Electronic Design

Portable Airspeed Measurement

Computer-compatible airflow instruments are widely available but are usually expensive, bulky, and mechanically fragile. This anemometer continuously converts airspeed in the range of zero to tens of meters per second into an RS-232-compatible data stream while overcoming those drawbacks. It’s battery-powered and, when combined with a laptop or notebook PC, consists of a fully portable airspeed measurement system.

The anemometer’s principle of operation is that of the familiar constant temperature hot-wire anemometer. In this case, the relationship between electrical resistance and the temperature of tungsten wire is used to monitor and regulate the temperature of a heated filament exposed to the airflow. The power needed to maintain a constant difference between ambient and filament temperatures then can be used to directly calculate airspeed via “King’s Law.” The law states that the rate of heat loss is proportional to the temperature differential between air and filament, multiplied by the square root of airspeed.

In this version (see the figure), comparator U1 monitors the ratio of the resistance of filament F1 (a denuded Radio Shack #272-1141 incandescent lamp) to reference 3. Whenever RWp × Th × Vw2/ RW, where Fp = pulse frequency, Th = heat pulse duration, Vw = pulse amplitude at the filament, and RW = filament resistance. Th is generated by a linear timing ramp produced by Q2’s collector current as it charges C2 to U2’s threshold voltage. Because Q2’s collector current is made proportional to Vw2 and to ambient temperature, Th is inversely proportional to these factors. This feature compensates the quantum of heat delivered by each pulse against variations in battery voltage and air temperature and keeps Fp proportional to the square root of airspeed. Maximum Fp (corresponding to ≈ 20 meters/s) is ≈ 1370 Hz.

Each filament heating pulse causes Q3 to transmit an RS-232 start bit to the COM port (formatted for 9600 baud, 1 start, 1 stop, 5 data, and no parity bits) of the connected computer. A simple software routine tallies these pulses and averages their frequency. Subtraction of an empirically derived zero offset from the average, squaring, and normalizing it with a suitable scaling constant produces the final airflow measurement.

Battery life is extended by applying filament power only when the COM port is “Open” and by the wide range of battery voltage (4.6 to 6 V) compatible with the accurate anemometer operation. As the battery finally does reach end of life and Vw drops below 4.5 V, Th becomes longer than 677 µs (the longest start bit compatible with COM-port framing requirements). The resulting “framing error” provides a reliable “low battery” warning.

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