If you’re a regular reader of our blog you’ll be familiar with the many benefits of local exhaust ventilation (LEV) for your business and employers and customers alike.
However, did you know that if you do have an LEV system, it’s also a legal requirement that you measure its airflow every 14 months to check your unit is actually functioning as intended?
And while that may the main reason why measuring airflow can be important, it’s not the only one.
In this post we’ll cover everything to know about measuring airflow, so that whatever industry you’re in, you, your people and your customers stay safe and protected.
What are the government guidelines for testing the airflow of LEV systems?
Government COSHH regulations for examining and testing LEV systems are lengthy and comprehensive. They include:
- Once an LEV system has been tested to a suitable standard, copies of all calibration and training certification must be issued and be kept on file for a minimum of five years. Examination and testing (TExT) of all assets by a competent engineer using calibrated handheld equipment
- Testing following specific HSE HSG258 guidance (control of contaminants in the workplace 2017), under COSHH regulations 7: prevention or control, 8: use of control measures and regulation, and 9: maintenance.
- Considering SFG20 (definitive standard for building maintenance) schedule reference 64-08 and a number of individual British Standards for fume cupboards, alongside the client’s COSHH risk assessment and manufacturer design specifications.
- Examining including face velocities, capture velocities, ductwork transport velocities and pressures as appropriate, as well as checking taps and lighting, along with alarms and gauges that monitor system performance, plus the surface level condition of the system.
- Visual inspection of fans, motors, ductwork and other associated components for condition, alongside filter age, condition and efficiency and exhausts and discharges.
For deeper detailed information, see the LEV FAQ page at HSE.gov.uk.
How is airflow measured? (The maths part)
Of course, to use any airflow calculations effectively, you first need to be able to gather accurate airflow data using the right equipment and know-how.
What is the most common way to measure airflow? (The equipment part)
Measuring airflow typically requires the use of a flow meter (otherwise known as an anemometer). These are relatively inexpensive pieces of handheld technology, however there is a small catch – an anometer needs to be accurately calibrated before it can be used.
Different ways to measure airflow (The practical part)
There are different ways to measure airflow depending on the type of LEV system you’re measuring.
Measuring duct AND face velocity speeds
LEV/Ductwork systems often have to extract not only light but sometimes heavy particulates like carbon fibre dust, food processing dusts like flour, and stone dusts given off by masonry work. They’re also excellent for extracting at-source dust and vapours given off by dental processes and nail filing in beauty salons – which are what make help our portable/mobile DentalAIR UVC® and SalonAIR® filtration systems so popular in those industries.
The simple way to measure airflow in ducting is to use a thermal anometer with a probe at the end of it. One way to do this is by making a hole in the ductwork and pushing the probe through in order to measure the linear airflow speed through the duct. . Our SalonAIR® and DentalAIR UVC® systems aren’t designed for a hole to be made in the duct, and therefore require measurement only at the open duct face.
Measuring the ‘open face’ of an extraction booth
Image credit: HSE’s HSG258 guide to controlling airborne contaminants at work
Measuring the airflow of a fume cabinet or spray booth can be done by splitting the unit’s large open face into a three by three grid and using a vane-type anometer to measure each area, before working out the average of each reading. You then need to make sure that average meets the minimum criteria given in the tables below.
What should your airflow capture velocity be?
You can’t explore everything to know about measuring airflow without covering the recommended capture velocities.
It’s important to keep in mind that the level of airflow needed depends on the particulate being extracted. Vapours require only a low duct speed. Solids like brick dust require a much higher duct speed.
The tables below, taken from the HSE HSG258 ‘LEV bible’ outlines the nature of the process being undertaken and the capture velocity your LEV system is required to meet.
Recommended minimum capture velocities for inside ducting
| Type of contaminant | Indicative duct velocity, m/s |
| Gases and non-condensing vapours | 5 |
| Condensing vapours, fume and smoke | 10 |
| Low or medium density, low moisture content dusts (plastic dust, sawdust), fine dusts and mists | 15 |
| Process dust (cement dust, brick dust, wood shavings, grinding dust) | Around 20 |
| Large particles, aggregating and damp dusts (metal turnings, moist cement dust, compost) | Around 25 |
Recommended capture velocities for large hood faces
| Contaminant cloud release | Example of process | Capture velocity range, m/s |
| Into still air with little or no energy | Evaporation, mist from electroplating tanks | 0.25 to 0.5 |
| Into fairly still air with low energy | Welding, soldering, liquid transfer | 0.5 to 1.0 |
| Into moving air with moderate energy | Crushing, spraying | 1.0 to 2.5 |
| Into turbulent air with high energy | Cutting, abrasive blasting, grinding | 2.5 to >10 |
Tips and tricks for measuring airflow
Hood proximity and exhaust volume
Image credit: HSE’s HSG258 guide
One of the most important tips and tricks for measuring airflow involves the placement of the anometer. The image above shows both the most and least effective working zones and capture zones for LEV equipment. You can also use this as a guide for effective placement of the anometer at the head of a ducted system.
Dealing with low airflow? Change your filters!
By far and away the most popular cause of low airflow measurements we see in filtered LEV systems comes not from a fault in the system, but from a simple mistake that’s incredibly easy to make!
If filters aren’t replaced at the end of their useful life, they can clog up, which can have the knock-on effect of slowing down the machine’s suction capabilities. So, if you are experiencing low airflow in something like a SalonAIR®, DentalAIR® or one of our other bespoke solutions, get in touch with us to find out the filters you need.
Still not sure how? We’ll measure your system’s airflow for you
Here at VODEX we know that measuring airflow can seem a little tricky if you aren’t an LEV expert. That’s why we offer a national service that can visit your workplace and check your LEV system for you.
If that sounds like something that would make your life easier, email us at sales@vodex.co.uk, or call us on +44 (0) 1489 899070.
