In gas detection are generally used to detection centrals or autonomous detectors.

With the use of centrals with multiple detection zones, exists the possibility for each channel of the central to be programmed to detect more than one specific gas, depending on the detector that is used – this schedule will be made indicating the alarm levels (up to 3 levels) to each gas.

The gases to be detected are of three types (depending on the detectors used):

• Flammable gases
• Toxics gases
• Oxygen

The units of measure for detecting these types of gases are:

• ppm (parts per million) for toxic gases
• %LEL (Lower Explosive Limit) for flammable gases
• %O2 (oxygen %) for Oxygen

Flammable Gases

For the detection of flammable gases detectors should be installed as close as possible to the source that hypothetically will produces them or in places where it is most likely the accumulation of gas, so that the probability of detection is the highest possible.

For gases heavier than air detectors should be positioned closer to the ground and for the lighter gases closer to the ceiling.

Air density – 30

• Methane gas – 16 times lighter than air
• Propane gas – 44 times heavier than air
• Butane gas – 58 times heavier than air

Toxic Gases

Detection of toxic gases usually intended to protect a large area and people running around. So the best position for placement of detectors of toxic gases is at the height of the respiratory tract of an adult. As it’s difficult to establish an exact height and because the gases develop in “cloud” shape, toxic detectors should be installed between 1.20m and 1.40m

Oxygen

Monitoring the oxygen level in a particular environment can sometimes be essential and critical especially in industrial environments.

The normal concentration of oxygen in air is 20.9% by volume in dry conditions.

The lack of oxygen is a major cause of sudden death, but the enrichment of oxygen in the atmosphere can also cause the flammability of certain materials, reasons why its level should be checked to ensure that it remains within the permissible levels of safety.

Most common types of gases

Carbon monoxide

Carbon monoxide “CO” is a colourless, odourless and highly toxic gas which is produced by incomplete combustion. Can be found in automobile exhaust fumes, stoves, heating systems, fire and, include, in smoke cigarettes. Other sources include the exhaust systems of defective water heaters, gasoline/diesel heaters and gas stoves poorly maintained.

Carbon monoxide interferes with the ability of blood to carry oxygen. Symptoms of poisoning by this gas are usually headache, nausea, convulsions and finally death by asphyxiation.

People who are exposed to gases from cars, as underground car parks assistants and fire fighters, are at greater risk for CO poisoning.

Because CO is lighter than air, detectors should be positioned just above the level at which it is expected that gas is formed (between 1.20 m and 1.50 m).

The installation of detectors too high or too close to windows, vents or other ventilation zones can prevent the gas from reaching the detector.

Carbon dioxide

Although carbon dioxide is present in the atmosphere with a concentration of 300 ppm, CO2 is a toxic gas with a maximum level of security of 5000 ppm (0,5% of the air volume).

This gas is formed during combustion and fermentation processes and is, along with methane, a major component released during the process of water treatment, is also used in greenhouses to increase the growth of plants and vegetables.

This colourless gas is difficult to be measured at low concentration thus is usually measured by infrared systems.

It is a gas heavier than air, so it tends to concentrate near the ground and there is some risks in poorly ventilated and too low oxygen level places.

Oxygen

The normal concentration of oxygen in air is 20.9% by volume in dry conditions, however, this level decreases in industry manufacturing processes, where oxygen is consumed by corrosion processes or similar reactions, or because it is diluted with other gases. Under normal circumstances the human body has no breathing problems to a minimum level of 19.5% oxygen.

For this reason, the level should always be checked to never fall below 18-19%.

The lack of oxygen is a major cause of sudden death, however, there is another aspect that is not less dangerous: it is the enrichment of oxygen in the ambient atmosphere – the oxygen becomes toxic at high concentrations and increases the flammability of materials, especially when the concentration exceeds 24% – this usually happens in the metallurgical industry during the welding operations in closed rooms, in case of leakages of the gas cylinder of O2.

Butane

Butane, better known as “cooking gas”, is odourless, so for safety and to perceive its presence, it’s mixed with a substance with a specific smell: this is where people place the famous “it smells to gas”. This gas produces asphyxiation because it expels the oxygen out of the environment.

Another interesting feature of butane is the fact that, unlike most of the gases, its density is approximately twice the density of atmospheric air, so tends to concentrate on the bottom of containers where it is stored.

It’s an extremely flammable gas and may even form an explosive mixture of air/vapour at room temperature.

Spills or leaks of this gas generate large volumes of highly flammable vapour and inhalation of high concentrations of vapour, albeit brief, can cause unconsciousness or even be fatal.

Propane

Such as butane, this gas is generally used as fuel gas for industrial, commercial and domestic use.

It is an extremely flammable gas, and may even form an explosive mixture of air/vapour at room temperature.

Spills or leaks of this gas generate large volumes of highly flammable vapour, inhalation of high concentrations of vapours, albeit brief, may lead to unconsciousness or even be fatal.

This gas is also odourless and is usually added to the gas an odorant agent to assist in detecting leaks.

Ozone