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LEV testing - hood and capture
You can have a powerful fan, sound ductwork and a clean filter - and still fail to capture the contaminant, if the hood is the wrong type or in the wrong place. The hood, and where its capture point sits relative to the source, is the single biggest factor in whether extraction actually works. Here is why the capture point makes or breaks a system, in plain terms. This is general information.
The short answer
It's natural to think of an extraction system's power as living in its fan - the bigger the fan, the better the extraction. But the truth is that the most decisive part of the system is the hood: the part that actually captures the contaminant at the source. If the hood doesn't capture the contaminant, nothing downstream can help - and whether it captures depends overwhelmingly on its design and placement. Here's why the hood and capture point make or break extraction. This is general information. The hood is where capture happens. The hood is the inlet of the extraction system - the part positioned at or near the source of the contaminant, where the system actually draws the contaminant in. Everything else in the system (the ductwork carrying the captured contaminant away, the fan providing the draw, the filter cleaning the air) only comes into play after the contaminant has been captured by the hood. So the hood is the critical interface: it's where the system meets the contaminant, and if it fails to capture, the rest of the system has nothing to work with. This is why the hood is the most important part to get right. Capture falls off fast with distance. The reason the capture point (where the hood sits relative to the source) is so decisive is that a hood's ability to capture falls off very quickly with distance. A hood creates a capturing air velocity in front of it - drawing air (and the contaminant in it) towards the hood - but that velocity drops sharply as you move away from the hood. For many hoods, the capturing velocity falls off roughly with the square of the distance: move twice as far from the hood, and the capturing velocity is a small fraction of what it was. So the zone in which a hood can effectively capture is small - close to the hood. A contaminant generated right at the hood is captured; the same contaminant generated even a modest distance further away is barely pulled in at all. This steep fall-off is the single most important fact about hood placement: the hood has to be close to the source. Direction matters as much as distance. Placement isn't only about distance - direction matters too. The hood has to be positioned so that the contaminant is generated within its zone of capture, and ideally so that the natural movement of the contaminant carries it towards the hood rather than away. Contaminants often move in a direction: fume rises on a hot plume, dust is thrown in the direction of the grinding or cutting, spray goes in the spray direction. A hood placed so that this natural movement carries the contaminant into it works with the contaminant; a hood placed so the contaminant is thrown or drifts away from it is fighting a losing battle, however close. So good capture point design puts the hood where the contaminant is generated and heading. Why a bad hood breaks the whole system. Put these together and you see why the hood makes or breaks extraction. A hood that's the wrong type for the contaminant, too far from the source, or facing the wrong way will fail to capture the contaminant - it never enters the system. And no amount of fan power or ductwork downstream fixes that: a bigger fan pulls harder through the ducts, but if the contaminant isn't within the hood's small capture zone, it isn't captured, so there's nothing for the fan to extract. So you can have a powerful fan, sound ductwork and a clean filter, and still fail completely, because the capture point is wrong. This is a common and costly mistake - investing in system power while neglecting hood placement, and wondering why the contaminant still escapes. Conversely, a well-designed hood, of the right type, placed close to the source and working with the contaminant's movement, captures efficiently - often with less airflow than a badly placed hood would need (because it's working with the contaminant, not against distance). So good hood design makes the whole system effective and efficient. What good design considers. So designing (or judging) a hood means considering: the type of hood suited to the contaminant and the task (an enclosing hood that surrounds the source is generally far more effective than a distant capturing hood, for instance); the position - as close to the source as practical, within the small effective capture zone; the direction - working with the contaminant's natural movement; and the airflow needed to achieve an adequate capturing velocity at the source (which a well-placed hood minimises). This is why an LEV assessment and design focus so much on the hood, and why testing checks the capture at the hood (the capture velocity at the source), not just the airflow in the ducts (covered in the hood-design and capture-velocity pages). The takeaway. So the hood and capture point - the type of hood and where it sits relative to the source - are the single biggest factor in whether extraction works, because capture falls off fast with distance and depends on direction, and because a contaminant not captured at the hood is never in the system for the fan and ducts to handle. Get the hood right and the system works, often efficiently; get it wrong and it fails, however powerful the rest. This is general information. This is general information.
