Explosive Gas Sensors and How They Work

Disastrous events can occur within industries that work with Flammable and Explosive gases; the smallest leak can very easily turn into an event that makes the evening news.?

Explosive gas sensors monitor the air for the presence of dangerous gases, alerting workers to hazards that might otherwise go unnoticed. They can help prevent disasters before they happen, detecting hazardous gases to prevent fires, explosions, and other risks to life and businesses.?

There are several types of sensors available for detecting explosive gases; what are the differences, and which is best for me and my needs?

Read on, and we will explain.?

What is LEL

An LEL sensor, short for Lower Explosive Limit, measures the concentration of combustible gases in the air, and detects if this accumulation of gases is approaching the Lower Explosive Limit.

If this limit is crossed and reaches 100% of the LEL threshold, the air becomes combustible and is within the threshold for ignition of fire or an explosion if paired with an ignition source such as a spark or flame.?

LEL gas sensors typically measure gas concentrations as a percentage of the LEL rather than a percentage of volume. For instance, if an LEL sensor shows a reading of 50%, it means the concentration of the gas is half of its lower explosive limit, which is still below the point where it could ignite.

For example:

• 0% LEL indicates no detectable combustible gas.
• 50% LEL is halfway to becoming explosive, and if it continues to increase it could reach explosive levels
• 100% LEL means the concentration is at the lower explosive limit (i.e., an ignition could cause an explosion).
• Alarm Settings: Alarms are set to go off around 10-20% LEL to allow ample time for evacuation or intervention.?

Types of Sensors Used for LEL Detection

LEL gases can be detected a number of ways, these depend on the gas detector used, your own personal needs, and which gases you are looking to detect for, below you will find your options, and how these can benefit you.?

Catalytic Bead (Pellistor) Sensors: These sensors have a catalytic bead that heats up when exposed to combustible gases. When these gases oxidise on the bead, they emit heat, resulting in a change in resistance. This change is quantified and expressed as a percentage of the LEL. Catalytic bead sensors are commonly used to monitor the LEL of hydrocarbons such as methane, propane, and other flammable gases. An added benefit is that they can detect multiple flammable gases including Hydrogen, but needs a concentration of Oxygen to work correctly.?

Low Power Catalytic Bead (Pellistor) Sensors: These are a specialised variant of traditional catalytic bead sensors designed to operate with minimal power consumption. This design makes them ideal for battery-powered devices such as portable gas detectors.

Infrared (IR) Sensors: IR sensors measure gas concentration based on the absorption of infrared light. Since combustible gases absorb infrared light at specific wavelengths, the sensor can detect their concentration in the air by measuring the absorption. IR sensors are commonly used in environments where oxygen levels may fluctuate or where catalytic bead sensors would be less effective. These sensors work incredibly well at detecting their specific gas, especially in inert atmospheres, but cannot detect Hydrogen.?

MPS (Molecular Property Spectrometer): MPS sensors is a new type of technology that detects gases based on the changes in thermal properties, molecular size, and energy transfer characteristics, rather than relying on traditional combustion or IR absorption methods.

The benefits of this sensor is that it can detect multiple gases, cannot be poisoned by other gases and there is no need for a correction factor, however it requires oxygen to function, making it unsuitable for an inert atmosphere.?

Filtered vs Unfiltered Sensors

Filtered LEL sensors: These refer to sensors that incorporate a physical or chemical filter to limit or modify the types of gases or particles that reach the sensing element. Filters can help eliminate unwanted cross-sensitivity to certain gases or environmental contaminants, leading to more accurate readings in specific applications.

Unfiltered LEL sensors: These are the same as previous, but operate without any filter, allowing them to detect a broader range of gases without restriction. This type of sensor may be more sensitive to a range of combustible gases but is also more susceptible to interference from contaminants and cross-sensitivity issues.

Call Point Safety on 01704 330 315, or email [email protected] and our experts can help you identify the best sensor technology for you.

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