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Cognizant Of Aquatic Respiration And Under On Respiratory Devices

Principles of Diving Breathing Apparatus

The breathing apparatus used by divers falls into two main groups: self-contained equipment, in which the diver carries a supply of breathing gas in cylinders, and surface-supplied equipment, where the breathing gas is supplied to the diver through a hose from the surface. 

Self-Contained Apparatus Self-contained diving apparatus has the benefit of making the diver far more mobile and giving them a much greater range of action. The apparatus can be used for breathing air but can also be designed to use other breathing gases of higher or lower oxygen content than air. Although this produces several advantages, it also includes some disadvantages, which will be discussed more fully below. Self-contained breathing apparatus can be further divided into Re-breather (Closed or Semi-Closed) Equipment and Open-Circuit Equipment.

Self-Contained Closed and Semi-Closed Circuit “Breathing Apparatus” 

In this apparatus, the breathing gas, which may be either pure oxygen or an oxygen-inert gas mixture, is stored in cylinders at high pressure so that it occupies a reasonably small volume. There are three main alternatives of the re-breather diving equipment although they all use a similar method of removing carbon dioxide from the breathing circuit. These variants are:

  • Closed Circuit Oxygen on Demand Pure oxygen is used as the breathing gas and the counter lung is kept topped up using a demand valve.
  • Semi Closed Circuit, Constant Mass Flow. Pure oxygen or a Pre-determined Gas Mixture is passed through a preset reducer that confirms a constant mass of gas is delivered to the counter lung.
  • Closed Circuit, Fixed Partial Pressure of Oxygen. A constant partial pressure of oxygen is maintained in the breathing circuit by the addition of oxygen. The amount of oxygen being added is controlled by Sensors in the breathing loop which monitor the oxygen content, generating the control signal which is used by the oxygen addition valve.

Self-Contained Closed and Semi-Closed Circuit ‘Constant Mass Flow’ Breathing Apparatus 

The principle of the first two diving equipment detailed above are similar and is outlined in 👇

