All You Need to Know about Cup Anemometer

Dr. John Thomas Romney Robinson of Armagh Observatory designed a simple type of anemometer in the year 1845. It comprised 4 hemispherical cups which were installed on horizontal arms fitted on a vertical shaft. The shaft was turned by the flow of air past the 4 cups in a horizontal direction at a speed that was almost proportional to that of the wind. As a result, the average speed of the wind was calculated by counting the number of turns made by the shaft over a specified timeframe. This device is also known as a rotational anemometer.

It is simple to see the 4 cups of the anemometer since these are arranged in a symmetrical fashion at the end of the arms. The hollow of one cup is always presented to the wind which is blowing at the rear part of the cup on the reverse side of the cross. The drag coefficient of a hollow hemisphere happens to be 1.42 on the hollow side and 0.38 on the spherical side. Therefore, it generates more force on the cup which is offering the hollow side to the wind. Due to the asymmetrical force, the torque which is produced on the anemometer’s axis makes it spin.

Hypothetically, it is important for the anemometer’s rotation speed to be proportional to that of the wind since the force which is produced on any object is proportional to the fluid’s speed which is flowing past that object. This consists of the turbulence which is generated by the device, thus increasing the drag against the torque which is generated by the support arms and the cups plus the mount point’s friction.

When the anemometer was first designed by Robinson, he declared that the cups actually moved 30% of the wind’s speed which was unaffected by either the arm length or the size of the cup. Although it was confounded by some autonomous experiments performed earlier, it was not correct whatsoever. An error was detected and every single experiment conducted previously had to be repeated.

John Patterson developed the 3-cup anemometer in the year 1926 and a cup wheel design was formulated by Brevoort & Joiner of the US in the year 1935. This particular device produced an error of less than 4% up to 97 km/h wind speed. It was found by Patterson that every single cup produced the optimum torque once it was at an angle of 45 degrees to the flow of the wind. This 3-cup anemometer also generated a more consistent torque and responded faster to gusts as compared to the 4-cup anemometer.

Afterward, in the year 1991, the 3-cup anemometer was modified by an Australian named Dr. Derek Weston for measuring both the speed of the wind and its direction. This individual included a tag in one of the cups that caused the speed of the cup wheel to increase and also decrease while the tag moved with and against the wind alternately. The direction of the wind is computed from all these modifications in the speed of the cup wheel while it is possible to calculate the speed of the wind from the average speed of the cup wheel. This type of cup anemometer is presently used in the industries on a wide scale right now instead of the 4-cup version.