Key points
The hood is where capture happens
The hood is the inlet of the extraction system - the part positioned at or near the source of the contaminant, where the system actually draws the contaminant in. Everything else in the system comes into play only after the contaminant has been captured: the ductwork carries the captured contaminant away, the fan provides the draw, the filter cleans the air - but all of that is downstream of capture.
So the hood is the critical interface: it's where the system meets the contaminant, and if it fails to capture, the rest of the system has nothing to work with. A perfectly good fan, duct and filter are useless if the contaminant never enters the hood. This is why the hood is the most important part of the system to get right - it's the make-or-break point where capture either happens or doesn't. The rest of the system supports the hood's capture; it can't substitute for it. So the hood is where capture happens - the critical interface with the contaminant - is the foundation: capture is the hood's job, and the system depends on it. Why the hood's placement is so decisive is next. So the hood is where the contaminant enters, or doesn't. This is general information. This is general information.
Capture falls off fast with distance
The reason the capture point - where the hood sits relative to the source - is so decisive is that a hood's ability to capture falls off very quickly with distance. A hood creates a capturing air velocity in front of it, drawing air (and the contaminant in it) towards the hood - but that velocity drops sharply as you move away. For many hoods, the capturing velocity falls off roughly with the square of the distance: move twice as far from the hood, and the capturing velocity is a small fraction of what it was.
So the zone in which a hood can effectively capture is small - close to the hood. A contaminant generated right at the hood is captured; the same contaminant generated even a modest distance further away is barely pulled in at all. This steep fall-off is the single most important fact about hood placement: the hood has to be close to the source of the contaminant, because its capturing reach is short. A hood placed too far from where the contaminant is generated simply can't pull it in - the capturing velocity at that distance is too weak. This is why 'close to the source' is the golden rule of hood placement (and why capture velocity is measured at the source, not at the hood face). So capture falls off fast with distance - the hood must be close to the source - is the key physics: the effective capture zone is small, so proximity is everything. Direction matters too (next). So the hood has to be right at the source to capture. This is general information. This is general information.
Direction matters too
Placement isn't only about distance - direction matters as much. The hood has to be positioned so that the contaminant is generated within its zone of capture, and ideally so that the natural movement of the contaminant carries it towards the hood rather than away. Contaminants often move in a direction: fume rises on a hot plume, dust is thrown in the direction of the grinding or cutting, spray travels in the spray direction.
A hood placed so that this natural movement carries the contaminant into it works with the contaminant - the rising fume goes up into a hood above it, the thrown dust goes into a hood in its path. A hood placed so the contaminant is thrown or drifts away from it is fighting a losing battle, however close: it's trying to pull the contaminant back against its own momentum or buoyancy. So good capture point design puts the hood where the contaminant is generated and heading, not just near it. For example, a hood for rising welding fume works best above and over the work (catching the plume); a hood placed to the side may miss the fume rising past it. So direction matters too - work with the contaminant's movement - is the second placement principle: position the hood to catch the contaminant's natural travel, not fight it. Why getting this wrong breaks the system is next. So place the hood where the contaminant is heading. This is general information. This is general information.
Why a bad hood breaks it all
Put these together and you see why the hood makes or breaks extraction. A hood that's the wrong type for the contaminant, too far from the source, or facing the wrong way will fail to capture the contaminant - it never enters the system. And crucially, no amount of fan power or ductwork downstream fixes that: a bigger fan pulls harder through the ducts, but if the contaminant isn't within the hood's small capture zone, it isn't captured, so there's simply nothing for the fan to extract.
So you can have a powerful fan, sound ductwork and a clean filter, and still fail completely, because the capture point is wrong and the contaminant escapes into the room instead of entering the hood. This is a common and costly mistake: investing in system power (a bigger fan) while neglecting hood placement, then wondering why the contaminant still escapes. The power was never the problem - the capture was. Conversely, a well-designed hood, of the right type, placed close to the source and working with the contaminant's movement, captures efficiently - often with less airflow than a badly placed hood would need, because it's working with the contaminant rather than against distance. So good hood design makes the whole system both effective and efficient. So why a bad hood breaks it all - fan power can't fix a missed capture - is the crux: capture happens (or fails) at the hood, and nothing downstream can rescue a contaminant that was never captured. What good design considers is next. So a missed capture can't be fixed downstream. This is general information. This is general information.
