[almitywife]

Part III

Manual Injection Rebreathers:

When I first got into rebreathers, it seemed like manual injection rebreathers were yesterday’s news…obsolete, unsophisticated and just not cool. At that point eCCR’s looked like the ultimate way to go. Two years later, it seems like quite a few people, myself included, are discovering just how valid and sophisticated an option mCCR’s actually are. There seems to be a surge in interest in manual systems and recently the number of available units has expanded exponentially. For these reasons I will go into considerable detail about them and why I think this trend is occurring.

“Manual” vs. “electronic” only refers to the o2 injection, as both eCCR’s and mCCR’s are becoming more and more electronically advanced. In-line deco, a HUD (heads up display) and ever more sophisticated and user-friendly computer monitoring systems are being added to each.

Even with the trend toward greater complexity, manual systems continue to adhere, with near religious conviction, to leaving out the solenoid/automatic set point controller. To lessen the need for frequent manual injection, a constant mass flow orifice is used to administer a continuous flow of o2 molecules into the loop at a rate that is tuned to just below the diver’s average metabolic rate, requiring the diver to pay constant attention and “top off” with manual injection to keep up with o2 metabolism and changes with depth. The mass flow rate is verified regularly during pre-dive set up and can be adjusted by increasing or decreasing the interstage pressure (IP) of the o2 first stage regulator. The intent is to have the diver custom adjust the IP between dives initially and thus get the flow rate of oxygen to nearly match their own minimum metabolic rate… some mCCR divers get this so fine tuned that they only have to add o2 a few times during the constant depth parts of the dive.

When first looking into rebreathers, many folks, having heard that they are “dangerous”, have a hard time getting over the idea that one’s life is really in one’s own hands with a manual system. The idea of manual injection steers many people away from mCCR’s as it does not sound as safe and reliable as an automated system. Training and experience with manual injection lead many to come to the opposite conclusion. Firm believers in mCCR's often conclude that manual injection is the corner stone to safer habits and ultimately reduces overall risk. They postulate that having to monitor and adjust loop o2 balance throughout the dive entrains one to the requirements in o2 injection rate, to become more instinctive about making the necessary adjustments to changes in dive profile, particularly when something unexpected happens. In practice I have found that adjusting and monitoring on my own o2 is surprisingly reassuring and intuitive, I feel plugged in to the process. I believe that giving the brain something to monitor that actually fluctuates, as opposed to an electronic set point controller, which nearly always says the same thing, keeps one alert and motivated to check the gauges frequently and make the necessary adjustment. I have found that after a while, this process actually becomes an enjoyable challenge, a skill which one is constantly honing. It is believed that this connectedness reduces the likelihood of critical lapses in attention, inadequate reflexive reactions in times of high anxiety and ensures that a malfunction would be detected in time to adequately respond. To sum up the sentiment, many believe that the only set point controller you can trust is the one between your ears. This theory that mCCRs are safer seems to be supported by the near zero fatality rate associated with them, though reasons for this low fatality rate have yet to be empirically determined. It is interesting to find that many eCCR divers are also so convinced of the value of manual injection that they advocate running eCCR’s on a low set point and inject manually most of the time. The one thing we all seem to agree on is that the brain is the most trust worthy monitoring system and must be fully engaged nearly continuously no matter what system you choose.

MCCR’s have been known as the less expensive option and while you can get started with them on more of a restricted budget, I have found that if you compare equally capable mCCR’s and eCCR’s (equal redundancy in electronics with equal depth capacities) that they come surprisingly close in cost, all things considered. It would seem that the current trend in mCCR's development is not so much focused on maintaining simplicity or being less expensive overall, but rather allowing the user to choose the level and type of additional electronic integration, often allowing for many of the amenities of eCCR’s while continuing to require the user to inject o2 manually and continuously monitor the effect of injection, pressure change and metabolism on the loop’s po2. Some mCCR users claim that regardless of whether they are safer that they have chosen to dive a mCCR for the fact that they are less likely to malfunction due to being simpler and they have a better chance at finding a remedy for in-field repairs should something need repair or replacement. MCCR’s tend to be more modular and field serviceable, reducing the prospects of missing dives or whole dive trips. MCCR’s can be configured so that the electronics can be swapped out in field without having to be a computer technician.

