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The measurement range of a device or any measuring instrument refers to the range of values for a measurement parameter within which measurement variations remain within certain predefined limits. In this context, it is important to understand the differences between the lower explosive limit (LEL) and parts per million (ppm).
What is the lower explosive limit (LEL)?
The LEL is the concentration of a substance in the air above which there is a risk of explosion. The abbreviation LEL stands for “lower explosive limit”. The LEL of flammable gases such as methane, ethane, propane and butane is of the utmost importance to us. The LEL is also known as the lower flammability limit.
Methane is measured in % LEL or % by volume (vol.-%). Methane is also measured in parts per million (ppm). Monitoring the lower explosive limit is essential for personal safety, as it can be stated that 100% LEL corresponds to a 100% risk of explosion. When working in an area above the LEL, an ignition source combined with oxygen can cause an explosion.
In Europe, the lower explosive limit (LEL) for methane is set at 4.4% by volume. Ignition is not possible below this limit, as the mixture of air and methane is too lean.
However, as measurements are only taken at specific points, it is necessary to work with high safety margins. For example, if we take a measurement in front of the body, the concentration in the room could increase significantly towards the ceiling.
The explosive limits chart
The LEL (Lower Explosive Limit) chart is a crucial tool for understanding the explosion risks associated with flammable gases and vapours in the air.
These limits are influenced by temperature and pressure conditions (therefore, these values are approximate). As pressure and temperature increase, the lower explosive limit tends to decrease, indicating that a lower gas concentration could lead to an explosion. Furthermore, an increase in pressure also helps to reduce the minimum concentration required for an explosion to occur.
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The lower explosive limit (LEL) of natural gas
Another reason for the low alarm thresholds is that natural gas is always a mixture of gases and contains other components besides methane. For example, propane and butane are often found alongside ethane. As these gases are explosive even at low concentrations, the LEL of natural gas is usually slightly below 4.4% by volume.
For this reason, alarm levels in Germany are set at 20% LEL or, in many cases, as low as 10% LEL for the first alarm.
The lower explosive limit (LEL) of methane
The LEL (Lower Explosive Limit) of methane is 4.4% by volume in air, which is the standard in Europe. This means that if the concentration of methane in the air is equal to or greater than 4.4%, there is a risk of an explosion occurring in the presence of a suitable ignition source, such as a spark.
What is ppm?
The abbreviation ppm stands for parts per million of that gas in a gas mixture or in the air. For example, if it is said that there are 100 ppm of carbon dioxide (CO2) in the air, this means that for every million parts of air, 100 parts are CO2.
When do we measure in LEL or % by volume or better in ppm range?
Volume % refers to one part per hundred, whilst ppm refers to one part per million.
The measurement range in which we operate in ppm is therefore considerably smaller than the measurement range for a measurement in % LEL or vol. %.
Even the smallest quantities of a gas such as methane can be detected and displayed in the ppm range using a highly sensitive sensor (1 vol.-% = 10,000 ppm).
As it is difficult to estimate values with many decimal places, for example 0.04 vol.-% or 0.008 vol.-%, the ppm measurement range is used for this purpose. In this case, 0.04 vol.-% would be 400 ppm.
To illustrate the measurement range, a folding ruler is used.
Comparison of the lower explosive limit (LEL) measurement range VS parts per million (ppm)
We use 100 vol.-% as the measurement range for the purity of the purge or vent gas. If an empty natural gas pipeline is filled, it can be brought back into service once it reaches approximately 100 vol.-%. The aim is to achieve a fully natural gas atmosphere within the pipeline so that there are as few disruptions as possible for consumers when it is brought back into service.
A reading of 10.000 ppm (= 23% LEL = 1 vol.-%) is measured in the LEL range for entry into confined spaces. This reading is important for assessing the clearance space. This applies when entering shafts and cavities or when a smell of gas is detected. From a measured value of 2.2 vol.-% = 50% LEL, the area must not be entered and work on a pipeline is prohibited.
What is the ppm range used for?
The ppm range is generally used to detect the lowest concentrations. This is the case for pipeline inspections or inspections of pipework in buildings. The measurement ranges are from 0 to 1,000 ppm or from 0 to 10,000 ppm, depending on the type of device. The measurement range usually depends on the type of sensors used in the device.
The 10 ppm test is for field testing (inspection of pipe networks), which is carried out using a carpet probe, for example, and measures extremely low concentrations of methane in the soil within a measurement range so small that it can barely be represented on a scale.
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What is the purpose of checking the lower explosive limit (LEL) in confined spaces?
The presence of flammable gases or vapours can be particularly dangerous in a confined space. If the concentration of a flammable gas reaches or exceeds the LEL, and an ignition source (such as a spark, fire or high temperature) is also present, an explosion may occur. It is therefore crucial to measure and monitor levels of flammable gases in confined spaces to prevent explosions and protect workers. Our OLLI multi-gas detection device is ideal for these scenarios.
In a confined space, LEL (Lower Explosive Limit) refers to the minimum concentration of a flammable substance in the air capable of forming an explosive mixture in the presence of an ignition source. In other words, the LEL indicates the lowest concentration of a flammable gas or vapour in the air, below which the mixture is not explosive. This is why it is extremely important to measure this value before entering an enclosed space.