What good design considers
So designing (or judging) a hood means considering several things together. The type of hood suited to the contaminant and the task: an enclosing hood that surrounds or partly encloses the source (like a booth or a partial enclosure) is generally far more effective than a distant capturing hood, because it contains the contaminant rather than trying to reach out and grab it - so the more enclosed, the better, where the task allows. The position: as close to the source as practical, within the small effective capture zone. The direction: working with the contaminant's natural movement (over rising fume, in the path of thrown dust). And the airflow needed to achieve an adequate capturing velocity at the source - which a well-placed, well-chosen hood minimises (a close, enclosing hood needs far less airflow than a distant one to achieve the same capture).
This is why an LEV assessment and design focus so much on the hood - it's where the effectiveness and efficiency of the whole system are decided. And it's why testing checks the capture at the hood (the capturing velocity at the source, whether the contaminant is actually drawn in), not just the airflow in the ducts - because good duct airflow with poor capture is a system that isn't protecting anyone (covered in the hood-design-the-single-biggest-lev-variable and capture-velocity pages). So what good design considers - type, position, direction and airflow - brings it together: a hood designed and placed to capture the specific contaminant at its source is what makes extraction work. So design the hood around the source, and the system works. This is general information. This is general information.
Questions
Because it's where the contaminant is actually captured - everything downstream (fan, ducts, filter) only works after capture, so if the hood doesn't capture the contaminant, the rest of the system has nothing to work with; capture is made or broken at the hood. The hood is the most important part of an extraction system because it's where the contaminant is actually captured. The hood is the inlet - the part positioned at or near the source, where the system draws the contaminant in. Everything else in the system only comes into play after capture: the ductwork carries the captured contaminant away, the fan provides the draw, the filter cleans the air - all of it downstream of the hood's capture. So the hood is the critical interface: it's where the system meets the contaminant, and if it fails to capture, the rest of the system has nothing to work with. A perfectly good fan, duct and filter are useless if the contaminant never enters the hood. This is why the hood is the make-or-break point: capture either happens there or doesn't, and the rest of the system supports the hood's capture rather than substituting for it. So getting the hood right - its type and placement - is the most important thing for the system working. So it's the most important because capture happens (or fails) there. So because capture is made or broken at the hood. This is general information. This is general information.
Because a hood's capturing air velocity falls off very quickly with distance - roughly with the square of the distance for many hoods - so the effective capture zone is small; a contaminant generated close is captured, but the same contaminant a modest distance further is barely pulled in, so proximity is essential. The hood has to be close to the source because a hood's ability to capture falls off very quickly with distance. A hood creates a capturing air velocity in front of it, drawing air and the contaminant in it towards the hood - but that velocity drops sharply as you move away. For many hoods, the capturing velocity falls off roughly with the square of the distance: move twice as far from the hood, and the capturing velocity is a small fraction of what it was. So the zone in which a hood can effectively capture is small - close to the hood. A contaminant generated right at the hood is captured; the same contaminant generated even a modest distance further away is barely pulled in at all. This steep fall-off means proximity is essential: a hood placed too far from where the contaminant is generated simply can't pull it in, because the capturing velocity at that distance is too weak. This is the golden rule of hood placement - close to the source - and it's why capture velocity is measured at the source, not at the hood face. So it must be close because its capturing reach is short. So because capture velocity drops off steeply with distance. This is general information. This is general information.
No - if the contaminant isn't within the hood's small capture zone, it isn't captured, so a bigger fan has nothing to extract; fan power pulls harder through the ducts but can't reach out and grab a contaminant the hood doesn't capture, so a badly placed hood fails however powerful the fan. No - a more powerful fan cannot make up for a badly placed hood. This is one of the most important (and most misunderstood) points about extraction. A bigger fan pulls harder through the ductwork, but capture happens at the hood, within its small effective capture zone - and if the contaminant is generated outside that zone (because the hood is too far from the source or facing the wrong way), it isn't captured, so there's simply nothing for the fan to extract. The fan can't reach out beyond the hood's capture zone and grab a contaminant; it can only extract what the hood has already captured. So a badly placed hood fails to capture the contaminant however powerful the fan is - the contaminant escapes into the room instead of entering the system. This is a common and costly mistake: investing in a bigger fan while neglecting hood placement, then wondering why the contaminant still escapes. The power was never the problem - the capture was. The fix is to correct the hood (type, position, direction), not to add fan power. So no - fix the hood, not the fan. So no; a missed capture can't be fixed downstream. This is general information. This is general information.
