Following on from one of the main debate streams that surfaced during the NACD conference, a few buddies and I recently discussed the “deliverables” of a CCR Cave Diving course.
One of them — and it seems a valid topic to present to “new” CCR cave divers as well as experienced OC cave divers signed up for a cave orientation course — is gas volume management for bailout scenarios.
Typically, OC cave divers have a pretty simple set of rules to govern how much gas they need to carry with them. For CCR divers, those rules are not as simple because there is an extra variable. And that variable is the diver’s gas consumption rate once he has bailed out: it will vary a lot!
To better understand that why this is, we have to consider the reasons that would drive a CCR diver to abandon “the loop” to breathe open-circuit.
One example — perhaps the worse-case scenario — is carbon dioxide poisoning. There are a bunch of possible events that could lead up to this, but for the time-being, let’s just take it as read that the diver has experienced one almighty pear-shaped CO2 breakthrough event, and has ALMOST left it until too late before bailing out. He is hyperventilating and is close to panic.
So, what consumption rate is best to use as a benchmark?
For most to the OC stuff I teach — and in lieu of real data — a SAC rate of 14 litres per minute is a good starting point. (That’s about half a cubic foot for those struggling with imperial units.) To find the actual consumption (RMV), that number would be multiplied by the depth or average depth expressed in bar and the product of that calculation by a number to represent the Dive Factor (workload, thermal stress, etc.). For most OC dives, a DF of 1.5 to 2 is OK. However, for a CCR diver battling back from the edge of CO2 oblivion, a DF of 3 is the minimum recommended stating point.
To put this into a real-world example, consider a CCR diver bailing out around a 40 minute swim from the mouth of a cave with an average depth of 25 metres.
Our 14 litre per minute consumption rate now gives us 14 X 3.5 (depth in bar) x 3 (DF) X40 (minutes to surface) which equals 5880 litres.
That is a lot of gas, and effectively requires the diver to carry more than two fully-charged 12 litre cylinders (aluminum 80s) as bailout. Is this realistic? Is it realistic to imagine that the elevated consumption rate experienced immediately following CO2 break-through would persist for the full duration of the exit swim? Also, is it wise for the diver to have no redundancy in the event of one of those two bailout regulators malfunctioning?
What do you think?