What is an anemometer?
An anemometer is an instrument that measures wind speed. The name of the instrument is derived from the Greek word “anemos” which means “wind”. Anemometers are mainly used in weather stations for meteorological purposes. But they are also used in industrial processes, in automotive industry, in aerodynamics and in various environmental applications. As any other instrument used for measurements, anemometers require a proper calibration in order to make sure that they measure correctly.
Before studying their calibration procedure, it is important to see the main types of anemometers and their principals of operation. These instruments are mainly classified into two major categories: those that measure the velocity (speed) of the wind and those that measure the pressure of the wind. Since there is a close connection between pressure and velocity, an anemometer of either of the two categories, can provide data for both wind speed and pressure.
Types of anemometers
One of the oldest types of anemometers is the cup or rotational anemometer. It has three or four cups, each mounted on one end of a horizontal arm. These arms (three or four) are attached to a vertical rod. As the wind blows it causes the cups to rotate, making the rod spin. The faster the cups rotate, the faster the wind speed is. Therefore, counting the turns of the rod over a set time period produces the average wind speed. Three cup anemometers are currently used as the industry standard for wind resource assessment studies.
This is also a mechanical anemometer measuring the velocity of the wind. It is very similar to the normal windmill in which the axis of rotation runs parallel to the direction of the wind thus making it horizontal. But the wind keeps on changing its direction. Therefore the axis has to change its direction; so an aerovane is also incorporated into the device. An aerovane consists of a propeller and a tail so that precise wind speed and direction measurements can be obtained.
Hot wire anemometers
This type of anemometer takes advantage of the fact that air cools a heated object when it flows over it. Hot wire anemometers use a very thin wire (in the order of some micrometers) electrically heated up to a temperature above the ambient. The amount of power needed to keep the wire hot is used to calculate the wind speed. The higher the wind speed, the more power is required to keep the wire at a constant temperature. Hot wire anemometers have extremely high frequency response and fine spatial resolution. This is why they are commonly used for detailed study of turbulent flows in which rapid velocity fluctuations are of interest.
Laser Doppler anemometer
They use a beam of light from a laser which is divided into two beams, with one propagated out of the anemometer. The velocity is determined on the basis of the amount of light that has been reflected off by the moving air particles. These types of anemometers are very accurate and they can measure even the slightest changes in airflow. They are also used in river hydrology.
In this type of anemometer, ultrasonic sound waves are used to measure the wind velocity. The speed of wind is obtained by sending sound waves between a pair of transducers and calculating the way their speed is affected. This type is quite popular in aircraft, scientific wind turbines, ship navigation and weather stations. Since they also do not have many moving parts they can as well be used in the automated weather stations.
They belong to the second category of anemometers which measure the wind pressure. 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.