Abstract:
A method and a sensing instrument are claimed for sensing atmospheric pressure. The instrument (24) comprises an enclosure (26) that has a shape exposed to a fluid medium, which fluid medium may be moving at a certain velocity relative to the sensing instrument. The sensing instrument has at least one port (28) open to the fluid medium for sensing the velocity of the medium and for measuring a pressure of the fluid medium. The measured pressure is corrected for the effects of the velocity of the medium according to the formula 
     
       P.sub.ATM =P.sub.M +K.sub.2 V.sup.2 
     
     where P ATM  is atmospheric pressure, P M  is measured pressure in the enclosure, K 2  is a constant that is a function of the shape of the enclosure, the shape of the port and the density of the medium and V is the velocity of the medium.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to barometers and specifically to a method and apparatus for correcting a sensed barometric pressure for errors caused by wind velocity and direction. 
     2. Prior Art 
     In the past, solutions to account for the recognized errors caused by wind velocity and direction in the barometric pressure sensed in an enclosure with ports open to the atmosphere have been less than satisfactory. Principally these solutions have revolved around providing mechanical baffles to dissipate the kinetic energy of the wind, leaving only the static or barometric pressure to be sensed in the enclosure. Such a device is described in the paper entitled &#34;Design and Testing of Barometer Inlets to Eliminate Wind Velocity Error in NOAA Data Buoy Installations&#34; by Ronald T. Miles, John F. Holmes and Joseph H. Greer and presented at the Instrument Society of America Conference at Niagara Falls, New York on Oct. 16-20, 1977. 
     The method of an apparatus made according to the present invention utilizes a definable relationship between the wind velocity and direction and the physical characteristics of the enclosure and the ports therein to provide a factor that is applied to the pressure that is measured in the enclosure to obtain an accurate static or barometric pressure. Thus the present invention eliminates the need for a baffle system to account for the effects of the kinetic energy of the wind. 
     SUMMARY OF THE INVENTION 
     A method and a sensing instrument are claimed for sensing atmospheric pressure. The instrument comprises an enclosure that has a shape exposed to a fluid medium, which fluid medium may be moving at a certain velocity relative to the sensing instrument. The sensing instrument has at least one port open to the fluid medium for sensing the velocity of the medium and for measuring a pressure of the fluid medium. The measured pressure is corrected for the effects of the velocity of the medium according to the formula 
     
         P.sub.ATM =P.sub.M +K.sub.2 V.sup.2 
    
     where P ATM  is atmospheric pressure, P M  is measured pressure in the enclosure means, K 2  is a constant that is a function of the shape of the enclosure means, the shape of the port means and the density of the medium and V is the velocity of the medium. 
     In another embodiment, the sensing instrument for sensing atmospheric pressure has a single set of ports open to the fluid medium for sensing a pressure of the fluid medium. The measured pressure is corrected for the effects of the velocity of the medium according to the formula 
     
         P.sub.ATM =P.sub.M +K.sub.3 V.sup.2 f(θ) 
    
