Anemometers are an important category of environmental sensor. Articles about their design, data capture, and linearization have comprised topics featured in EDN Design Ideas, several quite recently.
Less well covered, however, has been the topic of accurate, inexpensive, (even improvisational) methods for their testing and calibration.
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| Figure 1. | GO/NOGO thermal low speed airflow sensor, ON/OFF airspeed threshold is set by R4. |
The calibration method shown here is suited for sensitive, low air speed, miniature, solid-state thermal airflow sensors with full-scale ranges up to 250 fpm (2.8 mph), two of which are illustrated in Figure 1 and Figure 2. This type is particularly useful in applications like HVAC setup and forced-convection cooling airflow distribution measurement and monitoring.
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| Figure 2. | Linearized battery powered low speed thermal anemometer. |
The airspeed measured by any anemometer is relative to the instrument. Whether it’s the air or the anemometer (or both) that’s actually moving is irrelevant. This simple calibrator consists of a repurposed phonograph turntable capable of accurate operation at the traditional rotational speeds of 33.3, 45, 78 rpm, and of course, zero. See Figure 3.
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| Figure 3. | “Groovy” anemometer calibrator built from salvaged phonograph. |
Conveniently, the diameter of a standard phonograph record is one foot. So, an airspeed sensor mounted on the periphery of an ordinary discarded vinyl record will be moved through the air at:
Air speed (feet per minute) = π × RPM
33.3 rpm = 105 fpm
45 rpm = 141 fpm
78 rpm = 245 fpm
Connections between the rotating anemometer sensor, external (stationary) power supply, and instrumentation are easily provided by a simple slipring commutator improvised from standard 3.5-mm 4-circuit audio jack and plug. The former is supported by an inverted plastic funnel glued to the record while the latter is affixed to the tone arm. Some light lubrication on the plug may be beneficial in minimizing potentially problematic drag on the turntable motor.
A suitable counterweight positioned diametrically opposite to the sensor under test can help balance the turntable against static (weight) and dynamic (centripetal) forces acting on the rotating circuitry.
