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The Invisible Hazard

Published: 11th Mar 2013

For more than seven decades canaries in cages were used in the UK to test the atmosphere in coal mines. Canaries were used because they are sensitive to toxic gases such as carbon monoxide, which to a human is not only colourless and tasteless, but also odourless.

After a fire or explosion occurred in a mine, the rescuers would go into the mine carrying a canary in a small cage. If the bird showed any signs of distress, this was an indication that the atmosphere was unsafe and that miners needed to be evacuated. The canary would show signs of distress even if small quantities of the gas were present.

The practise of using canaries as gas detectors was phased out in 1986. Now we rely on much more sophisticated and humane methods to test air quality.

What is a confined space?

Along with a mine being classed as a confined space, we can also include any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space. A confined space is therefore any place which is substantially enclosed, but not always entirely, and where serious injury can occur from hazardous substances or conditions within that space.

Hazards in confined spaces

The UK Confined Spaces Regulations, 1997, name a number of hazards which are unique to working in a confined space. These can arise due to the combination of the space being confined, and the presence of substances and or conditions within the area. These risks are as follows:

1. Fires and explosion

As well as the risk of explosions from the presence of flammable substances, there is an additional risk of fire and explosions from excessive oxygen in the atmosphere; for instance, an oxygen cylinder which forms part of welding equipment leaking oxygen into the space. Risk of explosion can also come from airborne flammable contaminants being ignited. Additionally, fire or an explosion can also occur due to leaks from adjoining plant or processes that have not been isolated sufficiently.

2. Suffocation

Certain processes can lead to a lack of oxygen within the confined space, which can then lead to suffocation. Some causes are as follows:

• Purging – the space is purged with an inert gas to remove flammable or toxic gas, fumes, vapours or aerosols

• Natural biological processes which consume oxygen as in wells, storage tanks, sewers and also the process of fermentation that is carried out in vessels

• Rust formation consumes oxygen, so steel vessels that have been completely closed for a period of time can be depleted of oxygen

• Burning operations, e.g. welding

• Displacement of air when using liquid nitrogen

• Depletion of oxygen from workers breathing, in addition to inadequate replacement of air

• Free-flowing solids, such as sugar, flour and sand can bury a person and cause suffocation

3. Drowning

Workers can be drowned from liquids that have entered the confined space. Additionally, other injuries can be sustained depending on the liquid’s toxicity, or if it has a corrosive nature.

4. Overheating

A worker can overheat, leading to a dangerous rise in core body temperature due to the presence of excessive heat in the confined space. This can occur in furnaces for example, or boilers where the area has not been allowed to cool down before the worker has entered to undertake the work. This can also be exacerbated by wearing personal protective equipment (PPE). Heat stress can also occur from a slower heat build up; both cases can lead to heat stroke and unconsciousness.

5. Toxic gas, fumes or vapours

Toxic gas, fumes or vapours can be produced in a number of ways, some of which now follow:

• Fumes may be left over from previous processes, previous storage, deposits that have been disturbed, and from adjoining plants that have not been effectively isolated

• Gas fumes can build up in sewers, manholes, contaminated ground, or a leak from behind vessel linings or bricks

• Activities being carried out can lead to fumes and vapours within the space: brush and spray painting, welding, flame cutting, lead lining and the use of solvents and adhesives

• Work being carried out or equipment being used outside the space that produces fumes or vapours can also leak into the space, e.g. hot work on exterior surfaces, or exhaust fumes from a mobile plant

Testing the atmosphere for hazards

Now let us pick up where the canaries left off – detecting gas in the atmosphere. The air quality of the confined space needs to be tested before entering, and also while it's being occupied. As well as ensuring that the level of oxygen is acceptable, the presence of combustible gases and toxic gases needs to be ascertained. Samples should be taken at the top, middle and bottom of the area – this takes into account the varying densities of gases and vapours, and any varying concentrations.

Atmospheric monitoring process

The flowchart below shows the sequence of gas monitoring. Gases should therefore be monitored in the following order: 1. Oxygen 2. Flammable gases 3. Toxic gases 1. Oxygen

The reason why this order is necessary is that the majority of flammable gas sensors use a Wheatstone bridge* which requires a normal oxygen content to produce an accurate reading.

Even though the oxygen sensor can be used in environments where the possible presence of specific gases has not been identified, there are some gases that can poison oxygen sensors.

The manufacturer’s instructions should provide details on which gases could be detrimental to the efficient operation of the oxygen sensor.

The acceptable limits of oxygen are between 19.5% and 23.5%. The level of 20.9% is usually the standard used, as below this means there is something happening in the space which warrants further evaluation.

2. Flammable gases

Sensors that detect flammable gases are usually broad-range instruments that do the job of detecting the presence of flammable gas, but do not identify which flammable gas it is. Due to the fact that the calibration gas will not necessarily be the same gas/vapour that has been detected in the confined space, then response factors must be taken into consideration.

3. Toxic gases

Toxic gas sensors are usually chemical specific and therefore require identification of the suspected toxic gas before selecting the monitoring equipment.

