Although alluded to in your initial rebreather training, the idea of ‘optimal loop volume’ escapes some divers. Even among advanced rebreather divers, the loop appears to be managing them instead of them managing their loop. They learned to cope, instead of, to control it.
What is ‘optimal loop volume’?
The terms frequently used when explaining proper loop volumes are ‘optimal’ and ‘minimal’. For the purpose of this article, ‘optimal loop volume’ is achieved when:
During a deep breath you trigger either your ADV or collapse your counterlung before reaching the maximum of your inspiratory capacity (Inspiratory reserve volume).
This definition states that ‘optimal loop volume’ is based on the individual diver rather than on an absolute number. The idea is that while you breathe normally (tidal volume) you are not activating your ADV or fully collapse your counterlung unless you take a deep breath.
What is ‘minimal loop volume’?
For some the terms ‘optimal’ and ‘minimal’ are interchangeable, but others define ‘minimal loop volume’:
The volume you breathe during normal breathing (tidal breathing) only. Any increase in volume of breathing would activate your ADV or collapse your counterlung.
Whether you prefer ‘optimal’ vs. ‘minimal’ may be your choice. What you don’t want is diving with greater than optimal to maximum loop volume (maximum inflation of your counterlungs) where you can’t even trigger your ADV or collapse your counterlungs on a single breath. Your clue is this: The amount of gas you have to vent in order to “reach” your ADV, is excessive loop volume.
Learning to dive with ‘optimal loop volume’ takes a little practice. But in a short amount of time you instinctively “know” where you are and adjust accordingly.
Why does it matter and what are the benefits?
Affects total gas mixture: If you have a smaller volume of gas in your loop, changing the concentration of the gas will be much faster. It will also cost you less back gas. I.e. it will take you a lot less oxygen to increase your PO2 if you have a smaller loop volume than a larger one. Thus, if you want to bring up your PO2, exhale first before adding oxygen. The same principle applies for a diluent flush. You will bring down a high PO2 much more quickly if you have less volume to dilute. Likewise, your solenoid will fire less long/frequently during a dive if your loop volume is optimized.
Buoyancy: Are you struggling with your trim? Do you carry a lot of weight on the top? Are you kicking up stuff? These could be indications that you carry too much gas in your loop! If you want to ‘nail’ your buoyancy, you will do that much more successfully with an ‘optimal loop volume’. Little changes will have better response times. You go from “sloppy” to being “tight”.
Too much weight: carrying too much loop volume requires more weight to compensate. If you have too much weight, you will require more of your diluent for BCD /wing inflation. Do you really need those big tanks? Maybe you just need to optimize your loop.
Increased tank sizes: Excessive loop volume requires more gas to achieve desired loop concentration. Bigger tanks = more weight = more gas= narrow buoyancy range.
What if my loop volume is too low?
Breathing too low of a loop volume is quite uncomfortable. It makes you air hungry and short of breath, partially because you are retaining CO2. It leads to insufficient and ineffective breathing with underutilization of lung segments.
What happens if my loop volume is too large?
You might have difficulty exhaling. If you cannot exhale, you cannot inhale! This situation gives you the sensation that you “cannot breathe” and you start grabbing for your bail-out regulator unless you remedy your counterlung volume. Another hint is that your OPV goes off or you get puffy cheeks. But the biggest complaint is “I did not have enough gas to breathe”.
In closing, rather than being managed by our loop volume, why don’t we manage it? Even experienced CCR veterans are not immune to a runaway loop. It takes awareness and then a little practice getting right.