which translates to a velocity measurement. For heat transfer, the Peclet number Pe is defined as 
       Pe=LU/alpha,   [Equation 1]
 
     where L is the longitudinal length of the nanowire, U is the local flow velocity, and alpha is the thermal diffusivity of the fluid. Thus, a heated nanowire can be used to measure humidity or velocity of the fluid depending on the Peclet number. 
     However, a nanowire configured to measure a specific flow property of the fluid will still be affected to some degree by other flow properties of the fluid. The ability to compensate for effects caused by these other flow properties increases the accuracy of the measurement of the specific flow property being measured. For example, as described herein, the accuracy of a measurement of humidity may be increased by compensating for velocity effects on the nanowire, and the accuracy of a measurement of velocity may be increased by compensating for any temperature effects on the nanowire. 
     For some applications, real-time compensation is achieved by using at least two nanowires that are configured to measure different flow properties, such as temperature, humidity, or velocity, respectively, i.e., the resistance of each nanowire varies based on a value of a different respective flow property. The nanowires are operated in different respective modes of operation. For example, one of the nanowires may be operated with constant current anemometry (CCA) and another of the nanowires may be operated with constant temperature anemometry (CTA). A measurement obtained from one of the nanowires is used to correct a measurement obtained from another of the nanowires. For some applications, the measurements are made simultaneously in order to provide real-time compensation. For other applications, such as in a slow-changing or non-changing flow field, compensation can be performed without simultaneous measurements of the two flow properties. 
     As used in the present application, including in the claims, simultaneous measurements are to be understood as follows: when data are being measured at a measurement frequency f, simultaneous measurements are measurements taken in the same cycle, with a time interval between each measurement being less than 1/f seconds. Alternatively to simultaneously, measurements may be taken at time intervals of 5/f seconds. 
     As used in the present application, including in the claims, correcting a measurement is to be understood as increasing the accuracy of the measurement. 
     Reference is now made to  FIG. 1 , which is a schematic illustration of sensing apparatus in accordance with some applications of the present invention. For some applications, sensing apparatus  20  is provided. A power source  22  drives a sensor interface  24 . At least two nanowires  26  and  28 , e.g., held by probes  30  and  32  respectively, are coupled to sensor interface  24 . Optionally, additional nanowires  34  may be coupled to sensor interface  24 . Sensor interface  24  operates the nanowires and performs the compensation using a plurality of components, such as a micro-controller  36 , a feedback circuit  38 , a compensation circuit  40 , first nanowire-driving circuitry  42 , and/or second nanowire-driving circuitry  44 . First nanowire-driving circuitry  42  and second nanowire-driving circuitry  44  may be CCA circuitry  46  and CTA circuitry  48  respectively. Alternatively, first nanowire-driving circuitry  42  and second nanowire-driving circuitry  44  may be CTA circuitry  46  and CCA circuitry  48  respectively. 
     For some applications, sensor interface  24  performs the compensation by using an output measurement from a second one of the two nanowires, e.g., nanowire  28 , as an offset for a first one of the two nanowires, e.g., nanowire  26 , prior to measurement by first nanowire  26 . Alternatively or additionally, sensor interface  24  further includes a processing unit  50 , that uses an output measurement from second nanowire  28  to process measurement data output by first nanowire  26 . 
     For example, second nanowire  28  may be a temperature-measuring nanowire, and first nanowire  26  may be a velocity-measuring nanowire. Sensor interface  24  uses a temperature measurement obtained from the temperature-measuring nanowire to correct a velocity measurement obtained from the velocity-measuring nanowire. 
     Sensor interface  24  outputs the corrected measurements to a signal conditioner  56 , which outputs data to an analog-to-digital (A/D) converter  58 . The digital data may be stored in data storage  60 , and/or be sent as real-time feedback to a device, such as, for example, a gas meter  62 , a multicopter  64 , or a turbine  66 , further described hereinbelow. 