Yes - as much as distance; the hood should be positioned so the contaminant's natural movement (rising fume, thrown dust, spray direction) carries it into the hood, not away; a hood placed against the contaminant's travel fights a losing battle even when close, so work with the movement. Yes - the direction a hood faces matters as much as its distance from the source. The hood has to be positioned so that the contaminant is generated within its zone of capture, and ideally so that the natural movement of the contaminant carries it towards the hood rather than away. Contaminants often move in a direction: fume rises on a hot plume, dust is thrown in the direction of the grinding or cutting, spray travels in the spray direction. A hood placed so that this natural movement carries the contaminant into it works with the contaminant - a rising fume goes up into a hood above it, thrown dust goes into a hood in its path. A hood placed so the contaminant is thrown or drifts away from it is fighting a losing battle, however close, because it's trying to pull the contaminant back against its own momentum or buoyancy. For example, a hood for rising welding fume works best above and over the work (catching the plume), while a hood to the side may miss the fume rising past it. So good hood placement works with the contaminant's natural travel, not against it. So yes - direction is as important as distance. So yes; work with the contaminant's movement. This is general information. This is general information.
One suited to the contaminant and task (an enclosing hood that surrounds the source is far more effective than a distant one), placed as close to the source as practical, working with the contaminant's natural movement, with enough airflow for an adequate capture velocity at the source - a good hood is effective and efficient. A good extraction hood combines several things. The right type for the contaminant and task: an enclosing hood that surrounds or partly encloses the source (like a booth or partial enclosure) is generally far more effective than a distant capturing hood, because it contains the contaminant rather than trying to reach out and grab it - so the more enclosed, the better, where the task allows. The right position: as close to the source as practical, within the small effective capture zone (because capture velocity falls off fast with distance). The right direction: working with the contaminant's natural movement (over rising fume, in the path of thrown dust or spray). And enough airflow to achieve an adequate capturing velocity at the source - which a well-chosen, well-placed hood minimises (a close, enclosing hood needs far less airflow than a distant one for the same capture). Together, these make a hood that's both effective (it captures the contaminant) and efficient (it does so without excessive airflow). This is why hood design is the focus of LEV assessment, and why testing checks capture at the hood, not just duct airflow. So a good hood is the right type, close, well-aimed, with adequate capture velocity. So one suited, close, well-aimed and adequately powered. This is general information. This is general information.
Because capture at the hood is what actually protects people - good airflow in the ducts means nothing if the contaminant isn't captured at the source; so testing measures the capturing velocity at the source and whether the contaminant is drawn in, not just the duct airflow, to confirm the system does its real job. LEV testing checks capture at the hood because that's what actually protects people - it's where the system either captures the contaminant or fails to. Good airflow in the ductwork means nothing if the contaminant isn't captured at the source in the first place: a system can move plenty of air through its ducts yet fail to draw in the contaminant, if the hood is badly placed or the wrong type. So measuring only the duct airflow would miss the crucial question - is the contaminant being captured? That's why proper LEV testing measures the capture at the hood: the capturing velocity at the source (is it adequate where the contaminant is actually generated?) and whether the contaminant is genuinely drawn into the hood (often checked with a smoke test showing the air, and contaminant, flowing into the hood). This confirms the system does its real job - capturing the contaminant to protect the people working there - rather than just moving air. A test that checked only the fan and ducts could pass a system that isn't actually capturing anything. So testing focuses on the hood because that's where protection is delivered. So because capture at the hood is what protects people. So because capture at the source is the real test. This is general information. This is general information.
The hood and where it sits relative to the source is the single biggest factor in whether extraction captures the contaminant - so we test the capture at the hood, not just the airflow in the ducts, confirming the contaminant is actually drawn in at the source, which is what protects the people working there. Ask us to test your LEV. This is general information.
