Focus

The interest of the explosimeter for firemen

Date de l'actualité
2022-10-25

The Departmental Fire and Rescue Service (SDIS) has an important role in protecting people, property and the environment. Whether it is for the organization of the rescue means, the emergency rescue or the fire management, it must protect its teams during the interventions.

In order not to be exposed to certain risks, SDIS firefighters are equipped with an essential tool for their safety during interventions: the explosimeter.

1. What is an explosion?

The explosion is due to the presence of a fuel (flammable substance such as paper, wood, gasoline, gas, etc.) mixed with an oxidizer (oxygen of the air for example) in given proportions, which in the presence of a source of heat (source of ignition such as electricity, a cigarette, etc.), is going to be consumed very quickly and to cause ignition and/or explosion.

The association of these three elements constitutes the triangle of fire:

Some definitions :

- Flash point: temperature above which a liquid can ignite on contact with a heat source

- Ignition point: temperature at which a liquid compound emits enough vapour to form a mixture with air, the combustion of which is self-sustaining once it has started (flame, spark, etc.)

- Self-ignition point: lowest temperature at which a gaseous mixture (air-vapour) ignites spontaneously in the absence of an ignition source

- Activation energy: minimum amount of energy required to initiate a chemical reaction. When a flammable mixture is not at its auto-ignition temperature, an external energy called activation energy is necessary to start the combustion.

2. Ignition or explosive limits

The risk of explosion is evaluated according to the LEL (Lower Explosive Limit) and the UEL (Upper Explosive Limit). These are limits beyond which the concentration of gas in the air is sufficiently low or high not to cause a reaction.

> Lower Ignition or Explosive Limit (LEL or UEL): The LEL of a gas or vapor is the minimum concentration above which it can be ignited.

> Upper Explosive Limit (UEL): The UEL of a gas or vapor in air is the maximum concentration below which it can be ignited.

> Ignition zone (I.Z.): It is the dangerous zone, it is the range of compositions between the LEL and UEL.

3. What is an explosimeter?

3.a. Operation

There are three detection technologies for an explosimeter:

> Infrared Explosimeter: Infrared (IR) explosimeter values are expressed in % of air volume, not % LEL. IR sensors have a longer lifetime than catalytic ones, but are also more expensive. The measurement of an IR explosimeter is safe since in case of failure the detector automatically goes into alarm. However, an IR sensor does not work with hydrogen.

> The catalytic oxidation explosimeter: This is the most widespread and works as follows: thanks to an electric current, a filament is heated in a combustion chamber. When a gas enters this combustion chamber, an oxidation is triggered. The filament will heat up to a high temperature, and the electrical resistance of the filament changes the value of the electrical current. This type of explosimeter will therefore measure the quantity of gas present between 0 and 100% of the LEL.

> The MPS cell explosimeter: Innovative technology, the MPS sensor is safe, stable, resistant to poisoning and can accurately detect more than 14 gases, from a single calibration. There is no risk of false alarms or non-alarms.

"With the MPS cell, workers will no longer have to question their combustible gas sensor readings due to calibration for one gas and exposure to another. They will no longer face the risk of sensor poisoning. The MPS flammable gas sensor is the solution to the shortcomings of traditional lower explosive limit (LEL) sensors, such as false alarms, inaccurate readings, and limited ability to accurately detect multiple gases. Unlike traditional LEL gas detectors, the MPS sensor detects a dozen combustible gases in any environment."

Blackline Safety

3.b. Calibration

Generally, for an explosimeter with catalytic oxidation, i.e. having a catalytic sensor, it is a calibration with methane which is carried out. In addition, the alarm threshold is lowered for the detection of explosive gases and vapors.

Explosive sensors are generally set to two alarm thresholds: the first corresponding to 20% of the LEL, and the second to 40% of the LEL. However, if a calibrated methane detector is set to 10 and 20% LEL, the alarms are triggered earlier, but this does not affect the sensitivity of the explosimeter.

3.c. Precautions in use

When calibrating or using an explosimeter, several risks may arise. The necessary precautions must be taken to avoid altering the operation of the device and therefore its effectiveness.

Several factors are likely to influence the operation of the explosimeter and lead to erroneous responses:

Ambient humidity
Electromagnetic waves
High or very low temperatures
Silicone, lead or sulfur-containing substances

Caution! An explosimeter operating by catalytic oxidation does not indicate the presence of combustible gases in an inert environment, and therefore should not be used in an O₂ enriched atmosphere.

Did you know that?

TWENTY SECONDS is the time required for your explosimeter to react to flammable gas in the atmosphere. This reaction time depends on the gas concentration and the model of the explosimeter.

A firefighter's equipment and clothing identifies him or her as a firefighter. Uniforms, badges and equipment are an integral part of the firefighter's outfit and allow him to be recognized at first glance. But do you really know what a firefighter's equipment is? What personal protective equipment do they need?

The Departmental Fire and Rescue Services (SDIS), in other words the firefighters, encounter multiple problems in intervention, generally related to gases. Therefore, portable gas detection and respiratory protection are essential equipment for their interventions.

What protection against chemical risks?

During a fire, for example, firefighters are exposed to many chemical risks, generally related to the presence or lack of a gas.

Depending on the fuel (materials, chemicals, etc.), the toxicity of the smoke will be more or less important. The heat released by the fire as well as the quantity of oxygen will increase or decrease the risk.

It is also important to know that the oxygen present in the air tends to become rarified during a fire. So hypoxia, which is the lack of oxygen, can be a significant risk for firefighters. Indeed, it can lead to confusion or an inability to move, and therefore to leave the area in case of danger.

The presence of these risks also depends on the firefighter's missions. Between a firefighter entering a burning building or a firefighter responding to clean up the area after the fire is out, the presence of risk is different.

Portable gas detection

The firefighters' intervention bag usually includes an explosimeter, as well as a carbon monoxide (CO) detector to prevent the risk of carbon monoxide poisoning, the leading cause of death by poisoning in France.

Explosimeters or 4-gas detectors allow to measure, in addition to carbon monoxide, the risks related to the presence of fuels in the atmosphere, the O₂ content of the air for example or the H₂S concentration.

Portable gas detection, in general, but first and foremost the explosimeter, is essential for the protection of firefighters during intervention.

Respiratory protection

The Self Contained Breathing Apparatus, or SCBA, is the basic respiratory protection equipment for firefighters. It protects them from toxic gases and harmful particles that may be present during a fire.

The SCBA has a significant advantage for firefighters during interventions: its mobility. This equipment can be worn by the user himself, without hindering his movements and his work.

The purpose of SCBA is to create and maintain a breathable atmosphere, properly isolated from potentially toxic outside air. Non-breathable atmospheres can be the following: fire fumes, toxic sprays or atmospheres. The SCBA is therefore a Personal Protective Equipment that allows firefighters to protect themselves from the risks of a hazardous atmosphere.

A SCBA is, at least, composed as follows