     Reference is now made to  FIGS. 2-4 , which are schematic illustrations of sensing apparatus  20  in accordance with some applications of the present invention. For some applications, the circuitry inside sensor interface  24  includes a Wheatstone bridge  68 . A first nanowire  70  is coupled along a first leg  72  of Wheatstone bridge  68 , and a second nanowire  74  is coupled either to Wheatstone bridge  68  or to power source  22 . Second nanowire  74  acts as a compensating nanowire, further described hereinbelow. Signal conditioner  56  sends data to output circuitry  76 , such as A/D converter  58  ( FIG. 1 ), that outputs a measurement obtained by first nanowire  70  that is corrected by a measurement obtained from second nanowire  74 . First nanowire  70  and second nanowire  74  are configured to measure different flow properties of a fluid respectively, such as temperature, humidity, and velocity as described hereinabove. It is noted that use of a Wheatstone bridge in the circuitry is provided as an example, and the scope of the present invention includes other circuitry known in the art for measuring resistance. 
       FIG. 2  shows second nanowire  74  coupled along a second leg  77  of Wheatstone bridge  68 . First nanowire  70  is configured to measure a first flow property but the resistance of first nanowire  70  may also be affected by a second flow property. Second nanowire is configured to measure the second flow property, but substantially not the first flow property. Matching circuitry  80 , coupled to second nanowire  74  is configured (a) to maintain a nominal resistance of second nanowire  74  such that the overheat ratio of Wheatstone bridge  68  stays between 1.1 and 1.5, and (b) to regulate the current flowing through second nanowire  74 . An imbalance of resistance between the two legs of Wheatstone bridge  68  is referred to as the overheat ratio. The nominal resistance of second nanowire  74  is the combined resistance of second nanowire and matching circuitry  80  at an initial time t 0  before measurements are taken. A change in the second flow property affects the resistance of both first nanowire  70  and second nanowire  74  typically to substantially the same degree, and therefore does not cause a differential voltage across Wheatstone bridge  68 , while a change in the first flow property affects the resistance of substantially only first nanowire  70 , causing a differential voltage across the Wheatstone bridge that is detected, processed, and output as a corrected measurement of the first flow property. 
     For some applications, first nanowire  70  is a humidity-measuring nanowire that is also sensitive to velocity of the fluid, and second nanowire  74  is a velocity-measuring nanowire. In this case, matching circuitry  80 , coupled to second nanowire  74 , (a) maintains the nominal resistance of the velocity-measuring nanowire such that the overheat ratio in Wheatstone bridge  68  stays between 1.1 and 1.5, and (b) regulates the current flowing through second nanowire  74  in order to tune a velocity sensitivity of second nanowire  74  to match the velocity sensitivity of first nanowire  70 . Changes in heat transfer from the fluid by convection, i.e., changes in velocity of the fluid, typically affect the resistance of both first nanowire  70  and second nanowire to substantially the same degree, whereas changes in heat transfer from the fluid by conduction, i.e., changes in humidity of the fluid, affect the resistance of substantially only first nanowire  70 . Therefore, second nanowire  74  compensates the measurement of humidity measured by first nanowire  70  for any changes in velocity of the fluid. 
     For some applications, first nanowire  70  is a humidity-measuring nanowire that is also sensitive to temperature, and second nanowire  74  is a temperature-measuring nanowire. In this case, matching circuitry  80 , coupled to second nanowire  74 , (a) maintains the nominal resistance of the temperature-measuring nanowire such that the overheat ratio in Wheatstone bridge  68  stays between 1.1 and 1.5, and (b) keeps the current flowing through second nanowire  74  low enough for it to be a temperature-measuring nanowire. A change in the ambient fluid temperature will affect the resistance of both first nanowire  70  and second nanowire  74  to substantially the same degree, whereas changes in humidity of the fluid affect the resistance of substantially only first nanowire  70 . Therefore, second nanowire  74  compensates the measurement of humidity by first nanowire  70  for any changes in ambient fluid temperature. 
