Can you share some details of procedure for practicing CESA, say from 40'? Are a buddy and/or shot-line needed? Or can I just tell my buddy before we dive I'll be trying this - he could sit on the bottom and watch?
Thanks in advance,
Nigel
Pony bottle - what size?
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Can you share some details of procedure for practicing CESA, say from 40'? Are a buddy and/or shot-line needed? Or can I just tell my buddy before we dive I'll be trying this - he could sit on the bottom and watch?
Thanks in advance,
Nigel
LeadTurn_SD
Contributor
Hi Nigel,
I'm somewhat hesitant to talk too much about practicing CESA; I'm not an instructor of dive pro of any kind, just an "average" diver, but was taught to do CESA from depths that are generally deeper than what is taught today. My best recommendation would be to practice with a good instructor.
With that disclaimer out of the way:
1. A buddy is a good idea. He/she surfaces with you face-to-face. Do not leave you buddy alone (and vice versa).... you (and they) are not relieved from the regular duties of a buddy while do any types of drills, so do not get separated.
2. 60' per minute used to be the standard ascent rate. This is the rate I am coming up during a practice-CESA, not 30'/min. Most folks can "suspend breathing" for about minute without too much trouble (very uncomfortble for sure, but try it and you'll see it is possible). This means that most people can get to the surface from at least 60' if using a 60'/min ascent rate most of the way.... I feel slowing down in the final 15' or so is safer when practicing.
3. I've always practiced with an empty or near-empty BC for safety reasons so that as I'm swimming up I do not have to worry about the ascent "accelerating" too much (from expansion of air in the BC). In real life you are going to head for the surface with whatever air is in the BC, and be prepared for it to expand rapidly and vent as needed.... in practice my feeling is it is safer to keep the ascent rate well under control, and swimming up a slightly negative rig is a safer way to do this. Do know how to "flare" to slow down a too-fast ascent.
4. Some folks prefer the "Ahhh" sound while ascending; I prefer a slow release with an open airway; I guess more like whistling than singing, kind of a slow sigh. Keep your reg in your mouth.
5. If you are going to practice this, it should be at or near the beginning of the 1st dive of the day, so their is minimal nitrogen loading.
6. I don't use a shot line. But I shore dive in clear tropical water, and have no problem arresting my ascent if needed.
7. Go back to my disclaimer. I'm not an instructor, and you should really practice this WITH an instructor.
Best wishes.
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The answer is "it depends".
First of all, you want to work out your surface consumption rate - what is big enough for one person, may not be big enough for another. Try searching for "surface air consumption" or "SAC" to find out how to calculate how much air you breathe in a minute.
This is, of course, not a static thing - how much you breathe depends on many factors, including how stressed you are. I'd recommend finding out your working SAC - with my students, I get them to find a rock underwater, put their hands against it and swim "fairly hard" (RPE of 7/10 for the gym junkies) for two minutes. If you measure the drop in tank pressure, you can work out how much air you are breathing (I can tell you in metric units how to do this, but always get imperial calcs wrong!).
My gut feel is that a 13 cu ft tank would be fine for most people at the depths you are talking about. But also consider whether you may grow in terms of your - you may find in the future that a 13 cu ft is limiting. Why buy two tanks, if you can buy just one that works well for all circumstances?
I'd also suggest trying different tanks and seeing how they feel - particularly if you are carrying the pony as a slung tank. For example, I've recently swapped from a Catalina 19 cu ft to a Luxfer 30 cu ft. Even though it's larger, the Luxfer tank is actually much less obtrusive than the Catalina because it sits much more nicely (Catalina tanks are very tail heavy). Hope this helps.
LeadTurn_SD
Contributor
Hi Nigel,
I think I should have used a different word instead of "comfortable"
Important point before I continue: My opinion is that in a real OOA, you are NOT going to be starting with full lungs. You'll probably have just exhaled, then discovered you can't get air when you try to take your next breath, or maybe get a mouth full of water :shocked2: , then notice your buddy is 30' away and swimming away from you....
