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A Quick Guide to Atmospheric Testing in Confined Spaces


A 'confined space' is defined as a space which is enclosed by its nature and possesses the risk of death or serious injury from hazardous substances or dangerous conditions. By this definition a wide net is cast, with many obvious and some not so obvious examples e.g.

  • storage tanks

  • reaction vessels

  • enclosed drains

  • open-topped chambers

  • combustion chambers in furnaces

  • or even, poorly ventilated rooms

Some places may become confined when work is carried out or during fabrication, hence, vigilance is always necessary and dynamic assessment of risk always to be carried out.

In all circumstances you must carry out a suitable and sufficient assessment of the risks and decide on measures for safe working in compliance with the Management of Health and Safety at Work Regulations 1999, regulation. Most risk assessments will require the assessment and consideration of:

  • task

  • working environment

  • working materials and tools

  • suitability of those carrying out the task

  • arrangements for emergency rescue

If the assessment highlights potential for serious injury from work in confined spaces the Confined Spaces Regulations 1997 apply. Under these regulation key duties are imposed but not limited to:

  • avoid entry to confined spaces

  • if entry to a confined space is unavoidable, follow a safe system of work

  • put in place adequate emergency arrangements before the work starts

These are detailed extensively in Confined Spaces Regulations 1997

Atmospheric Hazards in Confined Spaces

Atmospheric hazards in a confined space are those that expose entrants to a risk such as death, entrapment, injury, or acute illness from one or more of the following causes:


An atmospheric oxygen concentration below 19.5% (oxygen deficiency), or above 23.5% (oxygen enrichment).

Combustible Gases

A flammable gas or vapor in excess of 10% of its lower explosive limit (LEL) yet still remaining below the upper explosive limit UEL). Lower Explosive Limit (LEL) Vs. Upper Explosive Limit (UEL)

  • The lowest concentration (air-fuel mixture) at which a gas can ignite is called lower explosive limit (LEL). Concentrations below this limit are too lean to burn.

  • The highest concentration that can be ignited is its upper explosive limit (UEL). Above that concentration, the mixture is too rich to burn.

RAE Systems’ one- to five-gas monitors read out in both % LEL and % by volume. For example the LEL of methane is 5% by volume, and the UEL is 15% by volume. When a confined space reaches 2.5% methane by volume this would be equal to 50% LEL. (5% methane by volume would be 100% LEL.) Between 5 to 15% by volume, a spark could cause an explosion. Different gases have different % by volume concentrations to reach 100% LEL. Some examples are: Propane’s LEL is 2.1% by volume; Pentane’s LEL is 1.5% by volume; Hexane’s LEL is 1.1% by volume and gasoline’s LEL is 1.3% by volume.

Toxic Gases

An atmospheric concentration of any toxic compound above the permissible exposure limit established by OSHA, NIOSH, ACGIH or HSE EH40. Common toxic compounds include, VOCs, carbon monoxide, ammonia, chlorine, hydrogen sulfide, sulphur dioxide, nitric oxide etc.

Monitoring a Confined Space Atmosphere

Monitoring the air inside a confined space is required prior to entering. Testing a confined space for atmospheric hazards should be done remotely before entering, and should be done in this order:

  • Oxygen. Ensure that proper oxygen levels are present.

  • Combustible gases. Ensure that combustible gases are not present.

  • Toxic Gases. Ensure that toxic gases are below the OSHA, HIOSH, ACGIH or EH40 permissible exposure limit. Common toxic gases in a confined space could be hydrogen sulfide (H2S) and carbon monoxide (CO), but other toxic compounds could be present. RAE Systems offers monitors to evaluate all these gases either individually or simultaneously.

  • VOC. VOC exposure is one of the most overlooked hazards in confined space entry. Volatile organic compounds (VOCs) are organic compounds evaporate easily at room temperature. Some familiar substances containing VOCs are vapours associated with fuels such as petrol, diesel, heating oil, kerosene, jet fuel, benzene, butadiene, hexane, toluene, xylene, and many others. All of these are potentially very harmful, and if present should be monitored to ensure that they are below safe limits.

In a confined space, it is important to take samples at the top, middle, and bottom to locate varying concentrations of gases and vapours. Highly concentrated gases can accumulate at the top or bottom of a confined space depending on whether they are less or more dense than air. Dilute gases and vapours in the ppm range distribute evenly throughout a confined space. It is especially important to sample at a distance from the opening because air intrusion near the entrance can give a false sense of adequate oxygen presence.

As the remote air monitoring is completed and the area is safe for entry, confined space entry permits should be completed and followed. After the initial entry, monitoring the air in the confined space should be continuous. A confined space entry attendant or hole watch should carry out the continuous monitoring. Conditions in a confined space can change without warning, due to leakage, toxic vapour release, or disturbing the contents of the space.

Monitors for Confined Space Entry

RAE Systems offers the latest and most innovative monitors available for Confined Space Entry (CSE). Datalogging is an option on all our multi-gas monitors.

The Q-RAE CSE Monitor is pre-set as an easy to use tool to comply with the atmospheric testing required. The QRAE3 offers the flexibility to work effectively in a confined space while remaining protected from combustibles, a number of application specific toxic gases inlcuding ammonia, chlorine, sulphur dioxide, hydrogen sulfide, carbon monoxide, and high or low oxygen levels.

The MultiRAE is an excellent personal protection monitor. It combines the traditional security of standard electrochemical and catalytic sensors with the next generation, broad-band protection of a photoionization detector (PID). Unlike the QRAE, this monitor offers a wide variety of interchangeable toxic sensors supplementable with a PID or CO2 sensor for applications such as confined space entry.

If you would like to know more about this application or require any assistance with monitoring technology please feel free to contact Environmental Science & Technology Ltd, on +44(0)1904 373 018 or

Disclaimer: This article contains only a general description of atmospheric testing in a confined space and equipment used to monitor a confined space. Under no circumstances should a confined space be entered or monitoring equipment used except by qualified and trained personnel, and after all instructions have been carefully read and understood and all precautions followed.

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