Gas Detection

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Crowcon manufacture a complete range of both fixed and portable gas detection equipment for personal and plant protection against gas hazards across the globe. Based on a solid foundation of expertise in understanding of the core physics and chemistry of gas detection, Crowcon works closely with international bodies to ensure products are designed to industry leading equipment certification and performance standards.


The name gas comes from the word chaos, which neatly summarises the main feature of the simplest state of matter. A gas is a swarm of particles moving randomly and chaotically, constantly colliding with each other and the walls of any container. The real volume of the particles is minute compared to the total space which they occupy and this is why gases fill any available volume and are readily compressed (Confined space). The average speeds of gas molecules are of the order of 100s of metres per second and they are colliding with each other billions of times per second. This is why gases mix rapidly and why they exert pressure.

This constant motion is easily demonstrated by releasing a small amount of odorous gas in a room. Within seconds the gas can be smelt in all parts of the room. These properties apply to substances, which are normally gaseous, and to vapours from evaporated liquids.

A volume of any gas at the same temperature and pressure contains the same number of molecules irrespective of what the gas is. This means that measuring gas by volume is very convenient. Gas measurements at higher levels are in % (volume) and at lower levels parts per million, ppm (volume).

Whilst different gases have different densities, they do not totally separate into layers according to their density. Heavy gases tend to sink and light gases tend to rise, but their constant motion means that there is continuous mixing (i.e they do not behave like liquids). This is why it is imperative to locate your Crowcon gas detecting sensors at the right location depending on the gas required to detect.

So in a room where there is a natural gas (methane) leak, the gas will tend to rise because it is lighter than air but the constant motion means that there can be some concentration at floor level. This will happen in perfectly still conditions but if there are any air currents, the mixing will be increased.

Air is a mixture of gases, typically:-

  • Nitrogen 77.2%
  • Oxygen 20.9% (Standard that is set on all Gas Monitors)
  • Water Vapour 0.9%
  • Argon 0.9%
  • Carbon Dioxide 0.03%
  • Other Gases 0.07%

Because its composition is reasonably constant, air is usually considered as a single gas, which simplifies the measurement of toxic and flammable gases for safety and health applications. So for any assistance regarding the Crowcon Gas Detection family, please contact us at Cebeco on (02) 9651 4220 or email us at

Crowcon – Gas Detection


Combustion of Gases and Vapours

Most organic chemical compounds will burn. Burning is a simple chemical reaction in which oxygen from the atmosphere combines rapidly with a substance, producing heat. The simplest organic compounds are those known as hydrocarbons and these are the main constituents of crude oil/gas. They are composed of carbon and hydrogen, the simplest hydrocarbon being methane, each molecule of which consists of one carbon atom and four hydrogen atoms. It is the first compound in the family known as alkanes. The physical properties of alkanes change with increasing number of carbon atoms in the molecule, those with one to four being gases, those with five to ten being volatile liquids, those with 11 to 18 being fuel oils and those with 19 to 40 being lubricating oils. Longer carbon chain hydrocarbons are tars and waxes.

The first ten alkanes are:

  • CH4 Methane (gas) - C6H14 Hexane (liquid)
  • C2H6 Ethane (gas) - C7H16 Heptane (liquid)
  • C3H8 Propane (gas) - C8H18 Octane (liquid)
  • C4H10 Butane (gas) - C9H20 Nonane (liquid)
  • C5H12 Pentane (liquid) - C10H22 Decane (liquid)
  • This catalog has no sub-catalogs.

(Crowcon have a gas monitor to suit all applications)

The above compounds are all known as aliphatics. Alkenes are similar but their molecular structure includes double bonds. (Examples are ethylene and propylene.) Alkynes contain triple bonds. (Example is acetylene) Aromatic hydrocarbons such as benzene have a ring molecular structure and burn with a smoky flame).

When hydrocarbons burn they react with oxygen from the atmosphere to produce carbon dioxide and water (although if the combustion is incomplete because there is insufficient oxygen, carbon monoxide will result as well). More complex organic compounds contain elements such as oxygen, nitrogen, sulphur, chlorine, bromine or fluorine and if these burn, the products of combustion will include other compounds as well. For example substances containing sulphur such as oil or coal will result in sulphur dioxide whilst those containing chlorine such as methyl chloride or polyvinyl chloride (PVC) will result in hydrogen chloride.

In most industrial environments where gas detection is required, there is the risk of explosion or fire because of the presence of flammable gases or vapours, a mixture of compounds is likely to be encountered. In the petrochemical industry, the raw materials are a mixture of chemicals many of which are decomposing naturally or are being altered, by the processes. For example crude oil is 'cracked' to produce many simpler materials.


In order for gas to ignite there must be an ignition source typically a spark, or flame or hot surface. For ignition to take place there must be an explosive mixture. This means the concentration of gas or vapour in air must be at a level such that the 'fuel' and oxygen can react chemically.

The power of the explosion depends on the 'fuel' and its concentration in the atmosphere. This is why you would use a Crowcon Combustible Gas Monitor to ensure safe levels are kept to protect staff, equipment and the surrounding environment.

Not all concentrations of flammable gas or vapour in air will burn or explode.

The LOWER EXPLOSIVE LIMIT(LEL): The lowest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). The term is considered by many safety professionals to be the same as the lower flammable limit (LFL). At a concentration in air lower than the LEL, gas mixtures are "too lean" to burn. Portable gas detectors in Australia have set 5%LEL as its first alarm.

The UPPER EXPLOSIVE LIMIT (UEL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Concentrations higher than UFL or UEL are "too rich" to burn.

Flammable liquids generally have a low FLASH POINT. This is the lowest temperature at which vapour is given off at sufficient rate to form an explosive mixture with air. Liquids with flash points below normal ambient temperatures automatically release vapour in sufficient volume to provide an explosive mixture, so leakage of such liquids is potentially as dangerous as a flammable gas leak.

VAPOUR DENSITY is a measure of the density of a gas or vapour relative to air. Gases or vapours with a vapour density of less than one are lighter than air and they tend to rise from the point of escape and may therefore be readily dispersed (or they may be trapped at a higher level).

Gases or vapours with a vapour density of greater than one are heavier than air and tend to sink to lower levels and will spread around forming concentrations between the LEL and UEL. Such heavy gases can remain trapped for a long time in ducts, inspection pits, etc, ready to explode as soon as a source of ignition is introduced.

When an explosive mixture of gas or vapour and air has developed, it can be ignited either by a spark of sufficient energy or if it is exposed to a surface at a sufficiently high temperature. The lowest temperature, which will cause a mixture to burn or explode, is called the IGNITION TEMPERATURE.

So for any assistance regarding the Crowcon Gas Detection family, please contact us at Cebeco on (02) 9651 4220 or email us at