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A self-contained breathing apparatus (SCBA), sometimes referred to as a compressed air breathing apparatus (CABA) or simply breathing apparatus (BA), is a device worn to provide breathable air in an atmosphere that is
immediately dangerous to life or health The term immediately dangerous to life or health (IDLH) is defined by the US National Institute for Occupational Safety and Health (NIOSH) as exposure to airborne contaminants that is "likely to cause death or immediate or delayed permanent advers ...
. They are typically used in firefighting and industry. The term ''self-contained'' means that the SCBA is not dependent on a remote supply of
breathing gas A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed ...
(e.g., through a long hose). If designed for use under water, it is also known as a
Scuba set A scuba set, originally just scuba, is any breathing apparatus that is entirely carried by an underwater diver and provides the diver with breathing gas at the ambient pressure. ''Scuba'' is an anacronym for self-contained underwater breathin ...
(self-contained underwater breathing apparatus). When not used underwater, they are sometimes called industrial breathing sets. Unofficial names include ''air pack'', ''air tank'', ''oxygen cylinder'' or simply ''pack'', which are mostly used in firefighting. An SCBA typically has three main components: a high-pressure tank (e.g., , about 150 to 374 atmospheres), a pressure regulator, and an inhalation connection (mouthpiece, mouth mask or face mask), connected together and mounted to a carrying frame. A self-contained breathing apparatus may fall into one of two categories: open-circuit or closed-circuit.


Types


Closed-circuit

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 rescue Mine rescue or mines rescue is the specialised job of rescuing miners and others who have become trapped or injured in underground mines because of mining accidents, roof falls or floods and disasters such as explosions. Background Mining ...
and in long
tunnel A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...
s, and going through passages too narrow for a big open-circuit air cylinder. Before open-circuit SCBA's were developed, most industrial breathing sets were
rebreather A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's breathing, exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. ...
s, such as the Siebe Gorman Proto, Siebe Gorman Savox, or Siebe Gorman Salvus. An example of modern rebreather SCBAs would be the
SEFA The SEFA is a make of backpack industrial breathing set formerly made by Sabre Safety. It is an oxygen rebreather. "SEFA" is an acronym for "selected elevated flow apparatus". Description It is in a shiny backpack stainless steel sheet casing w ...
.


Open-circuit

Open-circuit industrial breathing sets are filled with filtered, compressed air, rather than pure oxygen. Typical open-circuit systems have two regulators; a first stage to reduce the pressure of air to allow it to be carried to the mask, and a second stage regulator to reduce 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 fullface mask, regulator, air cylinder, cylinder pressure gauge, remote pressure gauge (sometimes with an integrated PASS device), and a harness with adjustable shoulder straps and waist belt which lets it be worn on the back. The air cylinder usually comes in one of three standard sizes: 4 liter, 6 liter, or 6.8 liter. The duration of the cylinder can be calculated with this formula: volume (in liters) × pressure (in bars) / 40 - 10 in minutes (the 10 is subtracted to provide a safety margin), so a 6-liter cylinder, of 300 bar, is 6 × 300 / 40 - 10 = 35 minutes working duration. The relative fitness, and especially the level of exertion of the wearer, often results in variations of the actual usable time that the SCBA can provide air, often reducing the working time by 25% to 50%. Air cylinders are made of
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, steel, or of a composite construction (usually
carbon-fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
wrapped.) The composite cylinders are the lightest in weight and are therefore preferred by fire departments (UK: fire and rescue services previously called
fire brigade A fire department (American English) or fire brigade (Commonwealth English), also known as a fire authority, fire district, fire and rescue, or fire service in some areas, is an organization that provides fire prevention and fire suppression se ...
s), but 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 tanks in a cascade storage system or from an
air compressor An air compressor is a pneumatic device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces m ...
brought to the scene.


Positive versus negative pressure

Open circuit SCBAs use either "positive pressure" or "negative pressure" operation. 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 perfectly, some leakage of ambient gas into the mask will occur, which can be a problem 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. As there is usually no air usage penalty in providing positive pressure, the older negative pressure type is, in most cases, an obsolete configuration and is only seen with older equipment. However some users refuse to use this technology as in case of a damage or loss of the facepiece the air will be released uncontrolled. The leakage rate can be so high that a fully charged SCBA will be drained in less than three minutes, a problem that does not happen with negative pressure SCBA systems.


Masks

The fullface masks of breathing apparatus designed for use out of water are sometimes designed in a way that makes them unsuitable for
scuba diving Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for " Self-Contained Underwater Breathing Apparatus", was coined by Chr ...
, although some may allow very shallow emergency submersion: * The seal at the edge of the mask is a wide tube with thin, flexible walls running around the edge of the mask, full of air at atmospheric pressure. On the surface it pushes against the edges of the wearer's face, causing a tight seal despite small variations in head shape. At more than a few feet depth pressure (underwater or in a caisson) this tube collapses, destroying the seal and making the mask leak. * Curved window which underwater would severely distort the image by
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
. The mask can have a large viewport, or small eye lenses. The mask might have a small orinasal breathing mask inside, reducing breathing deadspace. The mask can also incorporate a
two-way radio A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirection ...
communicator. Some early industrial rebreathers (e.g., the Siebe Gorman Proto) had a mouthpiece and attached
noseclip A noseclip or nose clip is a device designed to hold the nostrils closed to prevent water from entering, or air from escaping, by people during aquatic activities such as kayaking, freediving, Pelizzari, Umberto & Tovaglieri, Stefano (2001) ''‘ ...
instead of a mask.


