Method and apparatus for measuring gas flow speed

A method and apparatus for measuring the speed of a gas stream, particula suitable for use in detecting wind speed, utilizes the gas flow to create a negative pressure across a pressure sampling port oriented with its axis substantially perpendicular to the gas flow, the negative pressure being created by accelerating the gas flow stream lines over a surface of revolution containing the port. The negative pressure is compared with a reference pressure by means of a pressure transducer whereby the transducer output is representative of the speed of the gas. In a preferred form, the apparatus comprises a first pressure probe in the form of a sphere mounted on a tube projecting axially from a hollow cylindrical cup-like member with the sampling port formed in the base of the sphere and the cup-like member forming a second pressure probe with a pressure tapping internally of the cup providing the source of reference pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The invention relates to a method and apparatus for measuring gas flow 
speed by direct pressure measurement and finds particular application as a 
wind speed threshold detecting means. 
There is a need for the development of surface sensors for use in 
preventing wind-shear related accidents particularly at airports and it is 
for this application that the development of a gas speed measuring means 
in accordance with the present invention is particularly, though not 
exclusively, directed. In this respect, there is a need for surface 
sensors of high reliability that can be obtained at reasonable cost for 
use in large numbers. Further, it is desirable to employ wind speed 
detectors which will in the ideal, measure wind speed irrespective of its 
direction and which will be relatively insensitive to tilt or wind angle. 
Thus, the present invention is directed primarily to the provision of 
apparatus which is concerned with the measurement of gas flow speed alone 
rather than its direction of flow and which is substantially insensitive 
to tilt. 
Alternative applications for measuring apparatus in accordance with the 
invention include, for example, the detection of thunderstorm gust fronts, 
down-slope wind studies, wind caution signs for roads and bridges as well 
as wind machine protection and site surveys. 
2. Prior Art: 
It is a conventional expedient when measuring gas flow velocity, to measure 
the flow pressure by employing pressure measuring probes in the form of 
pitot/static tubes. In such arrangements, the tubes are generally aligned 
to sense the pressures associated with a gas stream flowing in a certain 
direction relative to the tube entrances and such devices have little 
sensitivity to variations in flow speed in other directions. This is 
acceptable in many pitot tube applications when measurement of 
substantially unidirectional streams is required, for example in aircraft 
where air speed or head wind or tail wind speed is measured. 
Devices have, however, been proposed having sensitivity to changes in 
direction as well as speed of a gas stream. For example, U.S. Pat. No. 
2,789,433 to Goudy, Issued Apr. 23, 1957, specifically discloses apparatus 
for measuring wind speed and direction and employs a substantially flat 
disc-like pitot head incorporating a number of pitot tubes, the entrances 
to which are spaced around the periphery of the head whereby each 
individual tube is particularly sensitive to gas flow in a particular 
direction. Opposed pairs of the tubes are grouped together to influence 
separate pressure transducers in the form of diaphragm switches and the 
outputs of the transducers are used to activate a comparison circuit, 
which dependent on the transducer outputs produces a field directly 
representative of the wind direction and strength. The device is 
relatively complex in its nature, however, and since the pitot tube 
inlets, which are the only source of pressure signals, are all spaced 
around the periphery of the disc-like head, would have little sensitivity 
to wind flowing in planes other than those generally parallel to the plane 
of the disc. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the invention to provide apparatus for use in measuring 
the speed of a gas stream, more particularly a wind speed detector, which 
is capable of measuring the speed of flow irrespective of its direction, 
while having a low sensitivity to the angle of the wind relative to the 
horizontal. 
It is a further object of the invention to provide apparatus for measuring 
wind speed which is simple and economical to manufacture and which can be 
used in diverse applications. 
The invention is based on the principle of utilizing a pressure sensing 
probe to create and register a negative pressure when exposed to a flow of 
gas, this pressure varying as a function of the speed of the gas, 
comparing the negative pressure with a reference pressure, which itself 
may vary with the speed of gas flow, and producing an output dependent on 
the difference between the compared pressures, the output being 
representative of the gas flow speed. To obtain the negative pressure, 
when the device is exposed to a gas stream, the probe is formed as a body 
of revolution with a pressure sampling port centered on the axis of 
revolution of the body and the body being used so that when it is exposed 
to a gas stream flowing in planes substantially perpendicular to the axis 
of revolution and the axis of the sampling port, the flow stream lines 
will be accelerated over the body thereby causing a negative pressure to 
be registered at the sampling port dependent on the speed of flow of the 
gas stream. By arranging the sampling port on the axis of revolution, the 
probe is rendered substantially omnidirectional insofar as it is 
independent of the general direction of flow of the stream within planes 
substantially perpendicular to the axis of revolution. 
Negative pressure signals from the probe are received by a pressure 
transducer of any conventional form which compares the negative pressure 
signals with a reference pressure conveniently obtained from a second 
pressure sensing probe having a different configuration from the first 
probe so as to register a different pressure from that registered by the 
first probe when the apparatus is exposed to a gas flow of a particular 
speed. The transducer output is thereby dependent on the difference 
between the probe pressures and is a function of the gas velocity. 
