An anemometer measures air speed. The cup type shown here turns because its cups interact differently with airflow on their concave and convex faces. A real instrument converts rotor motion into wind speed using calibration. This calculator compares two entered drag coefficients under equal-flow assumptions; it does not calculate wind speed, rotor rpm or volumetric airflow.
Anemometer Interactive Calculator
Vary the concave and convex cup drag coefficients to see the drag ratio that drives cup-anemometer rotation.
Equation Used
The worked example compares the cup faces using the drag-force equation. Because rho, wind speed, and cup area are the same for both faces, the force ratio reduces to Cd_concave divided by Cd_convex.
- Same wind speed, air density, and cup area act on both cup faces.
- Only drag-coefficient asymmetry is compared.
- Bearing friction and rotor inertia are neglected.
How a cup rotor responds to air
Three cups face the same way around the illustrated rotor. Their different response to airflow on opposite faces produces a mean driving torque. Bearings support the shaft, and a pickup can measure its rotation for a calibrated instrument.
In motion, each cup sees air velocity relative to its own movement, and its orientation changes around the circle. Steady rotor speed follows a balance of aerodynamic torque and losses. Comparing two drag coefficients at equal speed is useful for explaining asymmetry, but does not solve that complete rotor balance.
The animation’s rotation is illustrative. At equal entered coefficients, the comparison has no preferred driving direction. Its apparent speed is not a wind reading.
Other sensing principles
Vane or propeller instruments use a rotating sensor aligned with the airflow. Thermal instruments infer speed from heat transfer. Ultrasonic instruments compare sound travel along measurement paths. The measurement axes, response and calibration depend on the particular design.
Matching the measurement to the task
Weather monitoring, wind-energy assessment, ventilation measurement and research use different instruments and sampling methods. A sensor suitable for one setting may not have the environmental protection, response or low-speed performance needed in another.
Wind speed is not volumetric flow by itself. A duct-flow measurement needs an appropriate mean velocity across a known area. A wind-direction reading likewise needs a vane or a sensing arrangement capable of resolving direction; cup rotation alone does not provide heading.
For a crane, aircraft operation or other equipment limit, use the applicable procedures and the specified measurement system. This comparison contains no operational wind threshold or compliance assessment.
Coefficient comparison and calibration
The calculator gives Drag Ratio = Cdconcave/Cdconvex and Net Cd = Cdconcave − Cdconvex. Concave Share is 100Cdconcave/(Cdconcave + Cdconvex), and Convex Share is the corresponding percentage for Cdconvex. The two shares total 100%.
Under the stated equal air density, relative speed and projected-area assumptions, the drag relation F = ½ρv²CdA gives the same force ratio as the coefficient ratio. Net Cd is a coefficient difference, not force or rotor torque. The percentage shares are normalized coefficient values, not efficiency or fractions of measured wind speed.
A real cup instrument uses its specified transfer function. When v = af + b is applicable, the meaning of frequency f and the slope units must match the calibration: sensor signal cycles per second and rotor revolutions per second are not automatically interchangeable. This calculator has no frequency, slope or intercept inputs.
Calibration, site exposure, bearing condition, averaging and the logger interface affect a measurement. The drag comparison cannot establish uncertainty or diagnose a faulty instrument.
Worked example: cup-face asymmetry
Enter Concave Cd 1.4 and Convex Cd 0.4. Drag Ratio is 3.50, Net Cd is 1.00, Concave Share is 77.8% and Convex Share is 22.2%. These values compare the entered coefficients under equal-flow assumptions.
Change Convex Cd to 0.7 while keeping Concave Cd at 1.4. The ratio becomes 2.00, the difference becomes 0.70, and the shares are 66.7% and 33.3%. At 0.8 for both inputs, the ratio is 1, the difference is zero and each share is 50%.
As a separate calibration example, NRG publishes a consensus transfer function for its 40C of v = 0.765f + 0.35, with f the sensor’s electrical signal frequency in Hz and v in m/s. A 10 Hz signal gives 8.0 m/s by that function. It is not a conversion from ten rotor revolutions per second, nor does it assume eight pulses per revolution. Use the calibration appropriate to the actual sensor and measurement procedure.
Comparing anemometer types
| Type | Measurement principle | Consideration |
|---|---|---|
| Cup | Calibrated rotor motion. | Bearings, starting behavior, dynamic response and exposure. |
| Vane or propeller | Calibrated axial airflow response. | Alignment and the flow field at the sensor. |
| Thermal | Heat transfer from a heated element. | Probe orientation, fluid properties and contamination. |
| Ultrasonic | Sound travel along known paths. | Path geometry, environmental conditions and signal processing. |
Use model-specific range, uncertainty, response, protection and calibration requirements. A sensing principle alone does not specify price, service life or a recalibration interval.
Anemometer questions
No. They only give a ratio, difference and normalized shares. A real rotating instrument requires its calibration and a measurement of its output.
No. It is the concave coefficient divided by the sum of both coefficients, expressed as a percentage.
No. The sensor may produce multiple signal cycles or pulses per revolution. Use the frequency definition and units in its calibration; convert rpm to revolutions per second only when that is the required quantity.
Not by itself. Volumetric flow requires a suitable average velocity over the flow area and the proper unit conversion. A single point may not represent the whole section.
No. Height, terrain, buildings, weather variation, averaging and calibration can differ. Disagreement alone does not identify one sensor as defective.
No. Dynamic errors depend on the instrument and turbulent flow. Rotor inertia and aerodynamic response cannot be represented by one universal sign or percentage of error.
Use the specified sensor and logger interface. Unverified filtering can discard valid pulses or change the measured frequency; the correct treatment depends on the signal and maximum rate.
No. The sliders contain drag coefficients, not operational limits. Equipment-specific restrictions and measurement requirements remain separate.
References and further reading
NRG Systems: 40C anemometer describes the electrical frequency output and transfer function. Its post-calibration paper illustrates sensor-specific wind-tunnel calibration. Those functions are separate from the coefficient comparison on this page.
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