What's measured
Capture velocity is the speed of air at the point where the contaminant is actually released - far enough in front of the hood to grab dust, fume or vapour and pull it in before a worker breathes it. It is the number that decides whether extraction works, and it is not the same as the face velocity measured at the hood itself.
The short answer
Capture velocity is the speed of air at the point where the contaminant is released - far enough in front of the hood to grab the dust, fume or vapour and carry it in before anyone breathes it. It is the number that decides whether extraction really works, and it is not the same thing as the face velocity read at the hood.
The detail
Capture velocity is defined as the air velocity at the point of contaminant generation, upstream of the hood, needed to draw the release into the airstream and carry it away. It falls off very quickly with distance - move the source a little further from the hood and the velocity reaching it collapses - which is why hood position matters as much as fan power. The right value depends on how the contaminant leaves the process and how heavy it is: a gentle release into still air needs only a modest capture velocity, while an energetic release, or a heavy particle thrown into disturbed air, needs a much higher one. Published ranges run from roughly a quarter of a metre per second for still releases up to several metres per second for high-energy sources.
The common mistake is to confuse capture velocity with face velocity. Face velocity is the speed of air entering the hood opening; capture velocity is the speed of air out at the source, where the contaminant is. The hood's size, shape and face velocity together generate whatever velocity reaches the release point, so a healthy face reading does not guarantee adequate capture if the source sits too far away or a cross-draught is pulling the plume off course.
What it means for you
A thorough examination and test looks at whether the system still achieves the capture velocity it was designed to deliver, at the place the contaminant is released. The examiner measures airflow at the hood and uses the hood geometry and commissioning data to judge capture at the source, checking it has not drifted below the design figure as filters load, ducts leak or the fan wears.
If capture velocity at the source has fallen, contaminant escapes into the breathing zone even though the fan still runs and the hood still pulls. That is why the number is judged against the original design rather than against a vague sense that air is moving. Getting the source inside the effective capture zone - by moving the hood, the work, or both - is often the single biggest improvement available.
The service behind the guide
We test capture at the point your process releases contaminant, against the velocity the system was designed to deliver, with a clear report and remedial actions.
Questions
No. Face velocity is the air speed entering the hood opening; capture velocity is the air speed out at the point the contaminant is released. A good face reading does not guarantee adequate capture if the source is too far from the hood.
It depends on how the contaminant is released and how heavy it is. Gentle releases into still air need only a low capture velocity, while energetic or heavy releases need much higher values. Your design or commissioning data sets the figure.
Capture velocity falls off very quickly as you move away from the hood. A source sitting even a short distance too far away can drop below the velocity needed to catch the release, so hood position is critical.
A thorough examination and test measures airflow at the hood and uses the hood geometry and commissioning benchmark to judge the velocity reaching the source, comparing it against the original design at least every fourteen months.
Contaminant escapes into the air a worker breathes, even if the fan and hood still appear to work. The fix is usually to restore airflow, reposition the hood closer to the source, or bring the work into the effective capture zone.
Phoenix Duct Clean · by the numbers
We test capture against the velocity your system was designed to deliver, at the point your process releases contaminant, with a clear report and remedial actions.