A.A.R.G. rebreather diving standards.

• Background definitions.
• Common Core Requirements.
• Semi Closed Circuit.
• Fully Closed Circuit.

A.A.R.G. standards: background definitions.

FO2 = Fraction of Oxygen in a gas mixture.

Partial Pressure = The pressure exerted by any constituent gas in a mixture; the most significant measurement with respect to the physiochemical and physiologic behavior of a gas. As described by Dalton’s law, the total pressure of a gas mixture is the arithmetic sum of all the individual partial pressures of the constituent gases.

PO2 = Partial pressure of Oxygen.

C02 = Carbon Dioxide.

Loop = The loop describes the counterlungs, breathing hoses, C02 absorption canister/system and mouthpiece with upsteam and downstream check valves. Basically all gas filled parts of a rebreather that contains gas rebreathed by the diver.

Flushthru = technique of venting all the gas in the loop and the replacement of this gas from a high pressure gas cylinder. A common procedure used especially but not exclusively by semi-closed circuit divers.

Setpoint = the chosen operating PO2 of fully closed circuit rebreathers.

Hypoxia = physiological symptoms precipitated by insufficient oxygen.

Hyperoxia = physiological symptoms of excess oxygen.

Hypercapnia = physiological symptoms of increased carbon dioxide tension in lung gas and blood usually encountered in rebreathers due to scrubber failure/exhaustion and the resultant elevated CO2 in the breathing loop.

Scrubber = the carbon dioxide absorbent canister, a common component in all self contained rebreathers.

Mixed gas = in relation to rebreathers any gas mix containing >0 and <100% oxygen, the remainder of the gas mix made up of any single or combination of inert gas. Note air is classified as mixed gas as the FO2 in the loop rarely equals FO2 in diluent supply gas.

For the purposes of this standard, rebreathers will be distinguished as one of 2 types by the following classifications:

Semi closed circuit

Semi closed is defined as a mixed gas rebreather where the metabolised oxygen is replaced via a source of mixed gas. Designed to operate below the safe depth of pure oxygen rebreathers. Gas is pre-mixed in high pressure cylinders with oxygen levels to suit the depth of the dive.

Fully closed circuit

Fully closed is defined as a rebreather in which the metabolised oxygen is replaced from a source of pure oxygen. Designed for depths exceeding the safe depth of pure oxygen diving, a source of diluent gas is used to keep the PO2 within life support range at any depth. The monitoring/control of oxygen may be either electronic, which invariably involves 3 oxygen sensors, or cryogenic control physical gas laws.

Fully closed rigs have relatively stable PO2 in the loop, i.e. a variable F02 which decreases in direct and inverse proportion to increased absolute pressure. This is in contrast to semi-closed units in which the PO2 increases in direct proportion to depth.

A.A.R.G. has several ratings:

• Semi Closed Circuit (unit specific)
• Fully Closed Circuit (unit specific)

RATING REQUIREMENTS

Common Core

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Common core theory and practice requirements, semi closed and fully closed rating.

Theory, common core

the diver must demonstrate a sound understanding of the:

• effects of oxygen, both hypoxia and hyperoxia, including PO2 life support ranges.
• rate of oxygen metabolism in divers
• calculation of the duration of CO2 absorbent
• variables that effect canister duration
• disadvantages of using nitrox supply gas in rebreathers, especially at depths exceeding 20 meters, including ventilitory requirements of the diver, inert gas narcosis, decompression and thermal considerations and canister efficiency.
• physiological effects of elevated CO2 and the ramifications for the rebreather diver.
• physiological effects of hypoxia and hyperoxia including warning signs
• planning for adequate bailout to the surface, including all required decompression stops
• reasons why an OC second stage is required as the 1st option for safety in the event of a loop flood or scrubber failure.
• inherent difficulties of starting the CO2 absorbing reaction in-water and it’s implications for divers who have a second rebreather as their only bailout.

Practice, common core

• pre-dive preparation of all equipment.
• pre-dive checks including functioning of check valves, loop integrity via both positive and negative pressure checks, and scrubber functioning via pre-diving the loop for a minimum of 5 minutes.
• unit specific pre-dive checks
• in water bubble and leak checks.
• providing adequate buoyancy in the event of a loop flood. This is simulated by the addition of weight equal to the buoyancy of the loop volume of the rebreather being used, and where appropriate additional weight to simulate suit compression due to depth.
• a blue water ascent maintaining a safe ascent rate, and adequate buoyancy control. This is more difficult on rebreathers as there may be 3 pieces of equipment that need buoyancy control: the BC, drysuit, and the loop.
• the ability to plan for the bailout requirements of their dive partners, including any required decompression stops.
• choice of bailout gas(es) to provide a safe ascent to the surface with a PO2 of no less than 0.16
• re-filling the scrubber to minimise chance of channeling
• accurate record keeping of dive-time on each canister fill.
• cleaning of the unit between dives

Semi-closed circuit specific requirements

Note: if the divers rebreather is commercially manufactured the diver must have completed a manufacturer approved training course specific to that unit, if available.

Theory, semi closed:

the diver must demonstrate a sound understanding of the:

• calculation of theoretical FO2 in the loop with different flow rates and supply gases.
• importance of accurate pre-dive flow rate testing for constant mass flow semi-closed units
• duration extension of CO2 absorption advantage in semi-closed units.
• effects/risk of flow rate reductions caused by foreign particles in the constant mass flow orifice.
• calculation of equivalent depths, for decompression calculations

Practice, semi closed circuit rating

• For constant mass flow units, the diver must demonstrate competency in pre-dive flow rate testing. This includes introduction of back pressure equal to planned diving depth.
• For other types of semi-closed units, such as passive addition systems, the diver must demonstrate competency in the unit specific pre dive checks supply gas flow rate.
• the diver must demonstrate a flushthru before any ascent
• in the case of constant mass flow units with fixed intermediate pressure 1st stage regulator, a secondary cylinder with compensated intermediate pressure 1st stage, and a OC second stage regulator is required for bailout

Fully closed circuit specific rating requirements:

note: if the rebreather has not been manufactured by the diver, a requirement for this rating is a manufacturer approved training course specific to that rebreather, if available.

Theory, CC

• calculation of adequate diluent volume for the depth of the dive
• parameters for choosing a setpoint PO2
• ways to avoid PO2 spikes on descents, part of choice of correct diluent.

• partial pressures of gases
• mixed gas decompression theory
• the ramifications of using helium diluent
• the specific unit being dived
• the common causes of death and inherent weaknesses of the make of unit they are diving, and ways to avoid them.

Practice, CC

• the diver must demonstrate calibration of the oxygen sensors. The sensors must be calibrated to 1.0, with pure oxygen. If setpoints greater than 1 are used, the sensors must be calibrated at that setpoint using a pressure pot, or verified using pure oxygen at the relevant depth before descent. for example if the setpoint is 1.3, verify sensor readings with pure oxygen at 3msw.
• the diver must show competency in the manufacturers pre-dive checklist, if applicable.
• demonstrate an adequate loop flushthru with both diluent and oxygen, as they may differ in procedure due to placement of addition point in the loop.
• the diver must demonstrate the ability to control the unit in semi-closed mode, both with the aid of the oxygen sensors, and simulated without sensors using pre-dive manual addition semi-closed calculations.

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Last updated 1998 by Scott Leimroth ©