Built-in breathing system

In submarines the function is to supply a breathable gas in an emergency, which may be contamination of the ambient internal atmosphere, or flooding.

Like any other breathing apparatus, the dead space must be limited to minimise carbon dioxide buildup in the mask.

[3] When an externally vented BIBS is used at low chamber pressure, a vacuum assist may be necessary to keep the exhalation backpressure down to provide an acceptable work of breathing.

[1] This is common in therapeutic decompression, and hyperbaric oxygen therapy, where a higher partial pressure of oxygen in the chamber would constitute an unacceptable fire hazard, and would require frequent ventilation of the chamber to keep the partial pressure within acceptable limits Frequent ventilation is noisy and expensive, but can be used in an emergency.

[citation needed] When contamination of the internal atmosphere is not important, and where the external ambient pressure is higher than in the occupied space, exhaled gas is simply dumped into the internal volume, requiring no special flow control beyond a simple non-return valve.

The delivery and exhaust mechanism of a BIBS demand valve for this application is the same as for a scuba or SCBA second stage regulator, and these can be used for this purpose with little or no modification.

The exhaled treatment gas must be removed from the chamber to prevent the buildup of oxygen, which could present a fire risk.

The raised oxygen partial pressures in the blood may also help recovery of oxygen-starved tissues downstream of the blockages.

For serious cases resulting from very deep dives, the treatment may require a chamber capable of a maximum pressure of 8 bars (120 psi), the equivalent of 70 metres (230 ft) of water, and the ability to supply heliox and nitrox as a breathing gas.

The diver is then surfaced and pressurised in a chamber to 50 fsw (15 msw) within 5 minutes of leaving 40 ft depth in the water.

If this "surface interval" from 40 ft in the water to 50 fsw in the chamber exceeds 5 minutes, a penalty is incurred, as this indicates a higher risk of DCS symptoms developing, so longer decompression is required.

BIBS supply of breathing gas with higher oxygen content than the chamber atmosphere can solve this problem.

If the atmosphere in a saturation habitat is contaminated, the inhabitants can use the available BIBS masks during the emergency and be supplied with non-contaminated breathing gas until the problem has been solved.

[1] Submarine BIBS systems are intended to provide the crew with diving quality air or nitrox breathing gas in an emergency escape situation where the interior may be partly or completely flooded, and may be at a significantly higher than atmospheric pressure.