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LEV testing - the technical basics
An LEV system controls a hazardous dust or fume by moving air - and three different air velocities decide whether it works: the capture velocity at the source, the face velocity at the hood, and the transport velocity in the ductwork. Each does a different job, and an LEV test measures all three. Here is what each one means, and why all three matter.
The short answer
The three velocities each describe the air speed at a different point in an LEV system. Capture velocity is the air velocity out at the point where the contaminant is actually released - the speed the air must have there to draw the contaminant cloud into the hood before it escapes into the room. Face velocity is the velocity of the air entering the hood, booth or opening, measured across its face - a measure of how much the hood is drawing at its mouth. Transport velocity (also called conveying or duct velocity) is the air speed through the ductwork, which must be high enough to keep the captured contaminant moving along the duct to the collector without settling out and blocking the duct. An LEV test measures all three because each does a distinct, necessary job: capture gets the contaminant in, face confirms the hood is drawing, and transport ensures the duct carries it away. If any one is wrong, the system fails to control the hazard.
Capture velocity
Capture velocity is the most important of the three for control at source, and the one most often misunderstood. It is the air velocity at the point where the contaminant is released - at the actual source of the dust or fume, which is usually some distance in front of the hood. For the LEV to control the hazard, the air moving past that source point has to be fast enough to catch the contaminant cloud and draw it back into the hood, against the tendency of the dust or fume to drift away into the room. That required speed at the source is the capture velocity. If the air at the source is moving fast enough, the contaminant is captured; if it is too slow, the contaminant escapes into the room air where people breathe it.
The key thing about capture velocity is that it is measured at the source, not at the hood - and the two are very different. Air velocity falls off rapidly with distance from a hood, so the velocity at the source, even a short distance away, is much lower than at the hood face. This is why a hood that seems to be pulling strongly at its mouth can still fail to capture a contaminant released a little way in front of it - the velocity has dropped too far by the time it reaches the source. So capture velocity is the real test of whether the LEV controls the hazard where it is generated: it asks not how hard the hood pulls at its face, but whether there is enough air movement out where the contaminant actually is. That is what determines whether the contaminant is caught or escapes.
Face velocity
Face velocity is the velocity of the air entering the hood, booth, cabinet or opening, measured across the face - the mouth - of that opening. Where capture velocity is measured out at the source, face velocity is measured right at the hood, across the plane where the air enters it. It is a measure of how much the hood is drawing at its opening: a booth or hood with a good face velocity is pulling air in across its whole mouth at an adequate speed. Face velocity is often the most practical thing to measure directly, because the hood face is accessible and defined, and it is a standard check in LEV testing for enclosing hoods, booths and cabinets.
Face velocity matters because it indicates whether the hood is drawing properly across its opening - which is a necessary condition for the hood to work, and for enclosing hoods is closely related to the control they provide. For a booth or partial enclosure, an adequate and even face velocity across the opening means air is being drawn in all across the mouth, containing the contaminant inside and stopping it escaping out of the opening. So face velocity is a key measure for these enclosing types of hood - it confirms the hood is pulling in across its face at the design speed. It is not the same as capture velocity, which concerns the source out in front, but for enclosing hoods a proper face velocity is central to the control they give, which is why LEV testing measures it.
Transport velocity
Transport velocity - also called conveying or duct velocity - is the air speed through the ductwork, and it does a different job from the other two: it keeps the captured contaminant moving along the duct to the collector, rather than settling out inside the duct. Once the hood has captured the dust or fume, the contaminant is carried along in the duct air - but only if the air in the duct is moving fast enough to keep it airborne. If the transport velocity is too low, particularly for dusts, the contaminant drops out of the airflow and settles inside the duct, building up and eventually blocking it. So transport velocity is what ensures the captured contaminant actually gets carried away to the collector and does not accumulate in the ductwork.
This matters because a system can capture a contaminant well at the hood but still fail if the duct velocity is too low to convey it - the dust settles in the ducts, building up over time, restricting the airflow, and eventually causing blockages that reduce the whole system's performance. For dusts especially, there is a minimum transport velocity needed to keep the particular dust moving, which the ductwork must be designed and maintained to achieve. So an LEV test measures the transport velocity in the ducts to check that the system is carrying the captured contaminant away properly - that the duct air is fast enough to prevent the contaminant dropping out and building up. It is the third necessary piece: capture gets the contaminant in, face confirms the hood draws, and transport carries it away.
