I don’t think I’m going to do a ton of double-stage diving anytime in the near future.
I’m starting to think it’s kinda dumb. Down here, at least, where one is using all 80s; with more substantial backgas there’s more sense to it. When you do the math it starts to seem not worth the bloody effort here pretty quickly.
(Yeah, this is a math-y post. Feel free to slip into a coma immediately or go outside and play. Go get a nice lunch. Imagine shapes in the clouds. Feed birdseed to the ducks in the park. Live a little.) Let’s take as a given - for this example anyway - that we are using AL80s as both “backgas” and as two stages like the overwhelming majority of divers in the area do. All tanks are filled to 3000psi/210bar. Let’s also assume - because it makes the math easy - our diver has an RMV of .5cf/15l per minute. After the kerfuffle of shuttling all that crap to the water and strapping it to themselves our team goes through whatever dumb pre-dive acronym they happen to use. They determine a gas plan to drop each bottle at 1700psi/120bar and to turn at 2500psi/170bar. “Well just how the hell did they come up with that?” You just had to ask, didn’t you?
Using “half plus” to determine drop pressure is straightforward enough.
3000/2 = 1500 1500 + 200 = 1700 There’s our drop pressure. Turn pressure is another thing altogether. We need to reserve that volume in case of an OOG at the deepest point of penetration to get two divers all the way out of the cave. We plan for just in case of two failed stage bottles because… I dunno… one diver on the team never takes care of their gear, nor do they ever remember to shut off their stage bottles when they drop them. What a tool. We use 1300psi out of each stage. Baseline of a single AL80 is 2.5. Since there’s two of them we’ll use the “doubles” baseline (because it lets us skip a step) of 5. 1300/100 x 5 = 65cf So we need to reserve 65cf of backgas. 65/5 x 100 = 1300psi (that was weird, wasn’t it?)
So we’ve only got 1700psi to play with for our thirds. And, between the fact that I’m a mathematical dunce and pressure gauges aren’t that accurate, it’s best to just drop it to the closest number divisible by 3 (1500) and there we’ve got our useable pressure (500psi). Subtract that from our starting pressure and… Our turn pressure is 2500psi.
Metric is easier. “Half-plus” is generally implemented as 210/2 +15bar = 120 as a drop pressure, so 90 bar used. 90 bar x 11 litres = 990 litres used in each tank.
990 x 2 = 1980 litres used which needs to be reserved. There’s 4620 litres in backgas (22 x 210). 4620 total - 1980 reserve = 2640 available
2640/22 = 120 bar
That’s what we can figure thirds on.
120/3 = 40 bar useable. Subtract that from our starting pressure and… Our turn pressure is 170 bar.
Or you can just bypass all that shite and dive backgas to 1/6ths. Which is what I’d do. Because math is is the goddamn worst. Our team drops one bottle and everyone is very much enjoying finally being able to use their left arm again (or either arm if they’re on sidemount). Then they drop their second bottle and suddenly feel like a goddamn sea lion. Might as well be free diving they feel so streamlined. And this is exactly the moment that’s got me started on this fantastic voyage of the mind. You see, with the two bottles… with all that drag… with all that weight… Not only was the lack of arm mobility a pain in the ass, simply moving all that junk through the water is a chore. Yeah, with good buoyancy and trim and practice it’s easy-ish - but it still takes more effort to move that much stuff through the water than not.
So unless you want to strain yourself, you are swimming much slower than normal. Making your way through less cave in the same time.
What’s more - what is, in fact, critical - to maintain good buoyancy and trim with that much stuff you need to keep dealing with your drysuit and a TON of gas in your wing. Here in Mexico: up and down and up and down and up and down while shallow, where those gas volume changes are huge. This eats up a surprising amount of gas. For me, making constant adjustments, this burns through a little more than 150psi/10bar each hour. Our hypothetical diver’s RMV is about .5cr/15bar. Let’s say our dive averages 33f/10m, so our diver is breathing 1cf/min. And has, therefore, ditched their second stage bottle a little past an hour into the dive having used around 200psi/15bar out of backgas. Useable pressure was only 500 psi or 40 bar. Which is now down to only 300 psi or 25 bar before our diver hits turn pressure. It’s not even an intro dive anymore. At 33f/10m diver is looking at turning around about 15 minutes later. “But that’s still an additional 1/2 hour of dive time!” You cry, “That’s a hell of a cool, long dive!" Sure. It is. If our whole team is super proficient at stage ditching and donning, let’s say it’s around one minute for each bit of bottle handling. 65 minutes penetration on the stages. 15 minutes of penetration on backgas. Multiply by two to mirror those numbers for the exit. Squidge it a little to 5 minutes worth of stage manipulation. About a 165 minute dive. Pretty badass, right? Let’s look at the difference with only one stage.
Drop pressure is still 1700 psi as above. Turn pressure can be a little more liberal.
Still planning for OOG at max penetration and a failed stage (because that chucklehead still doesn’t doublecheck their gear)… So we use 1300psi out of one stage. Which works out:
1300/100 x 2.5 = 32.5 So the reserve out of backgas is only 1/2 of what it was before. 32.5/5 x 100 = 650 psi| Gauge tolerances/marking being what they are and erring on the side of “realising a stage is failed and getting our shit sorted might take an extra minute” let’s call that 700psi reserve. 3000 - 700 = 2300 psi to be used to calculate 1/3rds. Again, drop down to the nearest number divisible by 3 because it’s just easier that way. 2100/3 = 700 psi useable. Our turn pressure is 2300psi.
Metric is easier. It’s 50 bar useable. 160 is turn. It just is, OK? Sure, I can walk you through the math if you want, but I can see your eyes glazing over from here.
Our diver drops their one stage at about minute 33. Having used dramatically less gas dragging an extra bottle up and down through breakdown rooms, and in only half the time. So it’s just as likely that they’ve actually got near enough the full 700psi/50bar left to breath. So it’s likely our diver is going to hit turn pressure around 35 minutes later. Runtime minute 68 or so. A minute or so for handling the single stage bottle gives us a total runtime of something in the range of 138 minutes. A total difference of 27 minutes. 13 minutes of that in penetration time. Swimming 40f/12m per minute… about 500f/150m more cave give or take. Again, assuming you were swimming at that speed the whole time, and not slowed by extra drag. Somewhat counterintuitively, you hit that point of diminishing returns even more severely with a better breathing rate (I was doing all this in my head during yesterday’s dive using my own .4cf/11l per minute yesterday and was startled to find that it all worked out to a difference of only about 15 minutes total runtime/7 minutes penetration). Less gas breathed = more time in the water = more gas getting used for wing and suit = less gas around to be breathed. “But Roger… with a second stage you still have a ton of backgas to recalculate!” You’ve got that after using only a single stage anyway. Maybe an additional 100psi/5bar of useable pressure to recalculate made available by the second stage? If you’re recalculating from the cavern zone. But you’ll gobble that up dealing with the final stage drop right inside the entrance anyway. And you’ll still have one more tank to carry back to the car. Which sucks. So the moral of this story, if indeed there is one to be found: In the end, you’re going to see about the exact same amount of cave with one stage or two.
To hell with the second stage. If you think you need a second stage… what you really need is a rebreather. Or possibly LP104s.