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. 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.Īdditionally, PWM (pulse-width modulation) 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. Several ways of implementing this exist, and hot-wire devices can be further classified as CCA (constant-current anemometer), CVA (constant-voltage anemometer) and CTA (constant-temperature anemometer). As the electrical resistance of most metals is dependent upon the temperature of the metal, a relationship can be obtained between the resistance of the wire and the flow speed. Air flowing past the wire has a cooling effect on the wire. Hot wire anemometers use a very fine wire electrically heated up to some temperature above the ambient. 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. 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.

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. 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.

The other forms of mechanical velocity anemometer may be described as belonging to the windmill type or propeller anemometer. Three cup anemometers are currently used as the industry standard for wind resource assessment studies. Wind direction is calculated from these cyclical changes in cupwheel speed, while wind speed is as usual 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 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 degrees 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 cups over a set time period produced the average wind speed for a wide range of speeds. The air flow past the cups in any horizontal direction turned the cups in a manner that was proportional to the wind speed. In any form that an anemometer moves the wind will catch atleast one of the cups and cause it to rotate around the anemometer. It consisted of four hemispherical cups each mounted on one end of four horizontal arms, which in turn were mounted at equal angles to each other on a vertical shaft. John Thomas Romney Robinson, of Armagh Observatory. A simple type of anemometer,was invented in (1846) by Dr.

Who invented anemometer