Phoenix Journal · LEV Testing
Modern local exhaust ventilation can watch its own performance and raise a flag the moment airflow drifts. Here's how that self-monitoring works - and the legal duties it still can't replace.
LEV monitoring
A local exhaust ventilation system rarely fails all at once - it slips quietly, and by the time an operator notices fume hanging in the air the exposure has often already happened.
That slow drift is the whole problem with relying on people to spot a struggling extraction system. A partially blocked filter, a slackening drive belt or a damper that has crept out of position does not announce itself. The hood still hums, the fan still turns, and the numbers that matter - the ones that decide whether a worker is breathing clean air - stay invisible. This is exactly why the Health and Safety Executive, in its core guidance HSG258 (Controlling airborne contaminants at work), pushes so hard for indicators that make LEV performance visible at the point of use.
A "smart" LEV system takes that principle further. Rather than a single gauge an operator might forget to glance at, it watches itself continuously, compares what it sees against a known-good benchmark, and raises a flag the moment performance drifts outside safe limits. Below we explain what these systems actually measure, how they decide something is wrong, and where their honest limits lie - because a self-checking system still sits inside a legal duty that never goes away.
Almost every self-monitoring feature comes back to one measurement: static pressure inside the extract duct. A sample tube is fitted close to the point of extraction, and the indicator reads the negative pressure the fan is generating on the working side of the system. That single value is a remarkably good proxy for whether the hood is still pulling contaminant away as designed.
The reason it works is that pressure and airflow move together. When a filter loads with dust, resistance rises and airflow falls. When a hood gets partly blocked, a duct sags, or a fan belt slips, the pressure signature changes. HSG258-compliant indicators generally operate across a band from around 100 Pa to 2,000 Pa, sampling every 15 seconds or so and refreshing what the operator sees. The clever part is not the sensor - it is the benchmark. During commissioning, the system's healthy performance is recorded, and that figure becomes the reference the device measures everything against.
Some systems add a second layer by watching the pressure differential across the fan and across the filter separately. That distinction matters, because it tells you not just that performance has dropped but roughly where. A rising differential across the filter points at a clogged element; a falling differential across the fan points at the motor, the belt or the drive. Cloud-connected units go further still, logging readings over time so a gradual decline is visible as a trend line rather than a single bad morning. That is the difference between a system that says "something is wrong now" and one that lets you see trouble coming a fortnight out.
A smart indicator is not making a clinical judgement about worker exposure - it is applying a threshold. The common rule is straightforward: if the negative static pressure in the duct drops by more than about 20% below its commissioned benchmark, the device treats that as an unsafe condition. A green "safe" state gives way to a sequence of flashing red LEDs, an audible alert, or a message pushed to a dashboard, depending on the unit.
That 20% figure is deliberately conservative. It is set well before the system becomes dangerous, so the alert is a prompt to investigate rather than a declaration that people have already been overexposed. In practice a flag might mean any of the following:
Many indicators also carry a second, entirely separate flag: a built-in countdown timer, usually set to 3, 6 or 14 months, that reminds you when a thorough examination and test is due. It is worth being clear that this is a diary function, not a fault - the device is not testing itself against the law, it is simply nudging you before the legal clock runs out. The distinction matters because it is easy to read a calm green light as "we are compliant" when all it really confirms is that the pressure sits within range today.
This is also where it helps to be precise about terminology. A smart alert tells you that your engineering control - the LEV - has drifted, but it says nothing about general room ventilation or comfort airflow, which are different jobs entirely. If that boundary is fuzzy in your building, the difference between ventilation, extraction and LEV is worth getting straight before you decide what any given sensor is really protecting.
It is tempting to treat a self-checking LEV system as a compliance shortcut. It is not, and the HSE is clear on the point. Under COSHH Regulation 9, an LEV system used to control hazardous substances must be given a thorough examination and test at least once every 14 months - and for some higher-risk processes, considerably more often. A wall of green indicators does not shorten that duty or soften it. What smart monitoring changes is the quality of the fourteen months in between.
The reason is scope. A continuous pressure indicator watches one variable very well, but a thorough examination is a far broader assessment. A competent examiner measures capture and face velocities at the hood, checks duct transport velocities against the design, inspects filters and fans, reviews the commissioning benchmark, and forms a judgement about whether the system still adequately controls exposure to the specific contaminant - whether that is grease-laden vapour, wood dust, welding fume or a combustion gas such as nitrogen dioxide (NO₂). A sensor reading 20% low cannot tell you any of that; it can only tell you that today's pressure is not yesterday's. Equally, a system can pass its pressure check and still fail a proper examination on capture velocity or hood design.
Used well, though, the two reinforce each other. Smart indicators turn your logbook from a once-a-year ritual into a living record: operator pre-use checks become a glance at a light, weekly readings can be logged automatically, and a defect is time-stamped the moment it appears rather than reconstructed after the fact. That trail is precisely what an examiner and, if it ever comes to it, an inspector wants to see - evidence that control measures were effective every day, not just on the day of the test. When a smart system does throw a flag and a subsequent visit confirms a genuine shortfall, understanding the anatomy of a failed LEV test helps you turn that alert into the right repair rather than a guessed one.
The honest summary is this: a smart LEV system is an early-warning device and a diligent scribe. It shortens the gap between a fault appearing and a person noticing, and it makes your day-to-day compliance visible and defensible. It does not, and cannot, sign off your system as safe. That still takes a competent thorough examination & test - the smart kit simply makes sure you reach it with far fewer nasty surprises along the way.
Questions
No. Under COSHH Regulation 9 an LEV system controlling hazardous substances must still receive a thorough examination and test at least every 14 months, and more often for higher-risk processes. A smart monitor watches one variable - usually duct static pressure - and gives early warning between tests, but it cannot assess capture velocity, hood design or actual control of your specific contaminant. The two work together; one does not replace the other.
Most compliant indicators flag when negative static pressure in the duct falls by more than about 20% below the benchmark recorded at commissioning. That usually points to a loading filter, a slipping fan belt, a damper knocked out of position, or a blockage building in the ducting. It is a conservative prompt to investigate, deliberately set well before the system becomes dangerous, rather than proof that overexposure has already occurred.
Many HSG258-compliant airflow indicators re-sample duct pressure roughly every 15 seconds and refresh their display, so an operator is looking at a near-live reading rather than a stale one. Cloud-connected units also log those readings over time, which turns a series of individual samples into a trend you can watch, making a gradual decline visible days or weeks before it would trip an alarm.
Phoenix Duct Clean · by the numbers
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