     For some applications, first nanowire  70  is a velocity-measuring nanowire that is sensitive to temperature as well, and second nanowire  74  is a temperature-measuring nanowire. In this case, matching circuitry  80 , coupled to second nanowire  74 , (a) maintains the nominal resistance of the temperature-measuring nanowire such that the overheat ratio in Wheatstone bridge  68  stays between 1.1 and 1.5, and (b) keeps the current flowing through second nanowire  74  low enough for it to be a temperature-measuring nanowire. A change in the ambient fluid temperature will affect the resistance of both first nanowire  70  and second nanowire  74  to substantially the same degree, whereas changes in velocity of the fluid affect the resistance of substantially only first nanowire  70 . Therefore, second nanowire  74  compensates the measurement of velocity by first nanowire  70  for any changes in ambient fluid temperature. 
     For some applications, first nanowire  70  and second nanowire are both velocity-measuring nanowires that are sensitive to temperature as well. In this case, first nanowire  70  is disposed such that it is exposed to the velocity of the fluid, and second nanowire  74  is disposed such that it is generally shielded from the velocity of the fluid, and thus only exposed to the temperature. A change in temperature therefore affects both first nanowire  70  and second nanowire  74 , to substantially the same degree, whereas changes in velocity affect substantially only first nanowire  70 . Therefore, second nanowire  74  compensates the measurement of velocity measured by first nanowire  70  for any changes in temperature of the fluid. 
     Reference is now made to  FIGS. 5A-D , which are schematic illustrations of a velocity flow field and two nanowires in the field, in accordance with respective applications of the present invention. For some applications, such as is shown in  FIGS. 5A-B , second nanowire  74  is shielded from a velocity flow field  82  by being disposed such that an angle theta between second nanowire  74  and flow field  82  is less than 20 degrees, e.g., second nanowire may be parallel to velocity flow field  82 . For some applications, such as is shown in  FIG. 5C , second nanowire  74  is shielded from velocity flow field  82  by a porous structure  84 . Porous structure  84  prevents second nanowire  74  from being affected by the velocity of the fluid, while still allowing second nanowire to be affected by ambient fluid temperature. For some applications, second nanowire  74  may be disposed inside porous structure  84 . For some applications, such as is shown in  FIG. 5D , second nanowire  74  may be shielded from velocity flow field  82  by being disposed in a chamber  86  that is shaped to define exactly one opening  88 , having a diameter of 0.1-1 mm. Opening  88  allows ambient fluid temperature conditions to affect second nanowire  74  without facilitating a pressure differential that would cause fluid motion within chamber  86 . 
     Reference is now made to  FIG. 3 , which is a schematic illustration of sensing apparatus  20  in accordance with some applications of the present invention. For some applications, first nanowire  70  is coupled to first leg  72  of Wheatstone bridge  68 , and a second nanowire  90 , operated by its own nanowire-driving circuit  92 , e.g., matching circuitry along with either a CCA circuit or a CTA circuit, is coupled in series to power source  22 , between power source  22  and Wheatstone bridge  68 . First nanowire  70  is configured to measure a first flow property, and second nanowire  90  is typically configured to measure a second flow property. Compensation is performed by the voltage supplied by power source  22  to Wheatstone bridge  68 , and therefore to first nanowire  70 , being altered according to a resistance of second nanowire  90 , the resistance of second nanowire  90  varying based on the value of the second flow property. For some applications, second nanowire  90  may be a velocity-measuring nanowire that is shielded from the velocity of the fluid, such as described above with reference to  FIG. 2  and  FIG. 5 . 
