User:RexxS/Rebreather

Key points
All of those points can be sourced from Gene's reference. If we state the size and weight of an o/c cylinder (sourced to a vendor) and the same for one used in a rebreather (sourced to a vendor), then we ought to be able to say (9) that A is larger and heavier than B without requiring the reader to do more than a comparison. I think may not be violating WP:SYN by mentioning that (10) the lack of bubbles is advantageous to photography, covert operations, etc. (especially as we can source the military use to US Navy Divers Manual).
 * (1) Three types of rebreather: semi-closed circuit (SCR), fully closed circuit (CCR), and fully closed circuit oxygen rebreather;
 * (2) Rebreathers recirculate the exhaled gas, removing CO2 and replacing just the O2 that was used;
 * (3) CCRs offer the greatest advantages, but are most expensive;
 * (4) CCRs often use as little as one-seventeenth the gas that open-circuit does and that represents a significant saving in costs for helium (if used);
 * (5) CCRs minimise the amount of inert gas breathed and allow 30–40% less decompression time than open-circuit;
 * (6) CCRs produce almost no bubbles;
 * (7) Rebreathers require additional training beyond o/c training, and the cost of training may be significant;
 * (8) Rebreathers use cylinders which may contain up to 30 cuft (840 litres) of gas;

Proposed draft
A less common type of scuba set is the "rebreather", which differs from open circuit sets by retaining the exhaled gas and recirculating it, after removing carbon dioxide and replacing the oxygen used by the diver. The path followed by the gases is referred to as the "(breathing) loop".

Rebreathers may be classified into three common types:


 * Oxygen rebreathers supply 100% oxygen and only need one cylinder. They have no decompression requirements, but have depth limits of no more than 9 m because of the risk of oxygen toxicity.


 * Semi-closed-circuit rebreathers (SCR) recirculate most of the exhaled gases, but not all. Nitrox, heliox or trimix is supplied at a constant rate, balanced by exhausting gas at the same rate from the loop. They typically consume only one third the gas of open circuit. The SCR has the advantage of simplicity and only needing one cylinder. The amount and proportion of oxygen in the loop varies with the rate of flow, and with the rate of the diver's metabolism, and with any current rate of depth change. If the current supplied partial pressure of diluent is n bars, and if the diluent is nitrogen, that is equivalent to (12.5*n - 10) meters depth (with the water surface at sea level) on open-circuit air for decompression calculation.


 * Closed circuit rebreathers (CCR) recirculate almost all of the exhaled gases. By using electronics to monitor partial pressure of oxygen (ppO2), they can regulate the amounts of oxygen and diluent added to the loop, keeping the oxygen fraction close to a set point. The CCR has numerous advantages over other types, but it is complex and costly, needing electronics and a cylinder for the oxygen and a cylinder for the diluent.

A diver using a rebreather will consume between 0.3 liters per minute (L/min) and 3 L/min of oxygen, independent of depth, and rebreathers may be designed assuming an average of about 1 L/min. The respiratory minute volume for a diver using open circuit scuba is commonly given as 20 L/min at the surface, but increasing with depth. A deep dive using helium as the inert gas (heliox, 60% helium as diluent) with a CCR can consume about a seventeenth of the gas used by open circuit—depending the breathing efficiency of the diver—offering a considerable cost saving in the helium used.

Closed-circuit rebreathers commonly use two cylinders, each storing up to 24 cubic foot of gas (internal volume 3 liters) and weighing about 9 lb. A typical open circuit cylinder to hold 100 cubic foot of gas (internal volume 12 liters) will weigh about 29 lb and is much larger.

Since CCRs are capable of minimising the fraction of inert gases in the mix at all depths, they can offer 30–40% less decompression time than open-circuit. The large reduction in bubbles from a rebreather gives it an advantage for military use and for any recreational use where there is a need not to disturb marine life.

For safety reasons, all rebreathers require a good understanding of their maintenance and operation, and training for their use represents an extra cost beyond that required for open circuit scuba.


 * Note to ref#6: The source actually says "Decompression was 42 to 70 percent longer with open circuit." Can we assume the reader will accept that is equivalent to ""CCRs ... can offer 30–40% less decompression time than open-circuit."?
 * It seems the same to me, but check the arithmetic: "x% longer" means "(x+100)% as long"; 100/142 = 70.40% as long for decompression after rebreather diving, 100/170 = 58.82% as long, uhh, call it 30% to 40% shorter. Anthony Appleyard (talk) 21:42, 12 February 2010 (UTC)
 * Yes - that was just the calculation I used. --RexxS (talk) 09:17, 13 February 2010 (UTC)