As the electrical resistance of most metals is dependent upon the temperature of the metal ( tungsten is a popular choice for hot-wires), a relationship can be obtained between the resistance of the wire and the flow speed. Air flowing past the wire cools the wire. Hot wire anemometers use a very fine wire (on the order of several micrometres) electrically heated to some temperature above the ambient. In cases where the direction of the air motion is always the same, as in ventilating shafts of mines and buildings, wind vanes known as air meters are employed, and give satisfactory results. Hence, volumetric flowrate may be calculated if the cross-sectional area is known.
The speed of the fan is measured by a rev counter and converted to a windspeed by an electronic chip. 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.Ī vane anemometer thus combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument. Unlike the Robinson anemometer, whose axis of rotation is vertical, the vane anemometer must have its axis parallel to the direction of the wind and therefore horizontal. It may be described as a windmill or a propeller anemometer. One of the other forms of mechanical velocity anemometer is the vane anemometer. Three-cup anemometers are currently used as the industry standard for wind resource assessment studies & practice. Wind direction is calculated from these cyclical changes in cupwheel speed, while wind speed is determined from the average cupwheel speed. Weston added a tag to one cup, which causes the cupwheel speed to increase and decrease as the tag moves alternately with and against the wind.
The three-cup anemometer was further modified by the Australian Dr Derek Weston in 1991 to measure both wind direction and wind speed. The three-cup anemometer also had a more constant torque and responded more quickly to gusts than the four-cup anemometer. Patterson found that each cup produced maximum torque when it was at 45° to the wind flow. The three-cup anemometer developed by the Canadian John Patterson in 1926 and subsequent cup improvements by Brevoort & Joiner of the USA in 1935 led to a cupwheel design which was linear and had an error of less than 3% up to 60 mph (97 km/h). Every previous experiment involving an anemometer had to be repeated. Instead, the ratio of the speed of the wind and that of the cups, the anemometer factor, depends on the dimensions of the cups and arms, and may have a value between two and a little over three. This was apparently confirmed by some early independent experiments, but it was incorrect. When Robinson first designed his anemometer, he asserted that the cups moved one-third of the speed of the wind, unaffected by the cup size or arm length. On an anemometer with four cups, it is easy to see that since the cups are arranged symmetrically on the end of the arms, the wind always has the hollow of one cup presented to it and is blowing on the back of the cup on the opposite end of the cross. Therefore, counting the turns of the shaft over a set time period produced a value proportional to the average wind speed for a wide range of speeds. The air flow past the cups in any horizontal direction turned the shaft at a rate that was proportional to the wind speed. It consisted of four hemispherical cups mounted on horizontal arms, which were mounted on a vertical shaft. John Thomas Romney Robinson, of Armagh Observatory. Velocity anemometers Cup anemometersĪ simple type of anemometer was invented in 1845 by Dr. In 1994, Andrews Pflitsch developed the sonic anemometer. In 1991, Derek Weston added the ability to measure wind direction. In 1926, Canadian meteorologist John Patterson (Janu– February 22, 1956) developed a three-cup anemometer, which was improved by Brevoort and Joiner in 1935. In 1846, John Thomas Romney Robinson (1792–1882) improved upon the design by using four hemispherical cups and mechanical wheels. (1635–1703), developed their own versions, with some being mistakenly credited as the inventor. In following centuries, numerous others, including Robert Hooke Leon Battista Alberti (1404–1472) is said to have invented the first mechanical anemometer around 1450. The anemometer has changed little since its development in the 15th century.
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