Most people who work in mining or in confined spaces will know what the Lower Explosive Limit (LEL) is, and that they need an LEL monitor. But did you know that you can’t just take an LEL monitor from one application and move it to a new work site or process and expect it to protect you?
Here’s a few key points to help understand why…
What is the LEL? It’s the lowest concentration of a gas or vapour in air capable of producing a flash of fire in the presence of a source of ignition. Below the LEL, the gas mixtures are ‘too lean’ to burn. Safe Work Australia defines an atmosphere as flammable if the flammable gas/vapour/mist is likely to exceed 5% of its LEL
How do you measure it? LEL sensors are commonly found in confined space gas detectors, or you can use an explosivety meter also.
So why are they not all the same? Well, LEL sensors were originally designed to measure methane in coal mines. But what if it’s not methane that is your problem? Different gases burn with different heat – some gases burn hot and some burn relatively cool. For example, methane burns with twice the heat of propane.
So just say for example that you were in a coal mine one day and then took your LEL monitor into a tank that contained gasoline the next. Those two locations have very different flammable gases/vapours. I doubt you’d find much methane in the tank…but you’d probably find loads of hydrocarbons, which will respond differently on your LEL monitor (which for the sake of argument lets assume you have not adjusted at all since your mining visit…so it still thinks it is measuring methane). In fact – using that scenario as an example, the LEL monitor will only display 45% of the true reading when your in the tank…this means that you might think you are safe if are relying on the LEL monitor…when in fact you aren’t.
So how to I fix this? You need to understand the hazard you are trying to assess – ie: what is the flammable gas you are trying to protect you/your workers from? You can ‘correct’ your LEL readings by choosing calibration gases that are more appropriate to the gases you are measuring (such as propane), and you can use correction factors during calibration to correct the final reading shown on the monitor. To do this, you need to have a good understanding of your work area, the properties of the flammable gas, and the limitations of your equipment as a start.
Confused? Wondering where to go for help? Ask your friendly Occupational Hygienist…we love this stuff!