  • Both pieces of equipment use a fixed oxygen content gas. When the cylinder outlet valve is opened the gas flows through a reducing valve, which supplies a constant mass flow irrespective of depth, into a flexible breathing bag or counter lung positioned on the diver’s chest or back, at the approximate level of their lungs. 
  • The diver breathes the gas from the counter lung via a canister of carbon- dioxide absorbent and a breathing tube. When the diver exhales, their expired breath passes back into the counter lung through the carbon-dioxide absorbent, which removes the expired carbon dioxide but has no effect on the oxygen and nitrogen.
  • By breathing in and out of the flexible counter lung positioned level with their lungs the diver can breathe restfully at any depth, as the counter lung is subjected to the water pressure at the diver’s depth, and the breathing gas in the counter lung and the diver’s lungs will be at approximately the same pressure as that on the chest walls.
  • If more gas was supplied than the diver could use and there was no other outlet for the excess gas, the counter lung would fill up, and eventually it would be impossible for the diver to exhale against the excess pressure in the counter lung, which would soon burst. The same thing would happen when the diver ascended, because of the expansion of the gas in the counter lung with the external reduction of pressure.
  • To prevent this, a relief valve is fitted in the counter lung. The valve is normally held closed by a light spring and the external water pressure. The valve lifts when the pressure in the counter lung is slightly above that of the spring plus water pressure.
  • Pendulum and Two-Way Breathing. In the breathing apparatus described above, the diver breathes in and out through a single breathing tube and the carbon dioxide absorbent. This is known as pendulum breathing and has the disadvantage that a large ‘dead space’ exists between the mouthpiece and the canister; this means that the small amount of gas in the breathing tube at the end of an exhalation is drawn back into the lungs on the next inhalation without having been through the absorbent.
  • For this reason, the diver should avoid shallow breathing and make a conscious effort to breathe normally. By so doing they will make the ‘dead space’ as small a fraction as possible of the total volume of each breath, thus ensuring the removal of the maximum amount of carbon dioxide.
  • Pendulum breathing has the advantage that most of the gas passes twice through the absorbent at each breathing cycle; this not only ensures efficient absorption of the carbon dioxide but brings both sides of the granules into action and hence improves the endurance of the absorbent.
  • In military breathing sets the diver inhales through one tube and exhales through another, the flow being controlled by non-return valves in the breathing tubes. In this case, the gas passes only once through the canister at each breathing cycle. This is known as two-way breathing, which, provided the valves are close to the mouthpiece, has the advantage of cutting out most of the ‘dead space’.
  • Choice of Breathing Gas. The closed/semi-closed-circuit breathing apparatus may be used for breathing pure oxygen, an oxy-nitrogen mixture, or an oxy-helium mixture. The principle of operation and the gases used in the Closed Circuit, Fixed PO2 set such as CDLSE will be discussed later. For the Constant Mass Flow Diving Apparatus, the principles do not differ whichever gas is being used but the flow of gas and the method of using the apparatus do vary.
  • Oxygen Breathing. Pure oxygen would appear to be the ideal breathing gas. It supports life and will give the maximum endurance for a given amount of gas carried. However, oxygen becomes toxic with depth and it cannot be safely breathed at a partial pressure exceeding 2 bar abs. For safe diving to greater depths, oxygen must be diluted with an inert gas such as nitrogen/helium, so that its partial pressure at the depth involved does not exceed 2 bar absolute.
  • Pure oxygen may also be used in what is known as the ‘on demand’ technique, where no reducer is fitted. The diver fills the counter lung by merely opening and closing the cylinder valve; they then breathe from the counter lung until it ‘bottoms’, i.e. they cannot take a deep breath. They then refill the counter lung once more.
  • This method has the advantages of greater endurance and not leaving a trail of tell-tale bubbles on the surface. However, since this technique introduces hazards not present when a reducer is used, it is used only when conditions require that the presence of
    the diver remains undetected.
  • To ensure the safety of divers required to operate covertly, the “on-demand” technique has been modified in equipment so that oxygen is added to the counter lung automatically when the pressure in the counter lung falls below a preset value.
Closed Circuit, Fixed Partial Pressure of Oxygen, Re-breathing Apparatus
  • The Constant Mass Flow Re-breather, although more gas efficient than open circuit equipment, still has a continuous stream of gas being vented to the water. The ideal solution in terms of gas economy is the ‘Closed Circuit, Fixed Partial Pressure of Oxygen’ re-breathing apparatus where oxygen is added only when required to maintain a predetermined safe partial pressure and to which an oxy-inert gas diluent is added as required to maintain the ambient pressure and system volume.
  • The closed circuit, sensor-controlled re-breather is such an apparatus. Although the breathing circuit and CO2 removal system of
    this type of equipment is similar to those found in most re-breathing apparatus, the main differences lie in the methods by which oxygen and diluent gas are admitted into the breathing circuit. A simple diagram describing this type of equipment is at:👇
  • Oxygen partial pressure is monitored by specially designed electrochemical cells. In this equipment, the sensor consists of a sensing electrode, a counter electrode, a housing containing a basic electrolyte, and a Teflon membrane. Oxygen molecules diffuse through the Teflon membrane into the sensor where electrochemical reactions at the two electrodes produce an electron flow.
  • This tiny current, which is directly proportional to the oxygen level seen by the sensor, passes through a temperature-compensating circuit and the resultant voltage signal is then passed to an electronic control unit and a display unit.
  • The Control Unit compares the Sensor outputs against a preset value corresponding to the desired PO2 and then, if required, operates an O2 addition valve to add oxygen and if necessary activate an alarm indicating that the O2 content is outside the preset parameters. To ensure integrity, it is usual to have at least 3 O2 sensors. Logic circuits ensure that the best signal from the two is used for the various control and alarm functions.
  • Diluent (usually oxy-helium or oxy-nitrogen) is added to the breathing circuit as required by a demand valve, which comes into operation when the counter lung becomes deflated. As with the previously described re-breathers, a relief valve releases counter lung gas in the event of overpressure in the breathing circuit. Although not shown in the above-simplified diagram, it is usual to
    have manually operated bypasses and controls fitted so that the addition of oxygen and diluent can be achieved by the diver should the automatic control unit fail?
  • The major advantages of this type of equipment are the reduced decompression penalty due to a lower relative partial pressure of inert gas, and reduced gas consumption. Penalty due to a lower relative partial pressure of inert gas, and reduced gas consumption. When used in a self-contained mode, dive bottom times (durations) of up to 6 hours at any depth are easily achievable, provided that the diver has adequate thermal protection and CO2 absorbent.
  • The development of this type of apparatus has now progressed to the point where there are reliable units available in the commercial market, and they are beginning to see some commercial use as well as an obvious military application. The sports market has also shown a great interest in this type of re-breather, as the PO2 is maintained at a constant value regardless of ambient pressure. 
Self-Contained Open-Circuit Breathing Apparatus

