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LEV testing - diagnosis
A fail after a pass feels like bad luck or a harsh examiner. It is almost never either. Something in the system, or around it, has changed - and knowing the usual suspects tells you what to fix.
The short answer
An LEV system that passed at its last examination and fails at this one has not been treated harshly - its performance has dropped below adequate control since it was last checked. The usual causes are gradual wear - loaded filters, a tiring fan, leaking or blocked ductwork - or a change around the system, such as an altered process or an added workstation that robbed airflow. A new fail is a signal to find which.
The mindset
The first thing to understand about a fresh fail is that it is telling you something true. A thorough examination and test measures whether the system is controlling exposure, so a fail means that, on the day, it was not - regardless of how it performed a year ago. The instinct to blame luck or the examiner gets in the way of the useful question, which is what changed.
And something almost always has, because LEV does not hold its performance indefinitely. Between one test and the next, in normal use, systems drift and circumstances change. A fail is the point at which that drift or change has crossed the line from adequate to inadequate control - so it is a prompt to diagnose, not to dispute.
Cause one
The commonest reason is simple degradation. Filters load up and restrict airflow. A fan loses efficiency as its bearings wear or its blades foul. Ductwork develops small leaks or partial blockages. Flexible hoses perish or get crushed. None of these stops the system running, and none announces itself - so the system quietly loses capture over the months until a test finds it below par.
This is exactly why COSHH requires periodic testing rather than trusting a past pass: the test exists to catch the wear the eye and ear miss. A fail from wear is not a disaster - it usually points to a specific, fixable component - but it is a reminder that maintenance between tests is what keeps a system from drifting into a fail in the first place.
Cause two
The other common cause is change rather than wear. The process the LEV serves may have altered - a different substance, a higher throughput, a new task the hood was never designed for. The layout may have shifted so a source now sits at the edge of a hood's reach. Or, very often, a workstation or capture point was added, drawing airflow from the existing points and quietly reducing capture across the system.
That last one catches people out because it looks like an improvement. Adding extraction feels like more control, but a system has a finite amount of airflow to share - so a new capture point can starve the others, turning a system that passed into one that fails at the very points that were fine before. A change that seemed positive is often the reason for the fail.
Cause three
Sometimes the honest answer is that the system was close to the line last time and has simply tipped over. A previous pass at the edge of adequate, plus a little wear, is enough to produce a fail without any dramatic change. This is less comfortable, because it suggests the earlier margin was thinner than it looked - but it is still useful, because it points towards whether the system is fundamentally adequate or was always borderline.
This is where the quality of the earlier examination matters. A thorough test that recorded real figures lets you compare and see whether performance dropped a lot or a little. A superficial previous pass with no measurements gives you nothing to compare against - which is one more reason a proper examination is worth having every time.
What to do with a fail
The productive response to a fresh fail is to use the report to find the cause and act on it. Wear points to a repair - change the loaded filter, service or replace the fan, reseal the ductwork - after which a re-test should confirm control is restored. A process or layout change points to reassessing whether the system still suits the work, and possibly redesigning part of it.
It is also the moment to ask whether anything was added or altered since the last test, because if a new workstation starved the system, bolting on more extraction without balancing it will not help. A fail handled this way - diagnosed, fixed at the cause, and re-checked - turns a worrying result into a system you know is controlling exposure again, which is the whole point of testing.
Questions
Because its performance has dropped below adequate control since the last test. The usual causes are gradual wear - loaded filters, a tiring fan, leaking ductwork - or a change such as an altered process or an added workstation that robbed airflow.
No. A thorough examination measures whether the system is controlling exposure, so a fail means it was not on the day. Something almost always changed - a fail is information to diagnose, not misfortune to dispute.
Yes, commonly. A system has a finite amount of airflow to share, so a new workstation or capture point can starve the existing ones - turning a system that passed into one that fails at points that were fine before.
Filters loading and restricting airflow, a fan losing efficiency or fouling, ductwork developing leaks or partial blockages, and perished or crushed flexible hoses - all of which cut capture gradually and invisibly.
Sometimes. A previous pass at the edge of adequate, plus a little wear, can tip into a fail without a dramatic change. A proper earlier test with real figures lets you see how far performance dropped.
Use the report to find the cause and fix it - repair the worn component, or reassess the system if a process or layout changed - then re-test to confirm control is restored.
We test with measured figures and diagnose what changed, so a fail comes with a route to putting it right.
