Breathing apparatus

[3] McGraw-Hill Dictionary of Scientific & Technical Terms defines breathing apparatus as "An appliance that enables a person to function in irrespirable or poisonous gases or fluids; contains a supply of oxygen and a regenerator which removes the carbon dioxide exhaled", which is the description of any type or application of rebreather.

In other applications, when long endurance and reasonably light weight is required, it may allow a large saving of gas and be much simpler or lighter than the equivalent open circuit option.

[30] A mouthpiece, usually held in place by a bite-grip, and sealed by the lips, is common in scuba equipment, snorkels, and some types of escape breathing apparatus.

[31] A mouthpiece is simple and effective, with minimal dead space, and reliably seals without need for adjustment, but must be actively held in place by the user, and can cause jaw fatigue over long periods.

Breathing hoods with full length visors are commonly used with free-flow supplied air respirators for industrial work like in spray painting, boatbuilding, and woodworking workshops.

[36][27] Breathing apparatus may be used in various pressure regimes: hyperbaric for diving, tunneling, and caisson work, normobaric where the ambient atmosphere is unbreathable, or supplemental oxygen is needed for medical reasons, and hypobaric at high altitudes and in space.

Open and closed circuit, self-contained, and remotely supplied systems are all in common use, but gas composition choice is complicated by the possibility of oxygen toxicity and decompression requirements.

The possibilities of nitrogen narcosis and excessive gas density causing unacceptably high work of breathing make the use of helium as a diluent necessary for use at greater depths.

In these applications it is usual to use oxygen rebreather systems, as they are relatively safe, simple and efficient compared to open circuit, and do not inherently affect suit internal pressure.

The ambient pressure underwater varies with the depth, and diver attitude in the water can affect the variation in pressure between the lungs and the delivered gas in the mouthpiece by up to about 250 mm water (25 mb), but usually less, which can be positive or negative depending on the relative position of the lungs to the demand valve, exhaust valve of a free-flow helmet, or counterlung of a rebreather.

[9] The major categories of ambient pressure underwater breathing apparatus are: Two other types may also be identified: Breathing gas must be supplied for work in unbreathable normobaric atmospheres, which may be toxic, irritant, narcotic or hypoxic, and may include firefighting, damage control, exploration, and rescue work, and in normobaric environments where contamination of the person (hazmat environments) must be avoided.

[40] Depending on the nature of the hazardous atmosphere, the user may need to wear personal protective equipment to isolate the entire body from the environment (hazmat suit).

Smoke hoods and other escape respirators are used in many industrial environments where they may be needed to evacuate a building in a fire or other incident which may compromise the ambient air quality but there is likely to be sufficient oxygen remaining to sustain the necessary activity.

Systems providing a constant flow rate of open circuit oxygen at the nose or mouth will waste a lot of the gas to dead space and during exhalation.

The delivery of open circuit supplemental oxygen is most effective if it is made at a point in the breathing cycle when it will be inhaled to the alveoli, where gas transfer occurs.

The equipment must be lightweight and reliable in severe cold, including not getting choked with deposited frost from the exhaled gas, which is saturated with water vapour at body temperature.

[45][15] Although there is considerable similarity in the basic conditions in which aviation and mountaineering breathing apparatus is used, there are differences sufficient to make directly transferable use of equipment generally impracticable.

[46] Acute indications for therapy include hypoxemia (low blood oxygen levels), carbon monoxide toxicity, cluster headache and decompression illness.

[48][46] Partial pressures administered range from low flow rates giving slight increases over ambient air up to 2.8 bar absolute used in hyperbaric oxygen treatment of decompression illness and some other indications.

[55] At extreme altitude, above 5,500 metres (18,000 ft), one can expect significant hypoxemia, hypocapnia and alkalosis, with progressive deterioration of physiological function, which exceeds acclimatisation.

Pulse dose (also called intermittent-flow or on-demand) portable oxygen concentrators are the smallest units, which may weigh as little as 2.3 kilograms (5 lb) Their small size enables the user to waste less of the energy gained from the treatment on carrying them.

Space suits are often worn inside spacecraft as a safety precaution in case of loss of cabin pressure, and are essential for extravehicular activity (EVA).

It must be reliable and should not require constant attention or adjustment during use, and if possible performance should degrade gradually in the event of malfunctions, allowing time for corrective action to be taken with minimum risk.

[31] Diving helmets and most full-face masks do not allow the user finger access to the nose, and various mechanical aids have been tried with varying levels of comfort and convenience.

Siting the viewport close to the eyes helps provide a better view but is complicated by the need for sufficient clearance in front of the nose for a wide range of divers.

Cylindrically curved viewports can introduce visual distortions underwater that can reduce the effectiveness of the diver at judging distance, but are common in masks used in air.

To get consistent breathing effort for the range of postures the diver may need to assume, this is most practicable when the exhaust ports and valves are close to the mouth, so some form of ducting is required to direct the bubbles away from the viewports of helmet or mask.

Masks held in place by adjustable straps can be knocked off or moved from the correct position, allowing ambient atmosphere or water to flood in, and the loss of breathing gas.

They tend to have a large internal volume, and be heavier than demand helmets, and usually rest on the shoulders to prevent over-stressing the neck, so do not move with the head.

Access to the valves and pressure gauge is important for gas management, and it is helpful when equipment is shared by a team that the fit can be easily and quickly adjusted to suit the individual.