Flammable gases and vapors in industrial or confined settings pose a significant safety hazard, as uncontrolled ignition can lead to catastrophic fires or explosions. Effective monitoring requires a specialized instrument capable of accurately detecting concentrations before they reach dangerous levels. This measurement focuses specifically on the potential for combustion when substances are mixed with air, rather than toxicity. This article details the specific instrument designed for this purpose and explains the scientific principles behind its crucial measurement scale.
The Device Used to Measure Flammable Vapors
The instrument designed to measure the concentration of flammable vapors or gases in the air is known as a Combustible Gas Detector, often called an Explosimeter or LEL meter. Unlike instruments measuring toxic substances in parts per million (PPM), this device measures concentration as a percentage of its combustion potential. The resulting reading is displayed as a percentage of the Lower Explosive Limit, or % LEL.
This measurement unit is tailored to safety protocols, providing users with a direct indication of how close the atmosphere is to becoming a fire or explosion risk. The detector provides an early warning, triggering an alarm when the atmosphere approaches the minimum concentration required for ignition. Standard safety practice typically sets the first alarm at 10% to 20% of the LEL to allow time for ventilation or evacuation before a hazardous condition is fully realized.
Understanding the Explosive Limit Scale
For a flammable gas or vapor to ignite, its concentration must fall within the flammable or explosive range. This range is bounded by two distinct points: the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL). Combustion requires three elements: a fuel, an oxidizer (typically oxygen in the air), and a temperature high enough to sustain a reaction.
The Lower Explosive Limit (LEL) is the minimum concentration of the gas or vapor in the air that will support a flash of fire. Below the LEL, the mixture is considered “too lean” because there is insufficient fuel relative to oxygen to sustain combustion. For example, the LEL for methane is approximately 5.0% by volume in air.
The Upper Explosive Limit (UEL) is the maximum concentration above which ignition cannot occur. A concentration above the UEL is considered “too rich” because there is too much fuel and not enough oxygen to support the rapid chemical reaction of combustion. The explosive range is the span of concentrations between the LEL and the UEL.
Detectors are calibrated to measure from zero up to 100% of the LEL, which represents the point of immediate danger. Displaying the concentration as a percentage of the LEL standardizes the risk across different gases. A reading of 50% LEL for any gas means the atmosphere contains half the concentration necessary for ignition, simplifying the interpretation of the hazard for safety personnel.
How the Concentration Sensor Technology Works
The most common mechanism used in combustible gas detectors is the catalytic bead sensor, often called a pellistor. This sensor converts vapor concentration into a measurable electrical signal using a Wheatstone bridge circuit. The circuit contains two small ceramic beads, each wrapped in a fine platinum wire coil. One bead is coated with a catalyst, making it the active or detector bead, while the other is untreated and serves as a reference or compensator bead.
When flammable gas enters the sensor housing, the catalyst on the active bead promotes a flameless combustion reaction. This controlled burning rapidly increases the bead’s temperature. The temperature increase, in turn, causes a proportional increase in the electrical resistance of the platinum wire coil within the active bead.
The reference bead does not react with the gas, and its stable resistance compensates for changes in ambient temperature and humidity. The difference in resistance between the two beads creates an electrical imbalance in the Wheatstone bridge circuit. This voltage change is directly measured and translated by the detector’s electronics into the final % LEL reading.
Infrared (IR) Sensors
An alternative technology is the infrared (IR) sensor, which measures gas concentration by detecting the absorption of specific wavelengths of infrared light by the gas molecules. This method offers the benefit of not requiring oxygen to operate, unlike the catalytic bead sensor, which needs a minimum oxygen level to support the combustion reaction. However, the catalytic bead sensor remains the most widely deployed technology for general-purpose LEL measurement due to its versatility and ability to detect a broad spectrum of flammable gases.