Fan airflow is measured by determining how fast air is moving through a known area and then converting that information into a volume flow rate. In industrial ventilation and mine air surveys, this is done with specialised instruments and standard measurement procedures that minimise error and ensure repeatable results.
The most widely accepted method in ducts is a Pitot tube traverse. A Pitot tube measures the difference between total pressure and static pressure in a moving air stream, which can be converted to velocity. To measure fan airflow:
- Select a straight section of duct a suitable distance downstream of the fan or fittings so the airflow is reasonably developed and uniform.
- Divide the duct cross-section into a grid of measurement points according to recognised standards.
- At each point, insert the Pitot tube, read the differential pressure using a manometer or pressure transducer and convert each reading to local velocity.
- Average the velocities across all points, then multiply by the duct area to obtain the volume flow in m³/s or CFM.
Where Pitot traverses are impractical, engineers often use vane anemometers or hot-wire anemometers. A vane anemometer measures air speed directly as you move it across the duct face or roadway. In mine ventilation, surveyors perform traverses across roadways, walking or holding the instrument at multiple positions and calculating an average velocity. Multiplying the average velocity by the cross-sectional area of the roadway gives the airflow quantity through that section.
In some industrial systems, permanently installed flow stations or venturi nozzles are used. These devices have a known relationship between pressure drop and flow, calibrated by the manufacturer. By reading the differential pressure across the device and using the calibration curve, operators can obtain a continuous indication of fan airflow without performing manual traverses.
Whatever method is used, good practice includes correcting for air density when necessary, especially in high-altitude or high-temperature mines where actual density differs from standard conditions. Density affects the conversion from differential pressure to velocity, so instruments and calculations must reflect site conditions to avoid systematic error.
In addition, airflow measurement should be tied to clear documentation and quality checks. Recording instrument type, calibration status, measurement locations, traverse patterns and operating conditions helps ensure that airflow values can be trusted for safety-critical decisions such as gas dilution, cooling capacity and regulatory compliance.
In summary, fan airflow is measured by using Pitot tubes, anemometers or calibrated flow devices to determine air velocity distribution, multiplying the average velocity by the cross-sectional area and applying any needed density corrections. These methods form the backbone of quantitative ventilation surveys in industrial plants and underground mines.
