Anemometer and Airflow Measurement
Measuring air velocity and airflow at grilles, in ducts and at the equipment.
Tool
Technician
#tools#airflow#testing#balancingSafety
Keep hands, instruments and clothing clear of rotating blowers when taking readings inside equipment.
What it is
An anemometer measures air velocity, which becomes airflow once multiplied by area. Hot wire, vane and airflow hood instruments each suit different situations, and the balancing hood is the most direct way to read what a register actually delivers.
Airflow in cubic feet per minute = average velocity in feet per minute x free area in square feetThe free area of a grille is smaller than its nominal size, which is a very common source of error.
Getting a usable reading
- 1Decide what you are measuring: total system airflow, branch airflow or register delivery.
- 2For duct traverse, take a grid of readings across the duct and average them rather than taking one centre reading.
- 3Choose a location several duct diameters downstream of any fitting so the flow is not turbulent.
- 4For registers, use a hood where possible, since grille velocity readings need a correction factor that varies by grille.
- 5Cross check totals against static pressure and the manufacturer blower table.
- 6Record which method was used alongside the number.
Two methods beat one number
Airflow measured at grilles rarely adds up to airflow calculated from static pressure. When they disagree significantly, the difference is usually duct leakage, and that is useful information rather than a measurement failure.
Symptoms
Practice & Mastery
- Anemometer and Airflow Measurement Troubleshooting Scenarios2
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