Explosimeter: an essential tool

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What is an explosion?

A explosion is characterized by the presence of a fuel mixed with an oxidizer, in given proportions, which in the presence of a source of heat or ignition, will be consumed very quickly.

In order not to be exposed to this type of reaction, firefighters are equipped with this essential tool for their safety during interventions: the explosimeter Where multigas detector.

The explosimeter or multigas detector

This device measures the presence of explosive gases or from combustible vapors in the air (such as natural gas, butane, propane, hydrocarbons, solvents or alcohols) before the mixture becomes explosive. The risk of explosion is assessed on the basis of the LIE (Lower Explosive Limit) and some LSE (Upper Explosive Limit). These are limits beyond which the gas concentration in the air is low or high enough not to cause a reaction.

The majority of devices in service are catalytic oxidation explosimeters. They give a measure of the concentration of a flammable gas between 0 and 100% of the LIE.

Multigas detectors are calibrated on a reference gas, having specific explosive limits. In order to know the value of a gas other than the calibration gas, there are conversion curves.

It is important to know that each detector model has its own curves.

Take the example of an explosimeter calibrated on pentane.

In this case, when we measure pentane and the device displays 100%, we are at 100% of the LEL of the Pentane. For the same type of explosimeter, when measuring methane, if the device displays 44%, the LEL of the methane is 22%. To know the share of methane on this explosimeter calibrated on pentane, the formula is as follows: Displayed value x 0.5.

To use an explosimeter, you must perform a zero measurement in healthy zone and not in the presence of a flammable gas. This step is crucial, because it can then lead to negative values ​​and a underestimation of risk.

In addition, other factors are likely to influence the operation of the explosimeter and lead to incorrect answers:

Ambient humidity

Electromagnetic waves

High or very low temperatures

Silicone, lead or sulfur substances

Finally, the explosimeter does not detect the explosive risk associated with dust, it is even the opposite, because they clog the filter of the device. A dust explosion can occur when the five elements of the explosion pentagon are present (an oxidizer, a heat source, a fuel, a dispersion of dust, all in an enclosed space). Most dust is of organic origin like grains, flour, wood, etc.

In such an explosion, the first explosion Where primary explosion stirs up combustible wood dust accumulated on surrounding surfaces. When this dust ignites, it is a secondary explosion. Due to the greater amount of dust, the secondary explosion is often more powerful than the primary.