     Reference is now made to  FIG. 4 , which is a schematic illustration of sensing apparatus  20  in accordance with some applications of the present invention. For some applications, first nanowire  70  is coupled to first leg  72  of Wheatstone bridge  68 , and a second nanowire  94  is coupled in parallel to Wheatstone bridge  68 . First nanowire  70  is configured to measure a first flow property, and second nanowire  94  is typically configured to measure a second flow property. Compensation is performed by the current supplied to Wheatstone bridge  68 , and therefore to first nanowire  70 , being altered according to a resistance of second nanowire  94 , resistance of second nanowire  94  varying based on the value of the second flow property. Matching circuitry  95  regulates the current flowing through second nanowire  94 , thereby regulating the current flowing through first nanowire  70 . The total amount of resistance in matching circuitry  95  and second nanowire  94  determines how much current flows through Wheatstone bridge  68 , and therefore through first nanowire  70 . For some applications, second nanowire  94  may be a velocity-measuring nanowire that is shielded from the velocity of the fluid, such as described above with reference to  FIG. 2  and  FIG. 5 . 
     For some applications, such as when the first nanowire is a velocity-measuring nanowire and the compensating second nanowire is a temperature-measuring nanowire, the longitudinal length of the second nanowire is 0.6-9, e.g., 1-3, times the longitudinal length of the first wire. The respective cross-sectional areas of the velocity-measuring nanowire and the compensating temperature-measuring nanowire may be similar to each other, e.g., the same as each other. Alternatively or additionally, (a) a ratio of the width to the height of the cross-section of the first nanowire is typically 0.8-1.2 times (b) a ratio of the width to the height of the cross-section of the second nanowire, the cross-sections being taken perpendicular to a direction of current flow in the nanowires. 
     For some applications, such as when the first nanowire and the compensating second nanowire are velocity-measuring nanowires (e.g., as described hereinabove with reference to  FIGS. 5A-D ), the two nanowires may be similar in size, e.g., the same size, such that a volume of the first nanowire is 0.8-1.2 times a volume of the second nanowire. 
     For some applications, such as when the first nanowire is a humidity-measuring nanowire and the compensating second nanowire is a velocity-measuring nanowire, the second nanowire is substantially larger than the first nanowire. For example, a volume of the second nanowire may be 4-1000, e.g., 15-180, times a volume of the first nanowire. 
     Reference is now made to  FIG. 6 , which is a schematic illustration of sensing apparatus in accordance with some applications of the present invention. For some applications, sensing apparatus  96  is provided. In sensing apparatus  96  a single nanowire  98  is used to measure different flow properties by alternating nanowire  98  between at least two different modes of operation (which is unlike sensing apparatus  20  described hereinabove in which two nanowires are used, each being operated in a different respective mode of operation). Nanowire  98  is configured such that the resistance of nanowire  98  varies based on values of different flow properties when nanowire  98  is operated in different modes of operation respectively. When nanowire  98  is alternated between two different modes of operation, two different flow properties are measured. The measurements are output to signal conditioner  56 , and a compensation unit  100  (e.g., a compensation program or compensation circuitry) processes the output and uses the measurements of one of the flow properties to correct the measurements of the other flow property. Compensation unit  100  outputs corrected data  102  as well as compensating data  104 . Connection  101  between alternating current  110  and compensation unit  100  synchronizes compensation unit  100  with alternating current  110 . 
     For some applications, nanowire  98  is coupled to a leg  105  of Wheatstone bridge  106  in a CCA circuit  108  that alternates between driving nanowire  98  with a low current and a high current (e.g., between zero and a positive value). When low current is used, nanowire  98  remains cool and is sensitive to ambient temperature, thus measuring the temperature of the fluid. When high current is used, nanowire is heated and is sensitive to heat transfer from the surrounding environment, thus measuring the velocity of the fluid. For some applications, power source  22  alternates between the low and high current by using an alternating current  110  in the form of a square wave (such as is shown in  FIG. 6 ), a triangle wave, or a sinusoidal wave. When using alternating current  110  to drive nanowire  98 , the measurement frequency, at which the two different flow properties can be measured is typically less than half of the frequency of alternating current  110 . For some applications, power source  22  drives nanowire  98  with an alternating current having a frequency of 1-2000 Hz, e.g., 1-100 Hz or 100-2000 Hz. 