Usage

There are two major application areas for SCBA: firefighting and industrial use. A third use now coming into practice is medical; for example, the American National Institutes of Health prescribe use of SCBAs for medical staff during treatment of ebola. For firefighting, the design emphasis is on heat and flame resistance above cost. SCBAs designed for firefighting tend to be expensive because of the exotic materials used to provide the flame resistance, and to a lesser extent, to reduce the weight penalty on the firefighter. In addition, modern firefighting SCBAs incorporate a
PASS device A PASS device (Personal Alert Safety System) also known as a Distress Signal Unit (DSU) or ADSU (Automatic Distress Signal Unit), is a personal safety device used primarily by firefighters entering a hazardous or Immediately Dangerous to Life an ...
(personal alert safety system) or an ADSU (automatic distress signal unit) into their design. These units emit distinctive, high-pitched alarm tones to help locate firefighters in distress by automatically activating if movement is not sensed for a certain length of time (typically between 15 and 30 seconds), also allowing for manual activation should the need arise. In firefighting use, the layout of this breathing set should not interfere with ability to carry a rescued person over the firefighter's shoulders. The other major application is for industrial users of various types. Historically, mining was an important area, and in Europe this is still reflected by limitations on use in the construction of SCBAs of metals that can cause sparks. Other important users are
petrochemical Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable so ...
, chemical, and nuclear industries. The design emphasis for industrial users depends on the precise application and extends from the bottom end which is cost critical, to the most severe environments where the SCBA is one part of an integrated protective environment which includes gas-tight suits for whole-body protection and ease of decontamination. Industrial users will often be supplied with air via an air line, and only carry compressed air for escape or decontamination purposes.


Temperature effect on pressure

The pressure gauge's indicated gas pressure changes with ambient temperature. As temperature decreases, the pressure inside the cylinder decreases. The relationship between the temperature and the pressure of a gas is defined by the formula ''PV'' = ''nRT''. (See
Universal gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...
.) What is particularly important to understand from the formula is that the temperature is in
Kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
s, not degrees Fahrenheit. Consider the freezing point of water at 32° F (0° C, 273.15° K) and compare it to 96° F (35.6° C or 308.71° K; normal human body temperature is 37° C). While 96 is arithmetically three times 32, the difference in temperature from a scientific point of view is not threefold. Instead of comparing 32° F to 96° F, temperatures of 273.15° K and 308.71° K should be compared. The scientifically valid change in temperature from is by a factor of 1.13 (308.71 K/273.15 K), not 3. If an air cylinder is pressurized to 4,500 psi at 96° F and later the temperature drops to 32° F, the pressure gauge will indicate 4,000 psi (4,500/1.13). Stated differently, a drop in temperature of 10° F (5.5° C) causes a pressure decrease of about 82 psi (565 kPa). Failure to accurately account for the effect of temperature on pressure readings can result in underfilled air cylinders, which in turn could lead to a firefighter running out of air prematurely.


Regulation

In the United States and Canada, SCBAs used in firefighting must meet guidelines established by the National Fire Protection Association, NFPA Standard 1981. If an SCBA is labeled as "1981 NFPA compliant", it is designed for firefighting. The current version of the standard was published in 2018. These standards are revised every five years. Similarly, the
National Institute for Occupational Safety and Health The National Institute for Occupational Safety and Health (NIOSH, ) is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. NIOSH is part of the C ...
(NIOSH) has a certification program for SCBA that are intended to be used in chemical, biological, radiological, and nuclear (CBRN) environments. Any SCBA supplied for use in Europe must comply with the requirements of the Personal Protective Equipment Directive (89/686/EEC). In practice this usually means that the SCBA must comply with the requirements of the European Standard EN 137:2006. This includes detailed requirements for the performance of the SCBA, the marking required, and the information to be provided to the user. Two classes of SCBA are recognised, Type 1 for industrial use and Type 2 for firefighting. Any SCBA conforming to this standard will have been verified to reliably operate and protect the user from -30 °C to +60 °C under a wide range of severe simulated operational conditions. The Royal Australian Navy uses the open circuit compressed air breathing apparatus (OCCABA), a backpack-style, positive pressure breathing apparatus, for firefighting roles.


See also

* * * * * *} *


Notes


References

*


External links

* These references are related to NIOSH-certified SCBA with chemical, biological, radiological, and nuclear (CBRN) protection (SC/PD/CBRN): *
CBRN SCBA interim user guide and list of approved CBRN SCBA
*
CBRN SCBA standard development and publication leading to the first NIOSH-Certified SCBA with CBRN protection offered by the US DHHS/CDC/NIOSH/NPPTL on June 3, 2002
*
Image of 2-cylinder open-circuit SCBA
* Literature
Study of the physiological effects of wearing breathing apparatus
by RG Love and others.
Institute of Occupational Medicine The Institute of Occupational Medicine (IOM) was founded in 1969 by the National Coal Board (NCB) as an independent charity in the UK and retains this charitable purpose and status today. The "Institute" has a subsidiary, IOM Consulting Limited, whi ...
Research Report TM/94/05 {{DEFAULTSORT:Self-Contained Breathing Apparatus Firefighting equipment Industrial breathing sets Pressure vessels Breathing gases Functional masks Respirators