Why a test measures all three
The reason an LEV test measures all three velocities is that they are three links in a single chain, and the system only controls the hazard if all three are right. Capture velocity gets the contaminant into the hood; if it is too low, the contaminant escapes at the source and the rest does not matter. Face velocity confirms the hood is drawing in across its opening; for enclosing hoods, it is central to the containment. Transport velocity carries the captured contaminant away along the duct; if it is too low, the contaminant settles and builds up, undermining the system over time. Each is necessary, and a failure of any one means the system is not controlling the hazard properly, however well the others perform.
This is why an LEV thorough examination and test measures the velocities at the hoods and at the source, and the transport velocity in the ducts, along with other quantitative checks - to confirm that the whole chain is working. A test that only checked one velocity could miss a failure in another: good face velocity does not guarantee adequate capture at the source, and good capture does not guarantee the duct is conveying the contaminant away. So the test measures all three to verify each link in turn. For the person responsible for the LEV, understanding the three velocities makes the test report meaningful: it is checking that the contaminant is captured at the source, drawn into the hood, and carried away in the duct - the three things that together mean the system is protecting the people who rely on it.
The takeaway
An LEV system controls a hazardous dust or fume through three different air velocities, each doing a distinct job. Capture velocity is the air speed out at the source, where the contaminant is released - it must be enough to draw the contaminant cloud into the hood before it escapes, and because air velocity falls off rapidly with distance from a hood, it is much lower than the velocity at the hood face and is the real test of control at source. Face velocity is the speed of air into the hood across its opening - a key measure for booths and enclosures that the hood is drawing in properly. Transport velocity is the air speed in the duct, which must keep the captured contaminant moving to the collector without settling out and blocking the duct.
An LEV test measures all three because they are links in one chain: capture gets the contaminant in, face confirms the hood draws, transport carries it away - and the system only controls the hazard if all three are right. A failure of any one means the contaminant escapes at the source, or is not contained at the hood, or settles in the duct. So understanding the three velocities makes an LEV test report meaningful: it is checking that the whole chain works, from the source to the collector. That is what a properly performing LEV does - captures the contaminant where it is made, draws it into the hood, and conveys it safely away, so the people relying on it are protected.
Questions
The air velocity out at the point where the contaminant is released - the speed the air must have there to draw the contaminant cloud into the hood before it escapes into the room. It is measured at the source, not the hood, and because air velocity falls off rapidly with distance from a hood, it is much lower than the face velocity - which makes it the real test of whether the LEV controls the hazard where it is generated.
The velocity of air entering the hood, booth or opening, measured across its face - the mouth of the opening. It shows how much the hood is drawing in at its opening, and for enclosing hoods, booths and cabinets an adequate, even face velocity is central to containing the contaminant. It is measured right at the hood, unlike capture velocity, which concerns the source out in front.
The air speed through the ductwork - also called conveying or duct velocity - which must be high enough to keep the captured contaminant moving along the duct to the collector without settling out. If it is too low, particularly for dusts, the contaminant drops out of the airflow and builds up inside the duct, eventually blocking it and reducing the system's performance.
Because they are three links in one chain, and the system only controls the hazard if all three are right. Capture gets the contaminant into the hood, face confirms the hood is drawing in, and transport carries the contaminant away along the duct. A failure of any one - escape at the source, poor containment, or settling in the duct - means the system is not controlling the hazard, so a test checks each in turn.
Not necessarily. A hood can pull strongly at its face and still fail to capture a contaminant released a little way in front of it, because air velocity drops off rapidly with distance - so the capture velocity at the source can be far too low even with a good face velocity. And good capture does not guarantee the duct is conveying the contaminant away. That is why all three velocities are checked.
At an LEV thorough examination and test. Capture and face velocities are measured at the hoods and the source; transport velocity is measured in the ducts. These are part of the quantitative measurements a test takes, alongside static pressures across hoods and filters and volume flow, to confirm the whole system is capturing, containing and conveying the contaminant properly.
Our LEV thorough examination and test measures capture, face and transport velocity - checking each link in the chain, so you know the system captures the contaminant, draws it in, and carries it away.
