Self-contained breathing apparatus

Siebe Gorman Savox in a coal mining museum

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 and in long tunnels, 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 rebreathers, such as the Siebe Gorman Proto, Siebe Gorman Savox, or Siebe Gorman Salvus. An example of modern rebreather SCBAs would be the SEFA.

A person wearing an MSA brand breathing mask with a Nomex hood. This face piece attaches with a regulator to form a full SCBA.

SCBA packs carried on a rack in a firetruck

For underwater open-circuit breathing sets, see Scuba set#Types.

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 300bar, is 6 X 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%.

SCBA pack with PASS device (ADSU)

Air cylinders are made of aluminium, steel, or of a composite construction (usually carbon-fiber wrapped.) The composite cylinders are the lightest in weight and are therefore preferred by fire departments (UK: fire and rescue services previously called fire brigades), 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 brought to the scene.

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.

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.) What is particularly important to understand from the formula is that the temperature is in kelvins, 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.[3] The scientifically valid change in temperature from 32 to 96 °F (0 to 36 °C) 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 bottles, which in turn could lead to a firefighter running out of air prematurely.