1. Hand-Held Anemometer Uses a Savonius rotor
Easily portable - often used by military
Threshold of 0.75 knots
Accurate to 3% of full scale
Twin-tail vane. Accurate to ±2o
Operator error probably greatest cause of error

3. Propeller Anemometers
Flat Blade - Airmeters
Helicoidal - Aerovane
Bivanes

4. Airmeters Blades at 45o to wind Blades must point into the wind.
Then: and v = V.
Blades must point into the wind.
Responds well to light winds.
Accuracy of properly oriented airmeter ~ ±0.2-3% of reading.

5. Aerovane Gives both wind speed and direction.
Vane keeps propeller pointed into wind.
Usually have a lower starting threshold than cup anemometers.
Accuracy: 1-3% reading Vane: ±2o
Threshold: Impeller 2 mph
Vane: 5 mph

6. Propeller blade tip moves about 4 times faster than a cup anemometer.
Tend to underestimate low wind speeds
Tend to overestimate gusty wind speeds.
Less a problem than with cup anemometers.

7. Bi-Vanes UVW anemometer
Measures 3-dimensional winds: horizontal and vertical.
Threshold for best is about 0.5 mph.
Range: 0.5 to 112 mph

8. Starting and Stopping Threshold

9. Pressure Type Anemometer
Pressure-Plate Anemometers
Mayan Anemometer (sometime between 1200 and 1400 AD)
Small, light pith balls dropped from a basket and were blown downwind.

10. Leon Battista Alberti ~1450 constructed a swinging plate anemometer.

11. Leonardo DaVinci b. 1452
Probably drawn 1480’s ’s

12. Robert Hooke
Swinging Pressure Plate Anemometers all had a problem of Resonance Magnification.

13. Normal-Plate Anemometer
Resistance in AC circuit varies with the position of armature and pressure plate. Current required to keep armature and pressure plate in positon becomes a measure of the forc (wind speed) acting on pressure plate.
Not affected by Resonance Magnification.
Can measure gusty winds well.
Force on plate given by:

14. Micro-machined Force-balance Anemometer
Built by NASA’s Jet Propulsion Laboratory
2 cm X 2 cm.
Proof Mass hangs between two capacitor plates. As air moves the device, the proof mass attempts to remain constant. This changes the capacitance.
Measures very small air motions.

15. Pressure - Tube (Pitot Tube)
Dines Anemometer:

17. However, density is difficult to measure
However, density is difficult to measure. If we use the General Gas Law equation and substitute for density, we get: where,
DP = Pdynamic - Ptransverse, (a small value, about mb for a 20 mph wind)
Patmos = Atmospheric Pressure.
R = gas constant for dry air.
Tv = Virtual temperature of air. T is usually used with negligable error.

19. Possible Errors of Pitot Tube Anemometers
Tubing must be tight and not clogged (ice).
Changes in pressure must be accurately measured.
Must be directed into wind.
Low sensitivity at low wind speeds.
Compressibility of air may cause errors at high speeds (aircraft).
Density and humidity changes can affect results.