Anemometers are instruments dedicated to measuring the velocity of air and gases, essential for HVAC, ventilation, safety, process control, air quality and meteorology applications. There are several main technologies, each optimized for specific ranges and operating conditions: hot-wire anemometers, vane anemometers, Pitot tube anemometers, ultrasonic anemometers and vortex anemometers. The choice depends on the velocity range, the accuracy required, the duct geometry, the gas temperature and the presence of particulate.
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Hot-wire anemometers
Hot-wire anemometers use a thin metal wire (usually platinum or tungsten) heated to a controlled temperature; the air flow cools the wire by convection, reducing its electrical resistance. The electronics keep the wire temperature constant (or the current constant) and measure the power required, from which the flow velocity is derived using King’s law or dedicated equations. They are particularly accurate at low velocities (0.01–2 m/s), ideal for indoor climate measurements and for the flow rate of ventilation outlets. The dynamic response is excellent (hundredths of a second), but the wire is fragile and deteriorates in the presence of particulate.
Vane anemometers
Vane anemometers use a lightweight impeller (typically Ø 60–100 mm) mounted on a low-friction bearing; the air flow turns the vane at a speed proportional to the flow velocity. An optical or magnetic sensor counts the revolutions per second and the electronics calculate the linear velocity. They are rugged and suitable for medium to high velocities (1–30 m/s as standard, up to 80 m/s with special vanes). They are used for measurements at ventilation outlets, in air ducts, in spray booths and for the maintenance of HVAC systems. Vanes of various sizes are used according to the velocity typical of the application.
Pitot tubes
Pitot tubes measure the difference between the total (stagnation) pressure of the flow and the static pressure; Bernoulli’s equation gives the velocity: v = √(2·ΔP/ρ), where ρ is the air density. The typical Pitot tube is a metal tube (stainless steel) with two ports: an axial one for total pressure and a lateral one for static pressure. It is connected to a low-pressure differential manometer (typical range 0–1000 Pa or 0–2500 Pa). Pitot tubes are used for velocity measurements in large-section air ducts, in fan and compressor testing and in aeronautics.
HVAC applications
HVAC applications are varied: balancing air systems (checking the supply and return flow rate at every outlet in a building); testing air handling units; checking extraction hoods in professional kitchens (with a minimum capture velocity prescribed for food and environmental safety); monitoring spray booths (vertical extraction velocity to prevent the dispersion of solvents); commissioning cleanrooms (air velocity beneath HEPA filters, compliance with ISO 14644); energy efficiency inspections of air systems; maintenance of industrial ventilation systems.
Features and functions
Modern anemometers integrate: simultaneous measurement of velocity, flow rate (by entering the duct dimensions), temperature, humidity and pressure; time-averaging and statistical functions; alarm thresholds; memory for hundreds of measurements; USB/Bluetooth data output; PC software for archiving and generating balancing reports; interchangeable probes (hot-wire, vane, miniature probes for tight spaces, Pitot); average flow calculation over grilles for measurements at wide outlets.
Reference standards
The main standards are: ISO 5167 (for flow measurement with Pitot tubes and other restrictors); ISO 7726 (for ergonomics and the indoor thermal environment); EN 12599 (testing and commissioning of building ventilation systems); ASHRAE 111 (USA, HVAC commissioning). Anemometers are calibrated in reference wind tunnels with ISO 17025 traceability. The typical calibration interval is annual.
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