LEL Gas Detectors
Flammable gas detectors measure gases such as methane, propane, butane and hydrogen as a percentage of their lower explosive limit (LEL), giving a clear warning long before an atmosphere can ignite.
LEL and UEL explained
A gas can only burn between its lower and upper explosive limits. Methane, for example, has an LEL of about 4.4% by volume and a UEL of about 17%. A reading of 10% LEL methane is about 0.44% methane in air, a tenth of the way to flammable. Most monitors alarm at 10% and 20% LEL.
Catalytic or infrared sensors
Catalytic (pellistor): responds to almost all flammable gases, including hydrogen; needs oxygen and can be poisoned by silicones, sulphides and lead compounds.
Infrared: immune to poisoning and works in inert atmospheres, but cannot detect hydrogen; ideal for hydrocarbons such as methane and propane.
DSEAR and hazardous areas
Under DSEAR, employers must assess the risk from flammable substances, eliminate or control it and classify hazardous areas where explosive atmospheres may occur. Equipment used in those areas must be suitable for the zone, gas group and temperature class, which is why portable monitors carry ATEX certification.
Portable and fixed flammable gas detection
4-gas monitors include an LEL sensor for confined space entry.
Single gas LEL monitors suit hot work and gas engineering.
Fixed flammable gas detection protects boiler houses, gas meters and process areas.
Hydrogen needs catalytic or dedicated H2 sensors: hydrogen detectors.
Calibrate any brand: calibration and service
Alarm points for flammable gas
Portable flammable gas monitors usually alarm at 10% LEL and 20% LEL. At 10% LEL for methane, the concentration is about 0.44% by volume, well below the level at which it could ignite. Many organisations treat the first alarm as the point to stop work and leave the area.
Calibration gas and correction factors
An LEL sensor is calibrated on one gas, usually methane or pentane, but responds differently to others. If your hazard is a different gas or vapour, ask us about the correct calibration gas or correction factor so the reading means what you think it does.
Oxygen and sensor poisoning
Catalytic sensors need oxygen to work and can under-read in oxygen-deficient atmospheres. Silicones, lead compounds and high H2S levels can poison them, so bump test before every use. Infrared sensors avoid poisoning but cannot detect hydrogen.
Bump testing and calibration
A bump test exposes the detector briefly to a known gas to prove that the sensors respond and the alarms sound. It takes about a minute and should be done before each day's use. A full calibration adjusts each sensor against certified gas and is normally needed at least every six months, or sooner if a bump test fails, the monitor has been dropped or it has seen a high exposure.
Our calibration service covers any brand of portable or fixed detector at standard rates by instrument type: from £40 for a single gas monitor, £50 for a 4-gas monitor, £90 for a multigas monitor and £140 for a PID. Each instrument comes back with a traceable calibration certificate.
Training your team
A detector only protects people who know how to use it. Our gas detection training covers start-up checks, bump testing, reading alarms and what to do when one sounds. We run on-site courses anywhere in the UK from £750 a day, or a three-hour online session from £350.
Talk to our engineers on 01489 326031 or email sales@7solutions.co.uk. We supply, hire, calibrate and train across the UK.
Frequently asked questions
What does % LEL mean?
It is the gas concentration as a percentage of its lower explosive limit. For methane, whose LEL is about 4.4% by volume in air, 10% LEL is about 0.44% methane.
Why measure % LEL rather than % volume?
Because it shows directly how close the atmosphere is to becoming flammable, whatever the gas, which makes alarm levels consistent across different flammables.
Catalytic or infrared?
Catalytic sensors detect almost all flammable gases including hydrogen but can be poisoned by silicones and need oxygen. Infrared sensors resist poisoning and work without oxygen but cannot detect hydrogen.
What does DSEAR require?
Employers must assess the risk from flammable substances, eliminate or reduce it, classify hazardous areas where explosive atmospheres may occur and use suitable equipment in those areas.