Anemometers (Air Flow Measurement)

Anemometer

An anemometer, also known as a wind vane, is an instrument designed specifically for the measurement of gases either in a contained flow, such as airflow in an air-conditioning duct, or in unconfined flows, such as atmospheric wind. To determine the velocity, an anemometer detects change in some physical property of the fluid or the effect of the fluid on a mechanical device inserted into the flow. Wind velocity or speed is usually measured via a cup anemometer, an instrument with three or four small hollow metal hemispheres set so that they catch the wind and revolve about a vertical rod. An electrical device is used to record the revolutions of the cups and calculates the wind velocity.

An anemometer can measure the total velocity magnitude, the velocity magnitude in a plane, or the velocity component in a particular direction. The cup anemometer, for example, measures the velocity in a plane perpendicular to the axis of its rotation cups. If the cup anemometer is mounted with the shaft perpendicular to the horizontal, it will measure only the component of the wind that is parallel to the ground. Other anemometers, such as the pitot-static tube, are used with the tip aligned with the total velocity vector. Before using an anemometer, it is important to determine how it should be positioned and what component of the total velocity its measurement represents.

An anemometer usually measures gas flows that are turbulent. The cup anemometer, pitot-static tube, and thermal anemometer are mostly used to measure the mean velocity, while the hot-wire, laser Doppler, and sonic anemometers are usually used when turbulence characteristics are being measured. (The term "thermal anemometer" is often used to mean any anemometer that uses a relationship between heat transfer and velocity to determine velocity.)

Anemometers exist within two classes:

1) those which measure the velocity (speed) of the wind

2) those which measure the pressure (force) of the wind

As there is a close connection between the pressure and the velocity and a suitable anemometer of either class will give information about both these quantities.

Mechanical Anemometer

In 1450, the Italian art architect Leon Battista Alberti invented the first mechanical anemometer; consisting of a disk placed perpendicular to the wind. It would rotate by the force of the wind, and by the angle of inclination of the disk the wind force momentary showed itself.

Hemispherical Cup Anemometer

The hemispherical cup anemometer was invented in 1846 by Irish researcher, John Thomas Romney Robinson and consisted of four hemispherical cups. The cups rotated horizontally with the wind and a combination of wheels recorded the number of revolutions in a given time.

Sonic Anemometer

The sonic anemometer (invented by geologist Dr. Andreas Pflitsch in 1994) determines instantaneous wind speed and direction (turbulence) by measuring how much sound waves travelling between a pair of transducers are sped up or slowed down by the effect of the wind.

Root: The term anemometer is derived from the Greek word, anemos, meaning wind; and meter being an instrument that records or regulates the amount of something passing through it.

Do not forget, we always label the wind by the direction FROM which it is blowing.