Sustaining life in hazardous environments...
Sustaining life in hazardous environments...

Introduction to Gas Detection

12th September 2016
Introduction to Gas Detection

Gas Detection equipment plays a vital role in keeping workers safe while entering hazardous environments every day.

Gas Detectors are used in a wide variety of applications from chemical handling and confined space entry through to laboratory and research work. Whilst they are often used for monitoring a process, in this article we will look at how they are used to protect personnel.

Gases are broken down into three broad categories of risk:

  • Explosive – risk of explosion / fire from flammable gases
  • Oxygen – oxygen displacement leading to asphyxiation, or oxygen enrichment which increases the risk of flammability
  • Toxic – risk of poisoning from a toxic gas

Explosive Gasses 

Explosive gases

These gases are ones that could potentially explode if they are present in the atmosphere and ignition occurs. For an explosion to occur under normal circumstances a flammable gas needs to be present in a concentration above its Lower Explosive Limit (LEL). Below this concentration the mixture is too lean to create an explosion. All flammable substances also have an Upper Explosive Limit (UEL) above which the mixture is too rich to ignite. As an example, Methane has the LEL of 5.0% Volume in Air and an UEL of 15.0% Volume in Air.

A gas detector that contains a sensor capable of detecting flammable gas will be set to detect one specific gas. It will then be set to alarm or provide a warning when it detects the substance at 5.0% of the LEL and then again at 10.0% of the LEL. For example, if a detector was set to pick up Methane and the first alarm was set at 20.0% of the LEL of 5.0%, this would equate to 1.0% Volume in Air. This means that the detection equipment is warning the user well in advance of them actually encountering the gas in sufficient quantities to create an explosion. 


Oxygen is essential for our existence and is found in the atmosphere at 20.9%.

There are two reasons why the oxygen content of an area is monitored - oxygen displacement or oxygen enrichment.

Oxygen displacement occurs when another gas, usually an inert (unreactive) gas, is present in significant quantities. It displaces the present atmosphere and proportionately depletes the oxygen content. A lower oxygen content initially causes lack of concentration and drowsiness, but as it falls below 16% there is an increased risk of loss of muscular coordination, and below 6% death is very likely.

Oxygen enrichment, however, where an increased level of oxygen is present, will increase the explosive force of many substances and in extreme circumstances can lead to auto-ignition.

Typical oxygen alarm levels will be at 19% or 19.5% for monitoring depletion, and 23% or 23.5% for monitoring enrichment

Toxic GasesToxic Gases

Many gases can be harmful to our health due to their toxicity. In the UK the limits of toxic substances are defined by EH40/2005 Workplace Exposure Limits which detail toxic substances and the limits within which they can be safely worked with. There are usually 2 defined amounts relating to each substance. The first is the amount that a person can safely be exposed to for an 8-hour period usually known as the Time Weighted Average amount, and the second is the Short Term Exposure Limit which is the maximum amount a person can safely be exposed to for 15 minutes.

A gas detector will typically be set up to alarm at these 2 levels, so that affected personnel know when they have reached a level of a substance where it is now contributing to their Time Weighted Average, and then again when it reaches a level where it is at such a high concentration that they must ensure they do not allow exposure to it beyond 15 minutes.

Understanding the basics of why we detect gases will help you in appreciating why we should use this safety equipment correctly and that it is set up to detect the correct gas in each given scenario.

We will be posting further blogs and videos relating to this subject – so if there is anything specific you would like to see in a future post please do let us know.


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