If the testing of the atmosphere raises cause for concern, then correct where possible, forbid entry, or use the appropriate PPE to protect the workers.

Selecting a Gas Detector

1. Transportable or portable detectors

Transportable or portable gas detectors are routinely used to monitor the atmosphere of a confined space. Transportable gas detectors are defined by the British standard BS EN 573 (BSI 1999) as apparatus which is not intended to be portable, but which can be readily moved from one place to another. These may stay in place for hours or days; for example, when a fixed gas detector is out of service.

Portable detectors can be small, handheld devices, larger portable devices or personal monitoring instruments. A handheld device can be used for leak seeking or spot checks, while larger portable devices can be used in a number of different ways, including leak seeking, spot checks and local area monitoring. There are also personal monitoring devices which are commonly used in occupational health surveillance.

When a detector is used temporarily in one place to monitor the atmosphere, this is known as passive monitoring. When the detector is carried by the worker to conduct spot checks, local area monitoring or perhaps to check a vessel entry, then this is known as active monitoring. Transportable or portable detectors can be used for the former, and portable for the latter.

For the purpose of this article the term portable gas detector will be used interchangeably to mean transportable and portable detectors.

2. The units of measurement used

Portable gas detectors measure the concentration of the gas at the sampling point of the instrument using the following units of measurement:

• % volume ratio

• % lower explosive limit (LEL) for flammable gas

• Parts per million (ppm) for low level concentrations or mg/m3

• Parts per billion (ppb)

• % v/v – volume per volume

To be able to set an appropriate alarm level the operator carrying out the test must understand the importance of these units.

3. Comparison of gas detectors

The table below shows some of the most commonly used gas detectors.

Testing the atmosphere

1. Remote Sampling

The equipment used for remote sampling includes pumps, hand aspirating bulbs, tubing and probes. They are available in various configurations to suit sampling requirements. The maximum draw distance and draw rates vary depending on the manufacturer. The actual maximum tubing allowed is usually 100ft or less, and draw rates are around 1ft/sec. To avoid the intake of dust and fluid a filtration device can be used with the unit.

A dilution tube is an accessory used with a catalytic bead sensor to sample flammable atmospheres in areas of reduced oxygen concentration. Air containing oxygen is allowed into the sample stream by the dilution tube in measured amounts; this allows an accurate flammable gas level to be calculated using a conversion factor.

2. Horizontal entry situations

When a confined space has a cover or door the atmospheric testing should begin outside the entryway before the space is opened. In this way, hazards in the area that may have an impact on the confined space once it is opened may be assessed. Additionally, this method can also pick up on any gases/vapours that may be seeping through the opening.

3. Vertical entry situations

Monitoring from outside is also necessary for vertical entry before opening the cover. In the case when the cover has an access hole at the top, the probe can be used for the initial monitoring inside the space. In the event that there is no hole, then the cover would need to be removed and the monitoring would begin at the opening.

Flexible tubing can also be used for remote sampling. In this case the tubing must be lowered slowly into the space to allow sampling at all levels.

Obstructions at a low level may inhibit monitoring. If this occurs and the tubing cannot be positioned on the other side of the obstruction from outside the space, then monitoring would have to be carried out on entering the space. An example of a space with such divisions would be baffles within a mixing drum.

The person carrying out the tests should be trained in the equipment they are using and trained in the interpretation of the results.


The detector will have an alarm on it to indicate the detection of the gas/vapour in question. This alarm may be audible and/or visual, e.g. a flashing red light. The alarm is usually located on the main body of the instrument for portable gas detectors. In the case of passive monitoring it is essential that the instrument is located in such a way so the worker can definitely hear and see the alarm.

It is also essential for the detector to have an alarm to warn of a fault condition. It is possible for some detectors to fail in such a way that they falsely indicate a safe condition, when in fact this is not the case. Additionally, portable gas detectors will usually have a low battery alarm and where appropriate, a low fuel (hydrogen) alarm. These should be clearly distinguishable from the alarm to indicate an unsafe condition.

When detecting gases the alarm should be set at a level which is low enough to ensure the safety of workers but high enough to prevent insignificant activation. The manufacturer’s recommendations should be taken into account when setting limits.

Activation of alarms

There should be an emergency procedure in place in the event that the alarm is activated to warn of an unsafe atmosphere. It is essential that this procedure sets out clear instructions for appropriate personnel and also that it is part of the safety management system. Additionally, it should have been communicated to all necessary personnel and followed through with training and refresher courses.

Calibration and maintenance

All portable detectors used in confined spaces will need to be calibrated every time they are used – this being before as well as after.

It is essential that a maintenance schedule is set up for detection equipment. The frequency will depend on the type of detector and operating conditions; the manufacturer’s instructions should be consulted for guidance on setting this schedule.


One of the main risks with working in a confined space is the atmosphere. The air quality needs to be tested to ensure the hazard of toxic and explosive gases is controlled. Additionally, oxygen levels that are too high, or borderline acceptable, will bring about further risks that need to be controlled. Stringent risk management can reduce the risk to an acceptable level, and therefore keep workers safe.

Published: 11th Mar 2013 in Health and Safety Middle East

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Sharon Cave