Quick answer: The testo 405i measures air velocity, but the appropriate measurement geometry differs: the 405i is a hot-wire duct probe, whereas the 410i uses a small vane suited to accessible flow areas and outlets.
What makes the testo 405i different
The testo 405i is a hot-wire Smart Probe suited to low-speed air-velocity assessment and duct measurement; it is not a vane instrument. The app can calculate volume flow from an entered cross-section and representative velocity. For a vane-based comparison, see the testo 410i duct air-velocity guide.
Prepare and carry out the measurement
- Document the measurement objective: local velocity at a point or volumetric airflow across an opening.
- Select a measurement location with reasonably developed flow and orient the sensor as its manual describes.
- Record multiple points or a timed average rather than relying on a single maximum; identify the actual effective cross-sectional area.
- Calculate volume flow from representative mean velocity and area, then compare at the same fan/damper condition.
Model-specific use and interpretation
Choose an accessible traverse section, align the thermal sensor according to its directional markings and take multiple readings across the area. Note ducts with insufficient straight length or unstable flow.
Velocity in m/s multiplied by area in m² gives m³/s. For CFM, use the properly converted flow; the result is not the same as useful outdoor ventilation unless outdoor-air fraction is separately known.
A duct’s mean velocity, not a single central reading, must be used with area to estimate its volume flow.
Worked example (hypothetical)
Hypothetical mean velocity 2.0 m/s through a correctly established 0.15 m² opening gives 0.30 m³/s, or about 636 CFM. A central peak of 3.0 m/s would produce a misleading estimate if used for the whole area.
What affects reliability
Use multiple points rather than a single centerline reading. The instrument is particularly useful in small ducts or lower-speed airflow patterns, but probe orientation and insertion depth still matter.
A rough single-spot value is useful for troubleshooting a sudden difference, whereas a documented multipoint traverse is better for credible flow comparisons.
- Nonuniform velocity near bends, dampers and leakage points distorts simple averaging.
- Probe alignment and turbulence matter, especially at low velocity.
- Cross-sectional dimensions must be the actual air passage, not only the external casing dimension.
- Do not treat an informal measurement as a certified test-and-balance report.
Hot-wire orientation and traverse geometry
The 405i’s slender hot-wire probe is useful for measuring lower air velocities in ducts. Follow insertion and orientation instructions and avoid touching the sensing element. A traverse with several positions is preferable to one centreline value, particularly near elbows and dampers. Use the actual duct area and the correct averaging method in the app; a nominal diameter alone can misstate air volume if an obstruction reduces area.
Interpretation: Two equal-area readings of 1.2 and 2.8 m/s average 2.0 m/s; using only the faster reading would overestimate volume flow by 40% in that simplified illustration.
Limits and safety
Follow the exact instrument manual and the equipment manufacturer’s acceptance criteria. The example is not field evidence, a legal limit or a commissioning target. Work on energized electrical equipment, combustion systems, charged refrigerant circuits or hot surfaces requires qualified personnel and the site’s approved protective procedures. For related guidance, see Measure Negative Pressure in HVAC Systems: Reference Points, Testing.