An anemometer or windmeter is a device used for measuring wind speed, and is a common
weather station instrument. The term is derived from the Greek word anemos, meaning wind,
and is used to describe any air speed measurement instrument used in meteorology or aerodynamics.
The first known description of an anemometer was given by Leon Battista Alberti around
1450. Anemometers can be divided into two classes:
those that measure the wind's speed, and those that measure the wind's pressure; but as there
is a close connection between the pressure and the speed, an anemometer designed for
one will give information about both.
History The anemometer has changed little since its
development in the 15th century. Leon Battista Alberti is said to have invented the first
mechanical anemometer around 1450. In following centuries, numerous others, including Robert
Hooke and the Mayans, developed their own versions, with some being mistakenly credited
as the inventor. In 1846, John Thomas Romney Robinson improved upon the design by using
four hemispherical cups and mechanical wheels. Later, in 1926, John Patterson developed a
three cup anemometer, which was improved by Brevoort and Joiner in 1935. In 1991, Derek
Weston added the ability to detect wind direction. Most recently, in 1994, Dr. Andrews Pflitsch
developed the sonic anemometer. Types of anemometers
Velocity anemometers Cup anemometers
A simple type of anemometer was invented in 1845 by Dr. John Thomas Romney Robinson, of
Armagh Observatory. It consisted of four hemispherical cups, each mounted on one end of four horizontal
arms, which in turn were mounted at equal angles to each other on a vertical shaft.
The air flow past the cups in any horizontal direction turned the shaft in a manner that
was proportional to the wind speed. Therefore, counting the turns of the shaft over a set
time period produced the average wind speed for a wide range of speeds. On an anemometer
with four cups, it is easy to see that since the cups are arranged symmetrically on the
end of the arms, the wind always has the hollow of one cup presented to it and is blowing
on the back of the cup on the opposite end of the cross.
When Robinson first designed his anemometer, he asserted that the cups moved one-third
of the speed of the wind, unaffected by the cup size or arm length. This was apparently
confirmed by some early independent experiments, but it was incorrect. Instead, the ratio of
the speed of the wind and that of the cups, the anemometer factor, depends on the dimensions
of the cups and arms, and may have a value between two and a little over three. Every
previous experiment involving an anemometer had to be repeated.
The three cup anemometer developed by the Canadian John Patterson in 1926 and subsequent
cup improvements by Brevoort & Joiner of the USA in 1935 led to a cupwheel design which
was linear and had an error of less than 3% up to 60 mph. Patterson found that each cup
produced maximum torque when it was at 45 degrees to the wind flow. The three cup anemometer
also had a more constant torque and responded more quickly to gusts than the four cup anemometer.
The three cup anemometer was further modified by the Australian Derek Weston in 1991 to
measure both wind direction and wind speed. Weston added a tag to one cup, which causes
the cupwheel speed to increase and decrease as the tag moves alternately with and against
the wind. Wind direction is calculated from these cyclical changes in cupwheel speed,
while wind speed is as usual determined from the average cupwheel speed.
Three cup anemometers are currently used as the industry standard for wind resource assessment
studies. Vane anemometers
One of the other forms of mechanical velocity anemometer is the vane anemometer. It may
be described as a windmill or a propeller anemometer. Contrary to the Robinson anemometer,
where the axis of rotation is vertical, the axis on the vane anemometer must be parallel
to the direction of the wind and therefore horizontal. Furthermore, since the wind varies
in direction and the axis has to follow its changes, a wind vane or some other contrivance
to fulfill the same purpose must be employed. An vane anemometer thus combines a propeller
and a tail on the same axis to obtain accurate and precise wind speed and direction measurements
from the same instrument. The speed of the fan is measured by a rev counter and converted
to a windspeed by an electronic chip. Hence, volumetric flowrate may be calculated if the
cross-sectional area is known. In cases where the direction of the air motion
is always the same, as in the ventilating shafts of mines and buildings for instance,
wind vanes, known as air meters are employed, and give most satisfactory results.
Hot-wire anemometers
Hot wire anemometers use a very fine wire electrically heated up to some temperature
above the ambient. Air flowing past the wire has a cooling effect on the wire. As the electrical
resistance of most metals is dependent upon the temperature of the metal, a relationship
can be obtained between the resistance of the wire and the flow speed.
Several ways of implementing this exist, and hot-wire devices can be further classified
as CCA, CVA and CTA. The voltage output from these anemometers is thus the result of some
sort of circuit within the device trying to maintain the specific variable constant, following
Ohm's law. Additionally, PWM anemometers are also used,
wherein the velocity is inferred by the time length of a repeating pulse of current that
brings the wire up to a specified resistance and then stops until a threshold "floor" is
reached, at which time the pulse is sent again. Hot-wire anemometers, while extremely delicate,
have extremely high frequency-response and fine spatial resolution compared to other
measurement methods, and as such are almost universally employed for the detailed study
of turbulent flows, or any flow in which rapid velocity fluctuations are of interest.
