FAQ
TL;DR: Multiply air speed by duct area; 8 m/s in a 100 mm duct ≈ 227 m3/h. "you multiply the speed by the cross-section". [Elektroda, Chris_W, post #17764434]
Why it matters: For HVAC DIYers and technicians, this FAQ shows how to compute airflow from velocity and diameter and avoid measurement pitfalls.
- Core formula: Q = v × A; units must match (m/s × m² = m³/s). Multiply by 3600 for m³/h. [Elektroda, Chris_W, post #17764434]
- Example: v=8 m/s, D=100 mm → Q ≈ 0.0628 m³/s (≈226.2 m³/h). [Elektroda, any56, post #17762816]
- For stable readings, use a straight run ≈ 6×Dn to even the velocity profile. [Elektroda, gag70, post #20851006]
- In domestic checks, compensation error can exceed meter accuracy; manage method and sealing first. [Elektroda, gag70, post #17764240]
- Measuring at an anemostat with a funnel: use the exit opening’s cross-sectional area, not the anemometer’s fan. [Elektroda, Chris_W, post #20851361]
Quick Facts
- Core formula: Q = v × A; units must match (m/s × m² = m³/s). Multiply by 3600 for m³/h. [Elektroda, Chris_W, post #17764434]
- Example: v=8 m/s, D=100 mm → Q ≈ 0.0628 m³/s (≈226.2 m³/h). [Elektroda, any56, post #17762816]
- For stable readings, use a straight run ≈ 6×Dn to even the velocity profile. [Elektroda, gag70, post #20851006]
- In domestic checks, compensation error can exceed meter accuracy; manage method and sealing first. [Elektroda, gag70, post #17764240]
- Measuring at an anemostat with a funnel: use the exit opening’s cross-sectional area, not the anemometer’s fan. [Elektroda, Chris_W, post #20851361]
How do I calculate airflow (m3/h) from air speed and duct diameter?
Airflow Q equals velocity v times cross-sectional area A. For a round duct, A = π(D/2)^2. Example: v=8 m/s, D=0.1 m gives Q ≈ 0.063 m³/s = 227 m³/h. As one expert said, “you multiply the speed by the cross-section”. How-To: 1. Convert diameter to meters and compute A = π(D/2)^2. 2. Multiply by velocity to get m³/s. 3. Multiply by 3600 for m³/h. [Elektroda, Chris_W, post #17764434]
Which units should I use so the math works?
Use SI units: speed in m/s and diameter in meters. Compute area in m², then multiply v × A to get m³/s. Convert to m³/h by multiplying by 3600. Converting millimeters to meters before squaring prevents large numeric errors. [Elektroda, Chris_W, post #17764434]
What area do I use when measuring with a funnel on an anemostat?
Use the cross-section of the opening where the air exits. That could be the anemostat outlet or the funnel outlet if sealed. Do not use the anemometer’s fan diameter, because it does not define the flow area. [Elektroda, Chris_W, post #20851361]
Does temperature and pressure change the result?
Yes. The v × A method assumes constant air density. Pressure and temperature shifts change density and therefore the volume flow. “This calculation method is basically errors” if you ignore those factors in sensitive cases. For precision work, include compensation. [Elektroda, Anonymous, post #17762950]
How much straight duct do I need before measuring velocity?
Use about six duct diameters of straight run (≈6×Dn) to promote a uniform velocity profile. This reduces swirl and separation from fittings, bends, or grilles. With steadier flow, your single-point reading better represents the average velocity. [Elektroda, gag70, post #20851006]
Is a DN125 pipe okay for measuring at an anemostat?
Often no. Pressing a DN125 pipe to an anemostat adds wall friction and can reduce flow, giving low readings. A sealed cardboard box with the anemometer mounted centrally is better. Seal the box perimeter against the ceiling to prevent leakage. [Elektroda, gag70, post #20851006]
What’s the cross-sectional area of a 100 mm duct?
D=100 mm means D=0.1 m. Area A = π(0.05)² ≈ 0.00785 m². At 8 m/s, Q ≈ 0.063 m³/s or 227 m³/h. Multiply by 3600 to convert seconds to hours. These values align with the example calculations provided. [Elektroda, Chris_W, post #17764434]
How do I convert m3/s to m3/h?
Multiply by 3600, the number of seconds in an hour. Example: 0.063 m³/s × 3600 ≈ 227 m³/h. This unit conversion is standard in airflow reporting and matches the worked example. [Elektroda, Chris_W, post #17764434]
Can I compute using distance traveled in one second instead of velocity?
Yes. Distance per second equals velocity. Volume per second equals pipe area times that distance: V = πr²·d. Here r is half the diameter. Use meters consistently for r and d to get m³/s. [Elektroda, any56, post #17762627]
My 8 m/s, 100 mm result looks wrong—what am I missing?
Convert 100 mm to 0.1 m, then use radius r = 0.05 m. Compute V = π·0.05²·8 ≈ 0.0628 m³/s. That equals about 226.2 m³/h after multiplying by 3600. Unit mismatches or using diameter instead of radius cause errors. [Elektroda, any56, post #17762816]
What instruments work best for home ventilation airflow checks?
Use a thermoanemometer or a pitot (damming) tube. In typical domestic conditions, compensation error can exceed the instrument’s velocity accuracy. Focus on stable setups, good sealing, and repeatable technique before advanced corrections. [Elektroda, gag70, post #17764240]
Do I need pressure/temperature compensation for domestic checks?
Often not. For household ducts and common meters, the compensation error may be larger than the meter’s own accuracy. Prioritize consistent positioning and sealing. Add compensation only if your method is already repeatable. [Elektroda, gag70, post #17764240]