It is necessary that the second probe also be omnidirectional in its 
response, i.e., insensitive to changes in gas flow direction in planes 
substantially perpendicular to the axis of the body of revolution. To this 
end, the second probe is conveniently in the form of a cylindrical 
cup-like member having an internal pressure sampling source and the body 
of revolution forming the first probe, and preferably being in the form of 
a sphere, is mounted on a tube projecting substantially axially from the 
cup-like member through its open end with the sampling port in said body 
of revolution facing away from the cup-like member and being disposed 
substantially on the axis of the cup-like member. With this arrangement, 
the internal pressure sampling source registers a pressure which is less 
negative than that registers by the sampling port and which is 
substantially static gas flow pressure when the apparatus is exposed to a 
gas stream flowing in planes substantially perpendicular to the axis of 
the cup-like member. 
A device of the above character, when used as a wind speed detector is 
preferably mounted in a suspended inverted condition on a substantially 
vertical axis, with the body of revolution projecting below the cup-like 
member. In this state, the device is capable of accurately measuring the 
speed of winds in horizontal and near horizontal planes irrespective of 
the wind direction but becomes less accurate as the wind angle approaches 
the vertical tending to equalize the induced probe pressures.

DESCRIPTION OF PREFERRED EMBODIMENT 
The illustrated apparatus consists of a sphere 10 forming a first 
omnidirectional pressure sensing probe, a cylindrical, cup-like member 12 
forming a second omnidirectional pressure sensing probe and a pressure 
transducer 16 for receiving pressure signals from the respective probes 
and producing an output dependent on the difference between such signals. 
The sphere 10 is attached to the end of the tube 18 having its other end 
secured internally to the base of cylinder 12, the tube projecting from 
the open end of the cylinder and being coaxial therewith. A pressure 
sampling port 20 is formed on the axis of the sphere facing away from the 
cylinder, the port communicating through the tube with the transducer 16 
via a connection 22, to provide one pressure input source for the 
transducer. The transducer also receives pressure input signals from the 
interior of the cylinder via a connection 24 and has output connections 
26, 28 by which the transducer output, representative of the difference 
between the input pressures, can be fed to any suitable readout or display 
device of conventional form. The transducer itself can likewise be of any 
known form, mechanical, electrical or otherwise which operates to provide 
an output which is dependent upon the difference in pressure signals 
applied thereto. Further, while the transducer is illustrated as being 
located physically within the cylinder 12, this is not essential and the 
pressure signals from within cylinder 12 and from the port 20 can equally 
well be transmitted to a remotely located transducer. 
Suitable dimensions for a device as described, which can be made of metal 
or other materials, are for example, cylinder diameter 6 inches, cylinder 
depth 4 inches, overall tube length 7 inches sphere diameter 1 inch, 
sphere sampling port diameter 5/16 inches. Larger diameter sampling ports 
(e.g., over about 1.7 cms) will assist in deterring water collection and 
ice formation at the port. 
In use as a wind speed detector, the device is preferably mounted in an 
inverted condition on a substantially vertical axis. Winds flowing in 
generally horizontal planes, when encountering the sphere will have the 
stream lines compressed and thereby accelerated over the sphere surface 
whereby the sampling port 20 registers a negative pressure dependent on 
the wind speed. The pressure within the cylinder as detected by the 
connection 24 is less negative than that detected by port 20 at any given 
wind speed due to the shrouding effect of the cylinder and the device is 
substantially insensitive to changes in direction of substantially 
horizontal winds due to the symmetrical nature of the cylinder and sphere. 
As the general angle of the wind approaches the vertical, the two pressure 
readings tend to converge making the device less accurate in such 
conditions but it has been found that this effect is minimized by having 
the tube 18 and sphere 10 mounted coaxially with respect to the cylinder. 
Generally the wind speed is deduced from the equation 
EQU U=K (2.DELTA.P/.rho.)1/2 
where U is the horizontal wind speed, .rho. is the air density, .DELTA.P is 
the pressure difference between the two probes and K is a calibration 
constant. 
It will be appreciated that numerous modifications can be made within the 
scope of the invention as defined in the appended claims. For example, the 
probe port 20 can be screened internally to prevent ingress of insects, 
dust particles and the like and similar screening can be provided across 
the cup opening. Such screening increases the stability of the device and 
stability can also be improved by the provision of small corrugations 
about the rim of the cup. Further, sphere 10 can be replaced by another 
body of revolution such as a hemisphere with its hemispherical surface 
facing away from the cylinder and the cylindrical probe 12 can itself take 
other forms provided it is adapted to register pressures different from 
those registered by probe 10 at the same gas flow speeds and provided also 
that the probe is omnidirectional insofar as being substantially 
insensitive to change in gas flow direction in planes substantially 
perpendicular to the axis of probe 10. The device as illustrated however 
due to its simplicity in structure, provides a particularly economical and 
effective gas speed measuring device rendering it suitable for use either 
singly or in an array of such devices as a wind speed detector in airport 
or like applications.