An anemometer is an apparatus that is employed to determine the wind's velocity (direction and speed) and pressure. Most anemometers only measure one of these factors. Since wind velocity and wind pressure are closely connected; pressure, direction and speed can be derived mathematically from an anemometer that is only specifically devised to measure wind velocity.
Anemometers can be built from scratch utilizing some materials found at home and a few electrical and electronic parts that can be easily obtained from hardware and electronics stores. A home-made anemometer will usually have a main rotating shaft supported by bearings. On top would be at least three horizontal spokes, and at the end of each spoke would be cups to catch the wind. The main rotating shaft is attached to a dynamo which produces electricity when the main shaft rotates. The output in electricity then passes through a connected small circuit board and finally on to a measurement display.
The main component of the circuit board is a microcontroller unit, which is a vastly simplified cousin of the PC's microprocessor. It's available to consumers and electronics hobbyists and is found in various appliances like remote controls, power tools and even toys. The type of microcontroller often used for home-made anemometers is one that can release proportional voltage based on incoming electrical frequency. The rotation of the main shaft affects the electrical frequency put out by the dynamo; the resulting output voltage from the microcontroller is then registered on an attached measurement display, which can be as generic as a multimeter.
These types of home-made anemometers need to be calibrated, meaning a proportional relationship between wind speed and electrical voltage needs to be established. Wind speed is measured in kilometers per hour or meters per second, and although this is evident in the rotation of the main shaft, quantifying this would entail setting up a mechanical device that directly measures the number of rotations the shaft makes in a given time period. Mechanical means of measurement are usually prone to error when dealing with something as turbulent as wind. To test and calibrate your home-made anemometer, you can simulate various wind speeds by driving around in your car with the apparatus attached outside the car.
1. Mount the home-made anemometer on your car.
Make sure it's attached securely so that it won't fall off and get damaged. You will need the assistance of another person who will monitor the read-outs on the multimeter and take notes.
2. Drive the car at constant speeds.
Be sure you're driving on an even road and you do this on a non-windy day. As much as possible, the rotation of the shaft must be caused purely by the wind created by the car's motion and not by any other actual and naturally occurring wind. Drive the car at various constant speeds in constant periods of time. You can start at 20 kph and maintain that for 5 minutes for example, and then accelerate to 25 and so on and so forth.
3. Take down notes of the car's speed and the corresponding voltage read out.
At each degree of car speed, your assistant should take note of the voltage read out. You can shout out the reading on the car's speedometer to him so he can also take this down.
4. Graph the results and derive the proportional relation.
Now that you have correlative measurements of speed and voltage you can represent this data on a graph. This will then serve as your reference so that you can know how fast the wind is moving based on what you see on the multimeter.
Commercial anemometers and those that are of industrial grade have added features so accurate measurements on wind temperature and humidity can also be obtained. Handheld anemometers have even been released in the market and are geared for individuals such as wind surfers, sailors and kite fliers who need to have constant access to the precise and latest wind conditions.