An industrial version of the fine-wire anemometer is the thermal flow meter, which follows the
same concept but uses two pins or stings to monitor the variation in temperature. The
stings contain fine wires, but encasing the wires makes them much more durable and capable
of accurately measuring air, gas, and emissions flow in pipes, ducts, and stacks. Industrial
applications often contain dirt that will damage the classic hot-wire anemometer.
Laser Doppler anemometers In laser Doppler velocimetry, laser Doppler
anemometers use a beam of light from a laser that is divided into two beams, with one propagated
out of the anemometer. Particulates flowing along with air molecules near where the beam
exits reflect, or backscatter, the light back into a detector, where it is measured relative
to the original laser beam. When the particles are in great motion, they produce a Doppler
shift for measuring wind speed in the laser light, which is used to calculate the speed
of the particles, and therefore the air around the anemometer.
Sonic anemometers
Sonic anemometers, first developed in the 1950s, use ultrasonic sound waves to measure
wind velocity. They measure wind speed based on the time of flight of sonic pulses between
pairs of transducers. Measurements from pairs of transducers can be combined to yield a
measurement of velocity in 1-, 2-, or 3-dimensional flow. The spatial resolution is given by the
path length between transducers, which is typically 10 to 20 cm. Sonic anemometers can
take measurements with very fine temporal resolution, 20 Hz or better, which makes them
well suited for turbulence measurements. The lack of moving parts makes them appropriate
for long-term use in exposed automated weather stations and weather buoys where the accuracy
and reliability of traditional cup-and-vane anemometers is adversely affected by salty
air or large amounts of dust. Their main disadvantage is the distortion of the flow itself by the
structure supporting the transducers, which requires a correction based upon wind tunnel
measurements to minimize the effect. An international standard for this process, ISO 16622 Meteorology—Sonic
anemometers/thermometers—Acceptance test methods for mean wind measurements is in general
circulation. Another disadvantage is lower accuracy due to precipitation, where rain
drops may vary the speed of sound. Since the speed of sound varies with temperature,
and is virtually stable with pressure change, sonic anemometers are also used as thermometers.
Two-dimensional sonic anemometers are used in applications such as weather stations,
ship navigation, wind turbines, aviation and weather buoys. Three-dimensional sonic anemometers
are widely used to measure gas emissions and ecosystem fluxes using the eddy covariance
method when used with fast-response infrared gas analyzers or laser-based analyzers.
There are different technologies solution for Two-dimensional wind sensor.
Two ultrasounds paths: These sensors have 4 arms. These sensor are commonly used by
the wind sensor manufactured because it's easier to develop compare to the three ultrasounds
paths. The disadvantage of this type of sensor is that when the wind comes in the direction
of an ultrasound path the flow around the arms introduces aerodynamic turbulences which
perturbs the measures and the sensor will lose in accuracy.
Three ultrasounds paths: These sensors have 3 arms. These sensors are developed by some
wind sensors manufactured because it gives one path redundancy of the measure which improve
the sensor accuracy and minimize the aerodynamic turbulences.
Acoustic resonance anemometers
Acoustic resonance anemometers are a more recent variant of sonic anemometer. The technology
was invented by Dr Savvas Kapartis and patented by FT Technologies in 2000. Whereas conventional
sonic anemometers rely on time of flight measurement, acoustic resonance sensors use resonating
acoustic waves within a small purpose built cavity in order to perform their measurement.
Built into the cavity is an array of ultrasonic transducers, which are used to create the
separate standing-wave patterns at ultrasonic frequencies. As wind passes through the cavity,
a change in the wave’s property occurs. By measuring the amount of phase shift in
the received signals by each transducer, and then by mathematically processing the data,
the sensor is able to provide an accurate horizontal measurement of wind speed and direction.
Acoustic resonance technology enables measurement within a small cavity, the sensors therefore
tend to be typically smaller in size than other ultrasonic sensors. The small size of
acoustic resonance anemometers makes them physically strong and very easy to heat and
therefore resistant to icing. This combination of features means that they achieve high levels
of data availability and are well suited to wind turbine control and to other uses that
require small robust sensors such as battlefield meteorology. One issue with this sensor type
is measurement accuracy when compared to a calibrated mechanical sensor. For many end
uses, this weakness is compensated for by the sensors’ longevity and the fact that
it does not require re-calibrating once installed. Ping-pong ball anemometers
A common anemometer for basic use is constructed from a ping-pong ball attached to a string.
When the wind blows horizontally, it presses on and moves the ball; because ping-pong balls
are very lightweight, they move easily in light winds. Measuring the angle between the
string-ball apparatus and the vertical gives an estimate of the wind speed.
