Explosimeters, flammable gas detectors, LEL measurement

Explosimeters are flammable gas detectors for leak detection of explosive gases such as natural gas, LPG (butane, propane), pentane, hydrogen, hydrocarbons, alcohols, solvents, ammonia, etc. They detect flammable gases and measure their quantity in the atmosphere according to the Lower Explosion Limit (LEL). There are two sensor technologies for explosive gas detection: catalytic sensors (the most commonly used) and infrared sensors for severe environmental conditions.

Explosimeters or flammable gas detectors (fixed or portable) come in two large categories depending on the cell technology used.

Explosive gas detectors with catalytic cells 

Catalytic diffusion cells are the most widely used devices for combustible gases and vapor detections. There is a correlation between the flammable gases presence and the temperature through the measurement principle. This temperature rise has been then transformed into a quantifiable quantity: the LEL (lower explosive limit). Although this measurement technology has an excellent price/performance ratio, it has some disadvantages:

  • It can only work with a minimum oxygen content of more than 10% (totally ineffective in an inert environment).
  • It does not discriminate the explosive gases present (same principle as the weighing scale: it gives the weight but does not say who is on the scale).
  • Premature wear in case of frequent or permanent detection of explosive gases (even when there are low LEL values).
  • High poison-sensitivity, such as silicone, lead, or sulfur compounds.

Flammable gas detectors with infrared cells 

Infrared absorption cells are used essentially for some explosive gas detection (when the IR curve is available). It is a more expensive technology than catalytic diffusion technology, but it has many real technical advantages:

  • Very long life.
  • They are not subject to the permanent presence of flammable gas or cell saturation (measurement above 100% of the LEL).
  • Immunity against poisons like silicates or H2S.
  • Possible operation in inert atmospheres (without oxygen).
  • The measurement is almost infallible since the detector systematically goes into alarm as soon as there is a failure

On the other hand, IR sensors do not work with monochromatic cells such as hydrogen.

ATEX Explosimeter

The ATEX approval (for ATmosphère EXplosive) indicates the capacity of the equipment to integrate a high explosive risk zone by not favoring the triggering of an explosion (flame, spark, etc.). An ATEX explosimeter (or ATEX detector for explosives) approved for use in an explosive atmosphere guarantees maximum safety for all teams working in a hazardous area.

It is a portable version of personal protective equipment used in many sectors of activity, such as the oil and gas industries, heavy chemicals, metallurgy, water treatment, and sanitation. This PPE is also used to ensure safety during entry into confined spaces (as part of the CATEC system) and in public works. It is also an integral part of firefighter equipment.

In parallel, it is applied in a fixed version connected to a gas detection controller or a safety automaton. The flammable gas transmitter allows for securing entire installations subject to this type of risk.

LEL of the most common flammable gases 

Lower explosive limit expressed as a percentage of the total volume (%/vol.).

Acetone: 2.5% - Acetylene: 2.5% - Ammonia: 15.0% - Benzene: 1.2% - Butane: 1.9% - Ethanol 3.3% - Hydrogen: 4.0% - Carbon monoxide: 12.5% - Methane: 5.0% - Methanol: 6.0% - Pentane: 1.4% - Propane: 2.1% - Styrene: 0.9% - Toluene: 1.1% - Xylene: 1.1%.

For benzene, styrene, toluene, and xylene, see also the page detection of volatile organic compounds.