     A nanowire operating at low current has a lower frequency response than when operating at a higher current. Therefore, the frequency response of nanowire  98  is limited by the value of the current in the low state of alternating current  110 . In order increase the frequency response, alternating current  110  can be configured to alternate between two high currents that both heat nanowire  98 , but to different degrees. In this case, instead of nanowire  98  being cool during a low state  111  of alternating current  110  and heated during a high state  113  state of alternating current  110 , nanowire  98  is in a lower heated condition during low state  111  and a higher heated condition during high state  113 . For example, nanowire  98  may be heated to 200 degrees Celsius during low state  111  of alternating current  110 , and to 300 degrees Celsius during high state  113  of alternating current  110 . In the higher heated condition, nanowire  98  is more sensitive to velocity and less sensitive to temperature, and in the lower heated condition, nanowire  98  is less sensitive to velocity and more sensitive to temperature. Both the temperature and velocity can be solved for by having both measurements and the wire calibration curves for both operating modes. 
     Reference is now made to  FIGS. 7-8 , which are schematic illustrations of sensing apparatus, such as sensing apparatus  96 , in accordance with some applications of the present invention. 
     For some applications, a switching circuit  114  is used to alternate nanowire  98  between two different modes of operation. For example, switching circuit  114  may be used to switch between operating nanowire  98  with CCA circuit  116  and operating nanowire  98  with CTA circuit  118 . A compensation unit  120  (e.g., a compensation program or compensation circuitry) processes data output by nanowire  98  after the different flow properties have been measured, and outputs corrected data  121 . For some applications, switching circuit  114  switches between operating the nanowire using two different CCA circuits  122  and  124 , such as is shown in  FIG. 8 , that drive the nanowire with different respective currents. For example, first CCA circuit  122  drives the nanowire with a current of 5-50 mA and second CCA circuit  124  drives the nanowire with a current of 0.1-1 mA. Alternatively or additionally, first CCA circuit  122  drives the nanowire with a current which is 15-150 times the current driven by second CCA circuit  124  through the nanowire. 
     Various different practical applications may benefit from sensing apparatus such as the sensing apparatus described hereinabove. Some examples include, but are not limited to, drones, such as multicopters or fixed wing drones, aircraft in general, gas meters, and turbines. 
     Reference is now made to  FIGS. 9A-B , which are schematic illustrations of sensing apparatus, such as sensing apparatus  20 , coupled to a multicopter and a fixed-wing drone. For example, sensing apparatus  20  may be coupled to a nose or blade of multicopter  64  or a nose, wing, or tail of a fixed-wing drone  65 . Sensing apparatus  20  then provides real-time feedback of different properties of the airflow during flight. It is noted that sensing apparatus where a single nanowire  98  is used to measure different flow properties, such as sensing apparatus  96 , may be used for this application as well. 
     Reference is now made to  FIG. 10 , which is a schematic illustration of sensing apparatus, such as sensing apparatus  20 , disposed within gas meter  62 . Sensor interface  24  may be disposed inside gas meter  62 , and coupled to control circuitry  112 , which includes the operating components, such as the power source, CCA/CTA circuits, signal conditioner, compensation unit, and output circuitry. It is noted that sensing apparatus where a single nanowire  98  is used to measure different flow properties, such as sensing apparatus  96 , may be used for this application as well. 
     Reference is now made to  FIG. 11 , which is a schematic illustration of sensing apparatus, such as sensing apparatus  20 , disposed within a turbine. Sensing apparatus  20  may provide real-time feedback of flow properties within turbine  66 . It is noted that sensing apparatus where a single nanowire  98  is used to measure different flow properties, such as sensing apparatus  96 , may be used for this application as well. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.