Breathing apparatus & safety equipment

A breathing apparatus is a respiratory device worn to supply compressed air to the user. Breathing apparatuses are typically used by people exposed to hazardous atmospheric air or a lack of oxygen.

A self-contained breathing apparatus (SCBA), is sometimes referred to as a compressed air breathing apparatus (CABA) or a breathing apparatus (BA). A SCBA is a device worn to provide breathable air in an atmosphere that is immediately dangerous to life or health. They are typically used in the firefighting industry. The term self-contained means that the SCBA is not dependent on a remote supply of breathing gas (e.g., through a long hose). If designed for use underwater, it is also known as a scuba set (self-contained underwater breathing apparatus). When not used underwater, they are sometimes called industrial breathing sets.

A SCBA typically has 3 main components: a high-pressure cylinder (e.g., 207bar to 300bar), a pressure reducer, and an inhalation demand valve (including facemask), connected together and mounted to a carrying frame.

A SCBA may fall into 1 of 2 categories: open-circuit or closed-circuit.

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The closed-circuit type, also known as a rebreather, operates by filtering, supplementing, and recirculating exhaled gas. It is used when a longer-duration supply of breathing gas is needed, such as in mine rescues, in long tunnels, or when going through passages too narrow for a big open-circuit air cylinder.


Open-circuit industrial breathing sets are filled with filtered, compressed air, rather than pure oxygen. Typical open-circuit systems have 2 regulators. The first stage regulator reduces the pressure of air to allow it to be carried to the mask, and the second stage regulator reduces it even further to a level just above standard atmospheric pressure. This air is then fed to the mask via either a demand valve (activating only on inhalation) or a continuous positive pressure valve (providing constant airflow to the mask).

An open-circuit rescue or firefighter SCBA has a full face mask, regulator, air cylinder, cylinder pressure gauge, remote pressure gauge (sometimes with an integrated PASS device). It also has a harness with adjustable shoulder straps and waist belt which allows it to be worn on the back. The air cylinder usually comes in three standard sizes: 4 litres, 6.8 litres and 9 litres. The actual usable time that the SCBA can provide air varies, depending on the relative fitness of the user, and especially the level of exertion of the wearer. These factors often reduce the calculated working time by 25 to 50%.

Air cylinders

Air cylinders are made of aluminium, steel, or of a composite construction FRP Hoop Overwrapped or more recently, Carbon Fibre Fully Wrapped. The composite cylinders are the lightest in weight and preferred by users around the world. However they also have the shortest lifespan and must be taken out of service after 15 years. Air cylinders must be hydrostatically tested every 5 years. During extended operations, empty air cylinders can be quickly replaced with fresh ones and then refilled from larger bank cylinders in a cascade storage system or from an air compressor located on site.

Newer models of composite cylinders now have up to a 30-year life but there is a trade-off in that these cylinders are quite a bit heavier than their 15-year life cousins.

Positive versus negative pressure

Open circuit SCBAs use either ‘positive pressure’ or ‘negative pressure’ operations.

A negative pressure system relies on the internal pressure of the mask dropping to below the ambient pressure to activate flow. If the mask does not seal correctly, it may cause some leakage of ambient gas into the mask, which can be problematic with toxic or irritant smoke and fumes.

A positive pressure system slightly pressurises the interior of the mask and activates flow when the pressure difference is reduced, but still above ambient. If the mask leaks, there will be continuous flow to maintain the pressure, and no inward leakage is possible. With a good fit this is economical on gas and prevents contamination. If the mask falls off, the regulator will continuously expend gas trying to raise the pressure and may consume a significant amount of gas before it is corrected.

Although the performance of both types of SCBA may be similar under optimum conditions, this ‘fail-safe’ behaviour makes a positive pressure SCBA preferable for most applications. This is because there is usually no air usage penalty in providing positive pressure.