Choptima CCR Bailout
One big benefit of chest mount rebreathers, like the Dive Rite O2ptima CM, is that they attach to existing open circuit systems. You can use them with doubles or sidemount with very few changes. Traditional backmount units require stage style bailout – or backmounting bailout which drastically increases the unit’s weight. We only have so much real estate for stage-style cylinders and it’s difficult to jump off of boats with a lot of cylinders. Taking too many cylinders will also increase your drag underwater, increasing Co2 production and making things less fun. Chest mount rebreathers give you the ability to retain your core configuration which lends itself to carrying more bailout without the proportional increase in drag we have with stages.
Basic bailout planning
The basic idea of bailout planning is that you need enough gas to get to the surface of a “bad” situation, including any mandatory decompression. The definition of a bad situation varies between agency, instructor, and environment. There is a tendency to err on the side of less bailout because – especially on backmount units – bailout is heavy, bulky, and prohibits mobility. This tendency manifests itself in limiting the “bad situation” examples to simple and short, often using low SAC rates. There is a thing as “too much bailout” just as there is “too little bailout” and we should avoid those. We must strive for the Goldilocks’ perfect temperature oatmeal.
A simple way to think of our definition of “bad situation” for cave diving is – “the furthest point of penetration” which is the longest swim out. For ocean diving “the end of the dive, right before the ascent” when we have the longest decompression obligation.
We must also add in a higher-than-normal breathing rate, which will occur under stress. A CO2 hit will also drastically elevate our breathing rate. We cannot plan bailout based on our best Surface Air Consumption (SAC) rate diving in a single cylinder – that’s lying to ourselves. Some agency standards define gas consumption rates for training courses, one example is: 1.6 cubic feet/min. That’s a very elevated breathing rate for most divers and is designed to accommodate CO2 hits. Having seen CO2 hits and done the calculations after the dive – 1.6cuft/min is realistic, maybe even low.
What is Dilout
Rebreathers don’t use much diluent. That’s why backmount units will use small cylinders like 27cubic feet (3L) as onboard diluent. With the choptima we have offboard diluent, typically utilizing the diluent source as a bailout source. Since it’s both diluent and bailout it’s often referred to as “dilout”. Dilout diving has many benefits the most important being volume. Dilout diving also has downsides, the biggest being choice of gas. When we need to validate cells underwater, we flush the system with a known gas – diluent – and verify the cell readings. It is nice to have a diluent with low oxygen because we’re further away from our setpoint and are able to see the cells drop further. With a high oxygen content, we will be closer to our PO2 and we’ll see less (or maybe no) movement in PO2 during our flush. This gives us more confidence in the cell validation. Low oxygen bailout will increase our decompression obligation. This is a big deal in cave diving. If we choose to dive air or trimix we will be able to see cell validation easier but have inefficient bailout gas from a decompression perspective. There is not a golden standard that solves these problems with dilout gas selection. For cave diving I prefer having a proper bailout gas that won’t give me the added decompression obligations in the event I want to leave.
Redundancy in the Overhead
Anytime we’re in the overhead we need redundancy. This applies to bailout abilities. It’s not appropriate to have a single regulator/cylinder for bailout and diluent in the overhead. We must travel horizontally to exit, therefore redundancy in dilout is required. In the open water, it’s more acceptable to use a single cylinder for both diluent and bailout because of your ability to ascend directly, but in decompression situations, this isn’t a great idea.
Doubles
Banded and manifolded cylinders are an awesome pair-up for chest mount rebreathers like the choptima. Baby doubles, like 40cuft or 50cuft cylinders offer a ton of gas for many diving applications, typically much more than an average backmount CCR diver would take. Doubles make your profile tall with a chest mount CCR. This makes it ideal for shipwrecks as the hallways and hatchways tend to be taller than they are wide.
Typically, the gas in your doubles will be your bottom bailout mix. Decompression gases can be rigged as stages. For boat diving you’re able to carry more gas more efficiently using backmounted doubles and a chest mount CCR.
Independent doubles are common with chest mount units. This is to remove the potential failure point of a manifold. It comes with the disadvantage of not being able to breathe off both cylinders in the event of a regulator failure. This separation makes it similar to sidemount but in the position of doubles. The separation of the two cylinders provides a more distinct purpose for each cylinder. Having two different gases in the cylinders is inappropriate – especially decompression gases. A crucial part of decompression diving is validating gas switches via the cylinder, that is not possible if your deco gas is backmounted. The risk of an improper gas switch outweighs any benefit to different gases in backmounted independent doubles.
Sidemount
Sidemount cylinders have true redundancy, without the failure point of a manifold, but they don’t offer the ability to use gas from both cylinders from a single regulator. For true dilout redundancy, both cylinders should have the same gas – the same concept as independent doubles. It’s important to remember that chest mount units are compatible with sidemount but they are not sidemount units. Diving sidemount configuration doesn’t instantly give you the skills or experience to dive small spaces.
Single cylinder
The choptima can be rigged to dive with a single cylinder. This would work well for extended shallow dives or photography applications. In a decompression situation, we can stage rig extra gas as well as decompression gasses. This approach would only be ideal in areas of the world where doubles weren’t available. Even then it’s not ideal. For technical diving, we should use a more technical configuration.
There is a lot of choice and possibilities when thinking of bailout for the choptima. It’s important to become very comfortable with your system and not constantly swap between styles. Sometimes it’s better to dive a configuration you’re more comfortable with that isn’t the perfect configuration for that environment, for example, sidemount off of a boat.