     where P ATM  is atmospheric pressure, P M  is measured pressure in the enclosure, K 3  is a calibration constant, V is velocity of the medium and f(θ) is a variable that is a function of the shape of the enclosure and the shape of the ports and the direction of movement of the fluid medium, where θ is the direction of the movement of the medium relative to the port. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 top view of a prior art device. 
     FIG. 2 sectioned view of a prior art device taken along line 2--2 in FIG. 1. 
     FIG. 3 graph of typical static pressure variations around cylindrical enclosures made according to the present invention and in cross flow. 
     FIG. 4 side view of a 12 port cylindrical sensor. 
     FIG. 5 side view of a cylindrical sensor having a single set of static pressure sensing ports. 
     FIG. 6 is a sectional view taken along line 6--6 in FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Atmospheric or barometric pressure is the undisturbed or static pressure at the point of measurement. It is known that measurement of such pressure inside an enclosure that has ports open to the atmosphere may be subject to errors caused by wind velocity and direction. 
     FIG. 1 shows prior art device 10 which was designed to reduce the error introduced by the wind to the barometric pressure measurement. Enclosure 12 comprises a conduit capped on the end shown. Around enclosure 12 is an elaborate baffle comprised of layers of screen 14 wound concentrically around enclosure 12. The object is that wind, as shown for example by arrow 16 will expend its kinetic energy as it passes through the successive layers of screen such that only the atmospheric static pressure remains to be sensed at enclosure 12. This is more easily visualized in FIG. 2. The numbers in FIG. 2 correspond to those of FIG. 1. FIG. 2 shows enclosure 12 capped at the top and open at the thread bottom where measured pressure is extracted. Such pressure is admitted to enclosure 12 through ports 18. Here it can be seen that wind 16 may be impeded by the layers of screen 14 such that its kinetic energy may be dissipated prior to the pressure pick off at ports 18. It was found that wind 16 did not completely penetrate at the point of impact, but that some of such wind tended to flow around screen 14. This created a negative pressure on the far side of screen 16 which in turn induced an error into the pressure measured in enclosure 12. Accuracy was further degraded in inclement weather. In conditions of rain, a meniscus formed on screen 14 that effectively shielded enclosure 12 from atmospheric pressure. Screen 14 was additionally subject to severe icing problems. 
     Given an enclosure, such as the one shown in FIG. 4, having a plurality of ports placed equiangularly around the circumference of the enclosure such that the wind is able to substantially impinge directly upon a port regardless of wind direction, it has been found that the error (P M  -P ATM ) due to the wind is proportional to the impact pressure of the wind, q c . The measured pressure inside the enclosure is P M  and P ATM  is true atmospheric pressure. It has been further determined that the error coefficient, ##EQU1## is substantially a constant, K 1 , regardless of the velocity of the wind; accordingly the relationship yields the equation ##EQU2## for all wind velocities. K 1  for a sensor of the type illustrated in FIG. 4 is shown by line 20 on the graph shown in FIG. 3. FIG. 3 shows a plot of the constant K 1 , which equates to ##EQU3## plotted against flow direction angle, θ in degrees. In this graph, the sensor is an elongated cylinder of the type shown in FIGS. 4 and 5. Flow direction θ defines a plane normal to the longitudinal axis of the sensor and varying through directions represented by 0° to 360° that are conventional to compass card notation. It has been shown that for cylindrical sensors having a plurality of ports equiangularly spaced around the circumference of the sensor, that K 1  is negative and has substantially the same value regardless of the direction of flow. The twelve port sensor tested had a K 1  =-0.55. The value of K 1  for sensor having other configurations is dependent largely on the shape of the sensor and the number and shape of the ports. 
     Knowing K 1 , the sensor may be utilized to accurately obtain P ATM  by solving the above relationship for P ATM . The relationship ##EQU4## yields P ATM  =P M  +K 1  q c . Throughout the wind velocity range, impact pressure is q c  =1/2 ρV 2 , where ρ is the density of the fluid and V is the fluid flow velocity. Substituting this for q c  and establishing K 2  =1/2 K 1 , yields the equation P ATM  =P M  +K 2  ρV 2 . This can be further simplified to P ATM  =P M  +K 3  V 2 . This final equation assumes that ρ=ρ s1 , which is standard sea level density, a constant value. K 3  then equals K 2  ρ s1 . Wind velocity, measured by any convenient anemometer, is required to complete the equation to determine atmospheric pressure from the sensor. 
     Sensor 24, shown in FIG. 4 is a device the performance of which can be described by the above equation. Sensor 24 has an elongated, cylindrical housing 26. Housing 26 is comprised of tubing having a central chamber. A plurality of ports 28, in this case twelve, are equiangularly disposed around the circumference of housing 26. Ports 28 pass through the wall of housing 26 and connect the central chamber to the environment around sensor 28. 
     Housing 26 is mounted in base 30. Base 30 may be mounted to a supporting structure as desired. Tube 32 passes through base 30 and is connected to the central chamber of housing 26. The signal P M  is obtained from tube 32. 
     P M  is supplied to transducer 34, shown schematically. Transducer 34 changes the pressure signal to an electrical signal that is a function of P M . Such electrical signal is supplied to computer 36 along with an electrical signal that is a function of flow velocity V. Such signal may be obtained from a convenient anemometer. The value of K 3  for sensor 26 is stored in arithmetic unit 36. Arithmetic unit 36 is mechanized to perform the function represented by P M  +K 3  V 2  to yield an electrical signal output that is a function of P ATM . 
     The constant K 3  is a function of the shape of enclosure 26 and ports 28 of sensor 24 as shown in FIG. 4. Each sensor has a unique constant K 3 . Such constant may be determined under known conditions of P ATM , P M  and V. Testing to determine K 3  may be done, for example, in a wind tunnel where P ATM  and V are known and P M  is the measured pressure in the sensor at the known P ATM  and V. K 3  then equals ##EQU5## 