The principle of these air-breathing systems is outlined in 👇

  • Air from the supply cylinder flows through the reducing valve to the inlet of a demand valve, which opens only when the diver demands air by inhaling. When the diver inhales, air from the demand valve passes via an inhale breathing tube and non-return valve in the tube to the mouthpiece. The diver’s expired air, containing the carbon dioxide, passes through a second non-return valve and an exhale breathing tube to the surrounding water; thus no special arrangements are required for the removal of the carbon dioxide.
  • Reducer Flow. In open circuit equipment, the air supply is automatically adjusted for depth by the use of a hydrostatically compensated reducer. As the external pressure increases with depth the reducer automatically provides a corresponding increase in its outlet pressure. This unfortunately increases the mass flow of air which, while not interfering with the breathing system, is wasteful of air.
  • The pressure of the air supply alters slightly with the position of the reducer about the chest. When the reducer is higher, as when swimming normally, the supply pressure is less than when swimming on the back. In the latter case, since the reducer is deeper than the chest, it is at a higher pressure than the lungs and will supply air at high pressure.
  • Endurance Warning The consumption of air is dependent both upon the diver’s depth and work rate; consequently, there will be
    very large variations in the consumption of air, making it almost impossible to give any realistic estimate of the endurance of the set.
Surface-Supplied Diving

Various types of surface-supplied equipment are in use in different diving organizations, typically these include:

(1) Kirby Morgan Band mask Mk 18B.

(2) Kirby Morgan Superlight 17K – Diving Helmet.

(3) ESDS.


 In surface-supplied diving, the diver is supplied with gas through a panel, which incorporates a hand-wound regulator to reduce the high-pressure gas supply to low pressure, compensated as required, for the depth at which the diver is operating. Gas supply can also be made to the panel from an onboard system using a reservoir supplied via a compressor. Gas supply panels used in service diving are capable of use with either KMB 17K, 18B, or ESDS. 

An emergency gas supply is carried in a cylinder (bailout) attached to the diver’s BCS. This supply is independent of that supplied by the panel.


The introduction provides an overview of underwater breathing apparatus, categorizing them into self-contained equipment and surface-supplied equipment. Self-contained apparatus offers mobility and a wider range of action, while surface-supplied equipment uses gas supplied through a panel. Self-contained closed and semi-closed circuit apparatus store breathing gas in cylinders and use methods to remove carbon dioxide from the breathing circuit. 

The closed circuit equipment maintains a fixed oxygen content, while semi-closed circuit equipment ensures a constant mass flow. The conclusion mentions the advantages and disadvantages of different breathing gases, such as pure oxygen and diluents like nitrogen or helium. It also discusses the principles of open-circuit apparatus and surface-supplied diving equipment.

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