Anemometer

An anemometer is a device for measuring the velocity or the pressure of the wind, and is one instrument used in a weather station. The term is derived from the Greek word "anemos" meaning wind.

Anemometers may be divided into two classes, (1) those that measure the velocity, (2) those that measure the pressure of the wind, but as there is a close connection between the pressure and the velocity, a suitable anemometer of either class will give information about both these quantities.

The first anemometer was invented by Leone Battista Alberti in the 1400's.

Velocity anemometers

Cup anemometers

The simplest type of anemometer is the cup-anemometer, invented (1846) by Dr. John Thomas Romney Robinson, of Armagh Observatory. It consists of three or four hemispherical cups, mounted one on each end of a pair of horizontal arms, which lie at equal angles to each other. A vertical axis round which the cups turn passes through the center of the arms; a train of wheel-work counts up the number of turns which this axis makes, and from the number of turns made in any given time the velocity of the wind during that time is calculated. The cups are placed symmetrically on the end of the arms, so it is easy to see that the wind always has the hollow of one cup presented to it; the back of the cup on the opposite end of the cross also faces the wind, but the pressure on it is naturally less, and hence a continual rotation is produced; each cup in turn as it comes round providing the necessary force. The two great merits of this anemometer are its simplicity and the absence of a wind vane; but if used without electronic data logging equipment it is not well adapted to leaving a record of the actual velocity at any definite instant, and hence it leaves a short but violent gust unrecorded. Unfortunately, when Dr. Robinson first designed his anemometer, he stated that no matter what the size of the cups or the length of the arms, the cups always moved with one-third of the velocity of the wind. This result was apparently confirmed by some independent experiments, but it is very far from the truth, for it was later discovered that the actual ratio, or factor as it is commonly called, of the velocity of the wind to that of the cups depends very largely on the dimensions of the cups and arms, and may have almost any value between two and a little over three. This had the result that wind velocities published in many official 19th century publications were often in error by nearly 60%.

Hot-wire anemometers

Hot-wire anemometers use a very fine wire (on the order of several micrometers) heated up to some temperature above the ambient. Flow past the wire has a cooling effect on the wire. As the electrical resistance of most metals (tungsten is a popular choice for hot-wires) is dependent upon the temperature of the metal, a relationship can be obtained between the resistance of the wire and the flow velocity.

Several ways of implementing this exist, and hotwires can be further classified as CCA (Constant-Current Anemometer), CVA (Constant-Voltage Anemometer) and CTA (Constant-Temperature Anemometer). The voltage output from these anemometers is thus the result of some sort of circuit within the device trying to maintain the specific variable (Current, Voltage or Temperature) constant.

Additionally, PWM (Pulse-Width Modulated) 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.

Sonic anemometers

Sonic anemometers uses sound waves to measure wind speed and direction. They're capable of measuring the wind velocity in the X (east-west), Y (north-south), and Z (up-down) directions. Additionally, they can take measurements with very fine resolution, and the lack of moving parts makes them appealing to automated weather stations. Use of such anemometers is likely to increase in the future.

Windmill anemometers

The other forms of mechanical velocity anemometer may be described as belonging to the windmill type. In the Robinson anemometer the axis of rotation is vertical, but with this subdivision the axis of rotation 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 aerovane combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument. 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.

Pressure anemometers

The first designs of anemometers which measure the pressure were divided into plate and tube classes.

Plate anemometers

The simplest type of this form consists of a flat plate, which is usually square or circular, while a wind vane keeps this exposed normally to the wind, and the pressure of the wind on its face is balanced by a spring. The distortion 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 leaves a record via a data logger. Instruments of this kind are inaccurate for high wind readings, and are poor at recording variable winds.

Tube anemometers

Lind's anemometer, which consists simply of a U tube containing liquid with one end bent into a horizontal direction to face the wind, is perhaps the original form from which the tube class of instrument has sprung. If the wind blows into the mouth of a tube it causes an increase of pressure inside and also of course an equal increase in all closed vessels with which the mouth is in airtight communication. If it blows horizontally over the open end of a vertical tube it causes a decrease of pressure, but this fact is not of any practical use in anemometry, because the magnitude of the decrease depends on the wind striking the tube exactly at right angles to its axis, the most trifling departure from the true direction causing great variations in the magnitude. The pressure tube anemometer (fig. 1) utilizes the increased pressure in the open mouth of a straight tube facing the wind, and the decrease of pressure caused inside when the wind blows over a ring of small holes drilled through the metal of a vertical tube which is closed at the upper end. The pressure differences on which the action depends are very small, and special means are required to register them, but in the ordinary form of recording anemometer (fig. 2), any wind capable of turning the vane which keeps the mouth of the tube facing the wind is capable of registration.

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, no matter how far from the external part. 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 (16 km/h); and the opening of a window in rough weather, or the opening of a door, may entirely alter the registration.

Notes on wind measurements

In the tube anemometer also it is really the pressure that is measured, although the scale is usually graduated as a velocity scale. In cases where the density of the air is not of average value, as on a high mountain, or with an exceptionally low barometer for example, an allowance must be made. Approximately 1½% should be added to the velocity recorded by a tube anemometer for each 1000 ft (5% for each kilometer) that it stands above sea-level.

Anemometers, such as the one shown above at Deconism Gallery, are often used in conjunction with windmills, so that the wind speed and power generated by the turbine (windmill) can be logged together in a data logger.

Other types of anemometers include:

hot wire or hot plate sensors, which measure the cooling of a heated element immersed in the wind
ultrasonic sensors, which measure the Doppler shift of sound waves travelling across the moving air
laser anemometers, which measure the velocity of particles passing through fringes formed by the intersection of two laser beams

References

Dines, William Henry. Anemometer. 1911 Encyclopædia Britannica.

External links

Glossary Definition: Anemometer - AMS Glossary of Meteorology
Hot-wire Anemometer explaination
Kanomax USA Inc

1911 Britannica | Meteorological instrumentation and equipment

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