     A further embodiment of a sensor that lends itself to use with a constant as determined by the above method is shown in FIGS. 5 and 6. Such a sensor is as made substantially according to the invention disclosed in U.S. Pat. No. 3,646,811 held by the same assignee as the present application and which is incorporated herein by reference. The &#39;811 sensor is primarily a sensor of fluid velocity. Fluid velocity may be airspeed which comprises wind velocity or air vehicle velocity or both. The sensor shown in FIGS. 5 and 6 is improved to also determine atmospheric pressure. The method of determining such velocity is fully explained in the &#39;811 patent. Briefly, sensor 40 is comprised of housing 42 mounted in base 44. Housing 42 is an elongated tube capped at the end. The interior of housing 42 is divided by walls 43 and 45 into two chamber pairs as shown in FIG. 6. The chambers are numbered 46, 48, 50 and 52. Chambers 46 and 50 lie substantially in the X plane and comprise an oppositely facing chamber pair and chambers 48 and 52 lie substantially in the Y plane and comprise an oppositely facing chamber pair. Thus, there are two longitudinally extending chambers 46 and 50, which are diametrically opposed or on opposite sides of the quadrants defined by walls 43 and 45, and similar chambers 48 and 52, which are also diametrically opposed. 
     Each chamber has pressure ports that pass through housing 42 and connect the respective chamber to the environment around sensor 40. Ports 54 enter chamber 46, ports 56 enter chamber 48, ports 58 enter chamber 50 and ports 60 enter chamber 52. Ports 54 face generally in opposite direction from ports 58 and ports 56 face generally in opposite direction from ports 60. Tubes 46A, 48A, 50A, and 52A, shown in both FIGS. 5 and 6, convey measured pressure signals from chambers 46, 48, 50 and 52 respectively. 
     As disclosed in the &#39;811 patent, it has been determined that fluid velocity is determined according to the relationship V=C(ΔP 46 ,50 +ΔP 48 ,52)1/2, where V equals fluid velocity, C is a calibration constant, ΔP 46 ,50 is the pressure differential between chambers 46 and 50 and ΔP 48 ,52 is the pressure differential between chambers 48 and 52. Accordingly, the pressure signals from chambers 46 and 50 are provided to differential pressure transducer 62. The output of transducer 62 is an electrical signal that is a function of the pressure differential between chambers 46 and 50. Such electrical signal is also fed to arithmetic unit 64. The pressure signal from chambers 48 and 52 are provided to differential pressure transducer 66. The output of transducer 66 in an electrical signal that is a function of the pressure differential between chambers 48 and 52. Such electrical signal is fed to arithmetic unit 64. Arithmetic unit 64 performs the arithmetic functions necessary to generate an electrical signal that is a function of fluid velocity in accordance with the aforementioned relationship. 
     Fluid vector 71 is shown in FIG. 6. The direction of flow relative to the X axis is shown by the angle θ. It has been disclosed in the &#39;811 patent that θ may be determined by the relationship Cos  2  θ= ##EQU6## Arithmetic unit 64 performs the necessary arithmetic functions to determine flow direction, θ, as a function of the aformentioned relationship. 
     It has been determined that each of the four pressure determined above is dependent upon flow direction and magnitude. When the pressure sensed in a chamber, in the following example chamber 46, is utilized to represent atmospheric or barometric pressure, this dependence is represented by the relationship P ATM  =P M  +K 4  V 2  f(θ), where P ATM  is atmospheric or barometric pressure, P M  is the measured pressure in the chamber 46, K 4  is a calibration constant, V is fluid velocity and f(θ) is a variable that is dependent on wind direction but is independent of wind velocity. The f(θ) for the specific device shown in FIGS. 5 and 6 is shown as curve 22 on FIG. 3. This function is mechanized in arithmetic unit 64. Where arithmetic unit 64 comprises a programmable device, the programming of arithmetic unit 64 is a standard operation well within the skill of the ordinary programmer. 
     To utilize this characteristic to obtain atmospheric or barometric pressure, tube 70 is connected to one of the four tubes 46A, 48A, 50A, and 52A. In the embodiment shown, tube 70 is connected to tube 46A and receives the measured pressure signal from chamber 46. Such pressure comprises P M , the measured pressure and is the same pressure as the pressure conveyed by tube 46A. Such pressure is supplied to pressure tranducer 72. Pressure transducer 72 converts such pressure to an electrical signal that is a function of P M , the pressure sensed in chamber 46. Such electrical signal is fed to arithmetic unit 64. Arithmetic unit 64 performs the necessary arithmetic functions to determine P ATM  =P M  +K 4  V 2  f(θ). The values for V and θ being determined as indicated in the aforementioned description of the means of determining fluid flow velocity V and direction θ. 
     Where sensor 40 is utilized at a fixed ground site, it is convenient to orient the X axis of the sensor 40 and chamber 46 in a north direction so that wind direction, θ, is related to north. Where sensor 40 is utilized on a moveable platform, such as an air vehicle, it is convenient to orient the X axis of sensor 40 and chamber 46 to face forward so that wind direction, θ, is related the nose of the air vehicle. 
     The physical changes to the device of the &#39;811 patent are not complex. Nonetheless, the improvement substantially increases the utility of the device. No additional pressure sensing ports are needed in the &#39;811 device in order to obtain an accurate atmospheric or barometric pressure, thus eliminating the need for two sensors in applications requiring both wind and atmospheric pressure data. This can all be accomplished simply by adding a T to pressure line without modification of the sensor portion of the &#39;811 device. Further, as described in the &#39;811 patent, the pressure sensing ports are deiced, thereby avoiding one of the limitations of the prior art. Accordingly, it is believed that the present invention represents a substantial advancement of the art, satisfying the requirements for patentability.