anemometer interstellar
TRANSCRIPT
Anemometer: Basics
A common weather station instrument
The term is derived from the Greek word anemos, meaning wind
Describe any air speed measurement instrument used in meteorology or aerodynamics
Invented in 1846 byJohn Thomas Romney Robinson
Image: National Weather Service (NWS) Collection, Baltimore, Image ID: wea00920
Anemometer: Reason For Use
Two primary reasons to measure wind speed:
To determine feasibility of wind power development at a site As part of a wind turbine control system
“Is it worthwhile to turn the turbine into the wind
and start it?”
Animation: How the air moves forming Winds, V. Ryan © 2005
Anemometer: Types
Mechanical Type: Cup Anemometer
Non-Mechanical Type: Hot Wire Anemometers Ultrasonic Anemometers Laser/Doppler Anemometers
Pressure Anemometers Plate Anemometers Tube Anemometers
Propeller Type Anemometers
Image: Digital Anemometer, www.everflowscientific.com
Cup Anemometers
Most common wind speed measurement device
It consists of hemispherical cups, each mounted on one end of horizontal arms
And the arms were mounted at equal angles to each other on a vertical shaft
Image: www.directindustry.com
Cup Anemometers: Continued
The air flow past the cups in any horizontal direction turned the shaft in a manner that is proportional to the wind speed
On counting the turns of the shaft over a set time period produced the average wind speed for a wide range of speeds
Animation: Anemometer 2 from Barani Design and Wind101.net
In a Nutshell: Cup Anemometers
Many cup anemometers have a vane attached to measure wind direction
Advantages Disadvantages
Low Price Flexible Design Simple Installation
Most technicians understand operating principles and necessary connections
Moving parts wear out
Without provisions for heating, they don’t work well in snow or freezing rain
They don’t work well in rapidly fluctuating winds
Image: http://www.photolib.noaa.gov/nssl/nssl0161.htm
Hot Wire Anemometers
Uses a very fine wire (on the order of several micro-metres)
Electrically heated up to some temperature above the ambient
Air flowing past the wire has a cooling effect on the wire
“The electrical resistance of most metals is dependent upon the temperature of the metal”
Using above a relationship can be obtained between the resistance of the wire and the flow speed
Probe Specifications
“Tungsten is a popular choice for hot-wires”Tungsten or Platinum filament~1 mm long4-10 mm diameter
BenefitsGood spatial resolutionFlat frequency response
LimitationsFragileRequires clean flowCost (start at $300-400)
Image: www.tungstenringsco.com
Typical Specifications
Parameters Handheld/Economy Industrial Grade
Measurable velocities 0.2-20 m/s 0.2-90 m/s
Operating temp ranges 0-50 °C -40-200 °C
Velocity Accuracy ± 3% reading ± 1% reading
Time constant 200 ms 100 ms
Interfacing options Handheld reader, RS232
RS232, RS485, voltage, 4-20 mA, Modbus,
Profibus, etc.
Source: http://www.extech.com/instruments/product.asp?catid=1&prodid=43
Ultrasonic Anemometers
First developed in the 1950s Use of 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
Figure: 2D ultrasonic anemometer with 3 paths
Image: Anémomètre / girouette ultrasonique à trois chemins
Ultrasonic Anemometers
Advantage Disadvantage
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
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
Figure: 3D ultrasonic anemometerImage: Google
Laser/Doppler Anemometers
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 (or deliberately introduced seed material) 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
Plate Anemometers
Modern anemometers
Are simply a flat plate suspended from the top so that the wind deflects the plate
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
This is either read off on a suitable gauge, or on a recorder
Plate Anemometers: Continued
Advantage Disadvantage
They are used on these high places because they are in a plate shape; has a good measurement status on higher altitudes
Instruments of this kind do not respond to light winds, are inaccurate for high wind readings, and are slow at responding to variable winds.
Image: www.stormdebris.net
Tube Anemometers
A tube anemometer uses air pressure to determine the wind pressure, or speed
A tube anemometer measures the air pressure inside a glass tube that is closed at one end
By comparing the air pressure inside the tube to the air pressure outside the tube, wind speed can be calculated
Figure: Invented by William Henry Dines in 1892, The movable part (right) to be put on top of the fixed part (left)
Image: Wikipedia
Propeller Type/Vane Anemometers
The axis on the vane anemometer is parallel to the direction of the wind and therefore horizontal
Since the wind varies in direction and the axis has to follow its changes
Combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument
Image: nssl0161, National Severe Storms Laboratory (NSSL) Collection
Output from Anemometers
Signal conditioning is usually done within the instrument The output can be an electrical signal to a data-logger or
readout device: Pulse signal Voltage signal For example, 0-10 V corresponds to the velocity measurement range of
the instrument.
Current signal Typically, 4-20 mA corresponding to the instrument range. Eliminates voltage drop when the signal is transmitted over larger
distances.
Importance of Accurate Wind Speed Measurements The power obtained from the wind goes with the cube of
the wind speed A small error in the measurement results in a much larger
error in the predicted wind power
For example, a 5% error at a wind speed of 10 meters/sec leads to a 16% error in predicted wind power
10% anemometer error leads to 33% errors in power prediction
This could be disastrous if one is monitoring a site for feasibility of wind power development!