This article discusses a generic configuration for a typical rebreather being used for trimix diving. It is in no way stating that this is the correct configuration or otherwise, but it does describe one way of doing things and has a growing number of followers. Newcomers to rebreather diving on mix may get some useful ideas from it; the dyed in the wool can start their email wars about what is wrong with it. The essence of this configuration is
simplicity, some call it 'diving the unit' meaning to dive the unit and complete the dive without it having been functionally plugged into any off-board sling cylinders and without having any connections or switching utilised during the course of the dive.
The discussion avoids being RB brand specific and converses in generic terms. When first diving a RB with air as the diluent, it is the norm or standard to have 2 cylinders in the unit – one for oxygen and the other for the air diluent. The air diluent in this case would most likely feed the wing and dry suit as well. When progressing on to trimix and doing long deco dives, it is nice and I would suggest essential, to add a third cylinder to the unit that is dedicated to inflation, leaving the oxygen and diluent cylinders dedicated to the breathing gas in the loop and nothing else. Thus we have a 3 cylinder system, oxygen, diluent and the inflation gas, usually air.
The 3rd cylinder can be mounted off the backplate / harness under the diver's arm, on the side of the rebreather or as in my case, fixed to the underneath of my unit. If you are tall like me, this can be a plus. My 3rd cylinder is housed in a stainless cage I knocked up and bolted to the bottom of my unit; it raises my unit higher off the seat or bench and makes it a bit easier to wriggle into. The upside of a 3 cylinder system is that the configuration can be used for any depth; nothing needs to be done or undone, connected or unconnected or changed in any way just because one dive is in 10m and the next in 110m. Personally I have several diluent cylinders and use the appropriate one for the depth of the dive being undertaken.
The down side is that it adds weight to your unit and is a pain if you use your unit overseas as you have more units to transport or you must dive your unit configured differently to how you normally configure it. With the 3 cylinder set up, providing nothing goes wrong, you should be able to do any dive, including long deco sub hundred meter dives, using nothing more than the gas in the 3 cylinders and still have plenty of gas spare in each cylinder at the end of the dive. Mostly the off-board cylinders should be untouched and ready for the next dive.
Most commonly, dry suit inflation will come from the 3rd cylinder as that will use the most gas and the wing will be fed from an alternate source to ensure, very importantly, inflation redundancy. Tropical diving usually means wet suits or minimal protection so the only inflation required will be the wing and as in my case, the 3rd cylinder stays at home to reduce my baggage weight. Useful bits, still within the 'dive the unit' way of thinking, include a regulator on each of the 3 onboard cylinders.
Most units have a line switch on the oxygen regulator, the air regulator is usually tucked away but accessible and in the absence of a FFM or BOV, as in my case, the low profile regulator fed by the diluent is bungyed around my neck as a poor mans BOV substitute. The use of a FFM or BOV or combinations has much in its favour and was covered in more detail in an earlier article in this column. If not using a FFM, some form of restraint to help keep the mouthpiece in the mouth, is highly recommended.
Of course we still need our slings, no matter how conscientious we are about the integrity of our units, things may still go wrong and we need to be prepared to go to OC to retrieve the situation. If the unit becomes inoperable, it is comforting to know that you can directly breathe from any of the 3 cylinders in your unit, especially getting a sanity breath from your diluent regulator if you employ one in lieu of a BOV or an FFM setup. Beyond the sanity breath however it would be a case of getting on to the appropriate off board bailout cylinder asap as the onboard cylinders in most units will not keeping you going for very long in OC mode, especially at depth.
Imperative of course is the need to consider the depth you are at as to what gas/cylinder you bail out to. In the case of BOVs, they may be fed from the onboard diluent but are more often fed from an off-board diluent. Off-board cylinders would of course also have their own regulators as well as a BC feed each, these BC feeds are particularly important as they may be required to substitute for the onboard inflation or diluent cylinders in the event of their failure. As to the contents of the off-board cylinders, this always starts yet another war about what is right and what is wrong. Perhaps for the purposes of this discussion, we assume that the diver is doing a 'two sling dive', that he is primarily looking after himself, is not dependant on another divers for bailout gas and that there are staged cylinders on the shot line or deco station. Often these would be a rich Nitrox at 30 or 40m and a 100% O2 at the shallowest deco point. One bailout then would be bottom mix or some mix close to it. Most set the O2 in their onboard diluent to be slightly lean so that the unit can bring the O2 up to the selected set point whilst still being able to reduce the PP of O2 in the loop by flushing.
A bottom mix bailout then doesn't need to be quite as lean as the onboard diluent, most use virtually the same mix but with the O2 increased to an appropriate PP for the depth being dived. The other bailout would be somewhere between that of the bottom mix and of that which is attached deepest on the shot line and/or deco station. This mix is the one most debated, often the result of spending endless hours with your favourite gas planning tool attempting to smooth out some of the bigger PP transients when switching from one bailout to another. Some units have a breakout connector between the ADV and the diluent manifold, if this connector is used to connect to an off-board diluent cylinder then nothing really changes other than the loop is now sourcing diluent from the off-board cylinder instead of the onboard cylinder. This is a very useful thing to have but if following 'dive the unit' principles, should only be used when circumstances dictate it's use e.g. if something has happened to your onboard diluent or you wish to flush the loop with the off-board diluent.
If the ADV/diluent manifold breakout connector is not used and the off-board diluent is plugged directly into a manual feed, it must be remembered that the ADV will not be fed from the off-board and that the onboard diluent may still influence the loop gas composition, especially if the ADV is not isolated by its line switch. There are also arguments for and against a second oxygen cylinder.
Some argue for a second cylinder in that should the first fail for whatever reason then the backup may be utilised in order to keep the unit functional. Others argue against the second oxygen cylinder and say that should the first fail or run out you are in a bailout situation as it is an abnormal situation and should be treated as such. Another argument that I have heard concerns getting stuck in a wreck or something. If your slings are not part of the functioning operation of your unit then in the event of getting stuck or some situation where you simply need to remove one or both slings then you can do so without worrying about disconnecting anything and especially connecting them back up again correctly when you get yourself unstuck again; further to that, when arriving at the deco bars on a long deco dive, some divers like to hand off their slings to the support divers, again, if they are functionally separate to the unit, this procedure evokes no thought process.
A last little bit of kit that I've personally come to consider important is the John line. Mine is a stout bit of rope with a brass clip at both ends and a rope clamp on one end. Pre dive I have the clip only end clipped onto my harness, the line stowed in a pocket and the rope clamp exposed. Should I need to attach myself to a line I only need to grab the rope clamp, pull the line out of it's pocket and attach myself, quick, very easy and all with one hand.