A REPEATABLE METHOD
How to use it
- Confirm airflow direction, select the correct units and allow the instrument to stabilize in the room conditions.
- Use the exact terminal’s documented area and measurement method for supply or extract. Geometric free area and effective area are not interchangeable.
- Hold the vane square to the flow without blocking the grille and take readings on a repeatable grid across the face.
- Record point readings, terminal ID, area source, correction and fan setting. Average by the documented method; do not apply a correction twice or transfer it from a different grille.
- Repeat at other terminals and operating modes, then compare supply and extract totals with the design and local requirements.
INTERPRET THE MEASUREMENT
How to read the result
- For a hypothetical uniform, unobstructed cross-section: 1 m/s × 0.04 m² = 0.04 m³/s = 40 L/s = 144 m³/h. This is unit arithmetic, not a grille calibration.
- Swirl, angled vanes and proximity to the grille can make face velocity differ from duct-average velocity.
- Stable repeatability between tests is often more useful than false precision from one turbulent point.
- A matching probe, funnel and straightener setup may be needed at swirl outlets. Check the instrument and terminal instructions; a stable face reading alone is not a verified volume flow.
TROUBLESHOOT THE METHOD
Common problems and what to check
Readings jump around the grille.
Use a larger time average and a regular grid. Keep your body and hand out of the air path and avoid measuring during gusty outdoor conditions.
Calculated flow is much higher than expected.
Check the documented area and correction for this terminal and method; do not assume free and effective area are equivalent.
Supply and extract totals do not balance.
Repeat the traverse at every terminal and operating mode, check filters and dampers, and use a capture hood or professional balancing method when needed.
STOP BEFORE THE HAZARD
Safety boundary
- Keep probes, fingers and loose material away from accessible fans and moving dampers.
- Use safe access equipment for high grilles and do not remove covers from energized air-handling equipment.
- Airflow measurement does not confirm fire, combustion, pressure-balance or indoor-air-quality compliance by itself.
SMART BUYING GUIDE
What to look for when choosing
- Choose a velocity range and starting threshold suited to low residential air speeds, not just high industrial flow.
- A remote vane, time averaging and direct volume-flow entry can improve repeatability at awkward grilles.
- For commissioning many terminals, a calibrated capture hood may be better value than a premium hand vane.
Types of tool
Best for: Quick checks on accessible grilles and open air.
Best for: Repeatable traverses while the display stays visible.
Best for: Direct terminal-flow measurement and system balancing.
Common functions
Worth paying more for
- Good low-velocity response
- Time averaging and a remote probe
- Calibration documentation for balancing work
Usually not worth the extra cost
- Very high velocity range for household grilles
- Many unit modes without reliable low-speed response
- App features that do not preserve raw readings and conditions
Sources & instructions
- TROX · Grilles: effective area and measurement corrections (product-family example, PDF, pp. 16 and 18)
- Testo 100 mm vane probe · Averaging and funnel/straightener setup (model-specific example)
AI-generated topic illustration; do not use the depicted scale for measurements.
Put the measurements to work
Separate required fresh air, local exhaust, infiltration and heat recovery.
Turn a ventilation requirement into airflow and operating energyKeep air-change assumptions, room distribution, measured fan performance and electricity use as separate checks.