Home / LEV Testing / Guide
LEV testing - the key number
LEV testing produces a lot of numbers, but one matters more than the rest: capture velocity. It is the airflow at the point the contaminant is released that decides whether the system draws it in or lets it escape. Understanding this one number is understanding whether your LEV actually works.
The short answer
Capture velocity is the air velocity the LEV creates at the point the contaminant is released - and it is the number that decides whether the system works. To capture a contaminant, the LEV must create enough air velocity at the source to overcome the contaminant's own movement and draw it into the hood before it escapes. If the capture velocity is sufficient for the contaminant, the LEV captures it; if not, the contaminant escapes into the worker's breathing zone. It is the key measurement a test checks, against the levels guidance sets for the contaminant.
Why one number matters most
An LEV test measures several things, but capture velocity is the one that most directly decides whether the system works, because it is about the fundamental job of the LEV: drawing the contaminant in at the point it is released. All the other measurements - duct velocities, pressures, airflow volumes - matter as part of the system, but they matter ultimately because they determine whether there is enough air velocity at the source to capture the contaminant. Capture velocity is where the whole system's performance comes to a point: does it or does it not draw the contaminant in?
So understanding capture velocity is understanding the essence of whether an LEV works. The system exists to capture a contaminant at source before a worker breathes it, and capture velocity is the direct measure of whether it does that. A system can have impressive airflow figures elsewhere, but if the capture velocity at the source is insufficient, it is not capturing the contaminant - and if it is sufficient, it is. That is why capture velocity is the number to focus on: it is the one that translates most directly into the answer to the only question that matters, which is whether the exposure is being controlled.
What it actually is
Capture velocity is, precisely, the velocity of the air that the LEV creates at the point where the contaminant is released - at the source, in front of or around the hood. To capture a contaminant, the LEV has to move air past that source fast enough to catch the contaminant and carry it into the hood. Capture velocity is the measure of how fast the air is moving at that critical point. It is measured at the source, because that is where the capture either happens or fails - not deep in the ductwork, but right where the contaminant is generated and must be drawn in.
The reason this specific measure matters is that a contaminant does not sit still waiting to be captured - it is released with some movement of its own, and often disperses into the surrounding air. To capture it, the air velocity at the source has to overcome that movement and pull the contaminant into the hood before it escapes. So capture velocity is the velocity available at the source to do that overcoming and drawing-in. It is the air speed at the exact place and moment the LEV has to win its contest with the contaminant's own tendency to disperse - which is why it is the number that decides the outcome.
Enough or not enough
The crucial thing about capture velocity is that it has to be sufficient for the particular contaminant - and different contaminants need different capture velocities. A contaminant released gently into still air needs a lower capture velocity to draw it in; one released with energy, or into disturbed air with draughts and movement, needs a higher capture velocity to overcome that and still capture it. So there is no single universal figure - the required capture velocity depends on how the contaminant is released and the conditions around it, and recognised guidance provides the appropriate levels for different processes.
This is where the number decides whether the LEV works. If the measured capture velocity at the source meets or exceeds what the contaminant needs, the system is capturing it - the exposure is controlled. If the capture velocity is below what the contaminant needs, the contaminant escapes capture and disperses into the air the worker breathes - the system is not controlling exposure, whatever else it does. So the test of whether an LEV works is essentially whether the capture velocity is sufficient for the contaminant, judged against the levels guidance sets. Enough capture velocity means the LEV works; not enough means it does not.
How the test measures it
An LEV test measures capture velocity at the source and compares it to what the contaminant requires. Using instruments to measure the air velocity at the point of release, the test establishes what capture velocity the system is actually achieving there - the real number, at the real point. That measured figure is then compared against the capture velocity the contaminant needs, drawn from the recognised guidance (the HSE's HSG258, Controlling Airborne Contaminants at Work, provides capture velocity guidance for different processes) - so the test is judging the achieved capture velocity against the required one.
Alongside this measurement, the test observes whether the contaminant is actually captured - the qualitative check that confirms the measured capture velocity translates into real capture in the actual conditions. Together, the measured capture velocity against the required level, and the observed capture, tell whether the LEV is genuinely controlling the exposure. So when a test reports whether an LEV works, the capture velocity is at the heart of it - the measured number at the source, checked against what the contaminant needs, is the direct evidence of whether the system captures the contaminant. It is the key figure the test is built around.
The takeaway
Capture velocity is the air velocity the LEV creates at the point the contaminant is released, and it is the number that decides whether the system works. To capture a contaminant, the LEV must create enough velocity at the source to overcome the contaminant's own movement and draw it into the hood before it escapes. Sufficient capture velocity for the contaminant means the system captures it and controls exposure; insufficient means the contaminant escapes, whatever else the system does.
It has to suit the particular contaminant - how it is released and the conditions around it - so there is no single figure, and recognised guidance provides the appropriate levels. An LEV test measures the capture velocity at the source and checks it against what the contaminant needs, alongside observing the actual capture. So of all the numbers a test produces, capture velocity is the one to understand: it is the direct measure of whether the LEV does its fundamental job of drawing the contaminant in, which is exactly what deciding whether it works comes down to.
Questions
The air velocity the LEV creates at the point where the contaminant is released - at the source, in front of or around the hood. It is the speed of the air moving past the source, which has to be fast enough to catch the contaminant and draw it into the hood before it escapes.
Because it is the direct measure of the LEV's fundamental job - drawing the contaminant in at the point it is released. Other measurements matter as part of the system, but capture velocity is where it comes to a point: does the system draw the contaminant in, or let it escape?
No. A contaminant released gently into still air needs a lower capture velocity; one released with energy, or into draughty, disturbed air, needs a higher one to overcome that and still capture it. Recognised guidance provides the appropriate levels for different processes.
The contaminant escapes capture and disperses into the air the worker breathes - the system is not controlling exposure, whatever else it does. If the capture velocity meets what the contaminant needs, the system captures it and controls the exposure.
Using instruments to measure the air velocity at the point of release, establishing what the system actually achieves there, then comparing it against what the contaminant requires from recognised guidance such as HSG258 - alongside observing whether the contaminant is actually captured.
At the source - the point where the contaminant is generated and must be drawn in - not deep in the ductwork. That is where the capture either happens or fails, so it is where the velocity that decides the outcome has to be measured.
We measure the capture velocity at each source against what the contaminant requires - the key number that decides whether your LEV genuinely draws the contaminant in and controls exposure.
