I am not cave certified and I am in no way looking for advice on how to perform cave dives. I always dive within the limits of my training and experience, and implore others to do the same. I am, however, very interested in cave diving, and I ask these questions as an intellectual exercise. If I ever reach this level of training, I'm sure this will be covered in classes, but in any case it will be many years from now, so I ask now to satisfy my nerdy curiosity...
In the latest video on
BlueWorldTV, they dive in Jackson Blue and make a jump to a downstream passage off the main line. That immediately made me think about gas planning. I've read about strategies for diving downstream (siphons) using 1/6th of usable gas for the penetration, but what about dives with both upstream and downstream sections?
How would you approach gas planning on a dive like this?
- Recalculate when you get to the jump, so you use 1/6th of the remainder of your 1/3?
- Plan the whole dive at 1/6? Or an intermediate like 1/5?
- Other strategies?
Disclaimer-do on take anything you have seen on Blue World to date as anything close to resembling proper cave diving techniques.
Now, this is probably going to be a long post.
The "rule of thirds" came about back in the day with teams of 3 in a springing cave system, not dissimilar to what is encountered on the mainline in Jackson Blue. That rule has been used as gospel outside of its original intent but the theory is third in, third out, third reserve with the expectation being that the exit takes less time than the entrance due to the flow in the cave. This is not adequate in low-flow or no-flow caves and several good instructors will actually have you lights out air-share all the way to the exit where most people are unable to complete a safety stop before they suck the tanks dry.
When gas planning for a siphon, most people will just dive to sixths on the way in which allows them 5/12 each for exit/reserve. This is arguably far too conservative for caves like Peacock 3, Cow-Downstream , or Hole in the Wall-Downstream which are technically siphons but are usually barely perceptible flow rates.
Parallel lines is a bit of an anomaly because its siphon is created by an eddy from the main passage and it's not consistent in its flow depending on how the main spring is moving. The most typical dive utilizing the // lines will be to jump in, jump out, and actually exit in the main passage which needs to be treated and planned like a circuit. Typically you would go up the gold line until you hit turn pressure and place a cookie on the line in that spot. If you make it to the exit of // lines then you would drop your cookie at the marked jump point. Dive 2 would be to jump into // lines and kick up until you jump back onto the gold line. Provided you haven't hit your turn pressure you would then pull the line to jump from // to gold and then coast down until you hit your first jump line and pull it. Exit as normal.
Assuming you completed this successfully you would record the amount of gas used for each section of the cave and hold that in wet notes to be able to calculate required gas volumes for each of those sections and you can extrapolate how much gas you need, essentially recalculating your gas plan. This is also very useful if you are going to a spot in a cave and then hanging out there for some period of time to know when you actually have to turn back. Rather useful for things like photo sessions where you mark the gas you used to get in, say 400psi, and then you know you need to leave with no less than 800psi if the rule of thirds applies to that type of diving.
I'll say it because no one else will. The way this is typically dove if you plan on going past where // pops back out is that divers will blind jump into // lines because through most of the passage you can actually see and get back onto the gold line every 30-50ft or so, then blind jump back onto the gold line and carry on until they hit turn pressure and coast out on the gold line. The slightly less bad way is to jump into // and blind jump back onto the gold line but you should verify said gold line if you're going to do that knowing that it's not the way you're supposed to do it. If you've verified it though it still maintains a continuous guideline to the exit.
Before the dive you have to calculate all of this based on expected exit speeds, expected avg depth, expected gas consumption etc. to be sure if you should even bother attempting it. This is usually not done by open circuit divers because things like the rule of thirds is easy and means they don't have to do much in terms of dive planning, but it becomes critical if you are on CCR or on a DPV where you actually have to pre-plan your gas reserves.
) toward really understanding the Rock Bottom reserve strategy commonly used for open water. Once you have a solid grasp of that, you should be able to answer your original question yourself. Conceptually, gas planning for cave (or wreck or deco or even open-water) diving is not difficult -- it's merely the repeated application of those basics.