This type of anemometer is mostly used for middle-school level instruction which most
students make themselves, but a similar device was also flown on Phoenix Mars Lander.
Pressure anemometers
The first designs of anemometers which measure the pressure were divided into plate and tube
classes. Plate anemometers
These are the modern anemometers and are simply a flat plate suspended from the top so that
the wind deflects the plate. In 1450, the Italian art architect Leon Battista Alberti
invented the first mechanical anemometer; in 1664 it was re-invented by Robert Hooke.
Later versions of this form consisted of a flat plate, either square or circular, which
is kept normal to the wind by a wind vane. The pressure of the wind on its face is balanced
by a spring. The compression of the spring determines the actual force which the wind
is exerting on the plate, and this is either read off on a suitable gauge, or on a recorder.
Instruments of this kind do not respond to light winds, are inaccurate for high wind
readings, and are slow at responding to variable winds. Plate anemometers have been used to
trigger high wind alarms on bridges.They are used on these high places because they are
in a plate shape; has a good measurement status on higher altitudes.
Tube anemometers
James Lind's anemometer of 1775 consisted simply of a glass U tube containing a liquid
manometer, with one end bent in a horizontal direction to face the wind and the other vertical
end remains parallel to the wind flow. Though the Lind was not the first it was the most
practical and best known anemometer of this type. If the wind blows into the mouth of
a tube it causes an increase of pressure on one side of the manometer. The wind over the
open end of a vertical tube causes little change in pressure on the other side of the
manometer. The resulting liquid change in the U tube is an indication of the wind speed.
Small departures from the true direction of the wind causes large variations in the magnitude.
The highly successful metal pressure tube anemometer of William Henry Dines in 1892
utilized the same pressure difference between the open mouth of a straight tube facing the
wind and a ring of small holes in a vertical tube which is closed at the upper end. Both
are mounted at the same height. The pressure differences on which the action depends are
very small, and special means are required to register them. The recorder consists of
a float in a sealed chamber partially filled with water. The pipe from the straight tube
is connected to the top of the sealed chamber and the pipe from the small tubes is directed
into the bottom inside the float. Since the pressure difference determines the vertical
position of the float this is a measured of the wind speed.
The great advantage of the tube anemometer lies in the fact that the exposed part can
be mounted on a high pole, and requires no oiling or attention for years; and the registering
part can be placed in any convenient position. Two connecting tubes are required. It might
appear at first sight as though one connection would serve, but the differences in pressure
on which these instruments depend are so minute, that the pressure of the air in the room where
the recording part is placed has to be considered. Thus if the instrument depends on the pressure
or suction effect alone, and this pressure or suction is measured against the air pressure
in an ordinary room, in which the doors and windows are carefully closed and a newspaper
is then burnt up the chimney, an effect may be produced equal to a wind of 10 mi/h; and
the opening of a window in rough weather, or the opening of a door, may entirely alter
the registration. While the Dines anemometer had an error of
only 1% at 10 mph it did not respond very well to low winds due to the poor response
of the flat plate vane required to turn the head into the wind. In 1918 an aerodynamic
vane with eight times the torque of the flat plate overcame this problem.
Pitot tube static anemometers Modern tube anemometers use the same principle
as in the Dines anemometer but using a different design. The implementation uses a pitot-static
tube which is a pitot tube with two ports, pitot and static, that is normally used in
measuring the airspeed of aircraft. The pitot port measures the dynamic pressure of the
open mouth of a tube with pointy head facing wind, and the static port measures the static
pressure from small holes along the side on that tube. The pitot tube is connected to
a tail so that it always makes the tube's head to face the wind. Additionally, the tube
is heated to prevent rime ice formation on the tube. There are two copper lines from
the tube down to the devices to measure the difference in pressure of the two lines. The
measurement devices can be manometers, pressure transducers, or analog chart recorders.
Effect of density on measurements In the tube anemometer the pressure is measured,
although the scale is usually graduated as a velocity scale. In cases where the density
of the air is significantly different from the calibration value an allowance must be
made. Approximately 1½% should be added to the velocity recorded by a tube anemometer
for each 1000 ft above sea-level. Instrument location
In order for wind speeds to be comparable from location to location, the effects of
the terrain needs to be considered, especially in regard to height. Other considerations
are the presence of trees, and both natural canyons and artificial canyons. The standard
anemometer height in open rural terrain is 10 meters.
See also
Air flow meter Anemoi, for the ancient origin of the name
of this technology Anemoscope, ancient device for measuring or
predicting wind direction or weather Automated airport weather station
Particle image velocimetry Wind power forecasting
Wind run Notes
References
External links Description of the development and the construction
of an ultrasonic anemometer Robinson Cup Anemometer - Armagh Observatory
Animation Showing Sonic Principle of Operation - Gill Instruments
Collection of historical anemometer Prinicple of Operation: Acoustic Resonance
measurement - FT Technologies