Some mCCR’s are based on a design of only 2 o2 cells, diverging from the custom of having 3. The argument is that the brain can employ a much more sophisticated voting logic than a computer, by checking the cells at the beginning of a dive for current limitations and by flushing the loop with diluent to confirm which cell is responding properly when there is a discrepancy. Some manufacturers are beginning to produce their mCCR with only two cells but leave room for a 3rd. It is becoming common for the manufacturer to leave the option of additional cells and choice third party po2 monitor up to the customer. A 3rd cell is often added in this case with a completely independent po2 monitor, often combined with a decompression computer to more closely track decompression limits and as a comparison with the readings on the primary po2 display.

In response to the challenge of maintaining constant po2, most mCCR’s use a first stage on the Oxygen side that is plugged (as in non-pressure compensating) preventing it from changing much in flow rate of o2 molecules as pressure changes and depth increase. Manual injection, by the use of a simple inflator valve, is combined with a constant mass flow (CMF) valve, a nifty device that in simplistic terms meters out molecules of o2, which varies little with depth/pressure, rather than gas volume, which would fluctuate drastically in atomic density with changes in pressure. The combination of a mass flow orifice and non-depth compensating first stage helps make maintaining a more constant ratio of o2 in the mix easier for the diver. Because the spring in the first stage is blocked, however, there is a point with increasing depth where ambient water pressure will match the inter stage pressure of the first stage causing the o2 flow to cease…this is typically around 300 ft FSW. To give some buffer, a 250 ft depth rating is often given by manufacturers. Most mCCR’s are therefore considered “depth limited” in stock configuration. But the term “depth limited” is a relative term, especially if you are coming from the OC perspective, as 250 fsw is more than enough for many divers and even this can be overcome with off-board o2 from a standard, depth compensating regulator for the deep part of the dive. Both mCCR’s and eCCR’s require addressing high po2 in the same way…diluent is added, gas is expelled from the loop.

Some folks have concluded that since eCCR’s keep a more consistent po2 than someone diving a mCCR can and therefore that eCCR’s are better at fully maximizing the decompression advantages of CCR’s. Others say that the fluctuation in po2 typical of an mCCR diver’s manual injection is negligible once a diver gets good at maintaining a relatively constant po2. Critics of mCCRs say that the constant mass flow valve is a potential weak link in the chain, being prone to getting clogged. The typical rebuttal is that having a spare is cheap and they are easy to swap out and that such occurrences are rare, and since you are monitoring your po2 so much anyway that a problem would get noticed quickly, only requiring an increased rate of manual o2 injection.

Manufacturers of both mCCR’s and eCCR’s offer multiple size options (KISS: Sport and Classic, Megalodon: Standard and Mini Meg and Expedition, AP: Inspiration and Evolution). The size difference relates mostly to scrubber duration and o2 and diluent tank size. There are also front and back mounted counterlung options. Proponents of over-the-shoulder counterlungs say they offer better buoyancy, reduced work of breathing and reduced chance of Caustic Cocktail/more reaction time during a flood. Proponents of back mounted counterlungs enjoy the lack of clutter in their chest and report no difference in buoyancy… and they say the difference in WOB (work of breathing) is insignificant, except when turned on to one’s back, which is avoidable most of the time.

There are quite a few third party frames available for some CCRs, intended to help create flexibility in areas like tank sizes, weight trim and wing configurations.

A common variation in design for administering diluent to the loop is with the use of an ADV (Automatic Diluent Valve) or BOV (Bail-Out Valve). ADV’s come in two main styles, a regulator style that works just like a standard OC second stage (the purge button has been employed to allow for manual addition of diluent if needed) and a Schrader valve style that is more like a plunger that requires pressure on the plunger in order to administer gas.

Both mCCR’s and eCCR’s address the dangers of Co2 build in very similar ways. All use some kind of chemical absorbent that reacts and binds Co2, “scrubbing” it from the exhaled air. The most common type of co2 scrubber canister is Axial, where the exhaled air flows from one end to another. Radial scrubbers, one’s that flow from the inside to the outside of a cylinder are gradually becoming more popular as they can handle being flooded and still be breathable and they are also enjoyed for their increased efficiency…allowing one to use less co2 absorbent over time. There is a new kid on the block, the EAC or Extend Air Cartridge. Some rebreathers can be adapted to them and one is designed exclusively for it. See the portion on the Optima for details.