The 60'/min rate vs the 30'/min rate was meant to demonstrate that you can ascend from twice the depth in the same amount of time, safely. 60'/min was the old standard ascent rate, with no stops if within NDL. So performing a CESA at 60'/min would (in theory) still be "safe" in terms of DCS, assuming you are within NDL.
You are right, you WILL have enough air to exhale.
From deeper depths, my feeling is that the ascent rate is going to be considerably FASTER than 60'/min. While not something anyone ever PLANS for, it is worth thinking about what-if's... so for me, if forced to CESA from >60' I would swim up at whatever rate it took to get me to the surface without passing out. I know I can greatly exceed 60'/min and not embolize (many divers have done so), but I might get bent.... but I'll certainly drown if I don't get to the surface quickly enough.
I don't know how long I can go without breathing before passing out. I do know, while sitting in a chair and breathing normally, I can exhale, and then go for about a minute before it gets "uncomfortable". To me that is a more realistic "test", not how long you can hold your breath on a full inhale.
So if I'm somewhat confident that I have about a minute to safely reach the surface before passing out, and if I consider 60'/min or so an acceptable emergency rate of ascent, I know that my risk increases greatly if I get more than 60' away from the surface.
Best wishes.
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The vintage ponys were usually size D steel O2 bottles with 1/2" NPT thread post valves. They held about 14 CF. Sometimes we used size E O2 bottles, same diameter but about 25" long. Later in the vintage era, we started to use 14CF 2015psi aluminum ponys.
If serviced, the old regulator is probably just fine, but I would prefer a 70's Conshelf for a super reliable single hose regulator that you can still get new parts for.
My redundant air now is usually 1/3 to 1/2 the size of my main back gas, solo or with a buddy, vintage or not. Sometimes when I dive with an instabuddy, I carry 2 ponys.
If serviced, the old regulator is probably just fine, but I would prefer a 70's Conshelf for a super reliable single hose regulator that you can still get new parts for.
My redundant air now is usually 1/3 to 1/2 the size of my main back gas, solo or with a buddy, vintage or not. Sometimes when I dive with an instabuddy, I carry 2 ponys.
derwoodwithasherwood
Contributor
Your answer got me thinking about gut feelings and rules of thumb. An oft-used rule of thumb is the rule of thirds -- the final third being the reserve for emergencies. In that vein, would it be reasonable to say that a good rule of thumb for the redundant emergency air supply is that a pony should be 1/3 the capacity of the main cylinder? One could argue that anything less wouldn't be fully redundant, right?
I would think that since the dive plan already takes the size of the main cylinder into consideration, basing the pony size on the main cylinder size should be a valid approach. Of course, perhaps I should have thought of this before I settled on a 19. It doesn't quite measure up to a 72, never mind my 95. Ooops.
One of the problems with rule of thirds is that it can be both ludicrously conservative, or ludicrously aggressive.... depending on depth, time, type of dive.
I tend to apply rule of thirds in a slightly different way - ignoring the pony issue for now. First off is to calculate your "minimum gas reserve". This is the amount of gas needed for two divers to ascend "safely" to the surface from the maximum depth of the dive. I tend to pad the calculation, to include a minute at depth, a minute at half depth and three minutes at 5m - but you can redefine "safely" as you choose.
Let's say for the sake of argument that your minimum gas worked out at 70 bar for a given tank. You deduct the minimum gas from your starting pressure to give you your available gas. So a 210 bar fill gives you 140 bar available.
You then decide whether your dive is all usable, half usable or thirds usable. All usable would mean you can just pootle around until you hit your MGR pressure and surface where ever you like. Half usable is "it would be nice to return to the start point, but not essential". Thirds is cases where you *must* return to the starting point, e.g. a cave or wreck penetration. It's a lot better (IMHO) than straight thirds.
Now, to the pony. Basically your pony should be sized at half the MGR or larger. So if your MGR was 70 bar for a 10 litre tank, that's 700 litres. Your pony will need to hold at least 350 litres. Assuming a fill pressure of 200 bar, that's a 1.75 litre pony (that's marginally smaller than a 13 cu ft tank). Obviously your MGR depends on depth, so this isn't a general recommendation for size of a pony!
The way I teach my self-reliant diver course is that all divers in the team calculate MGR for backgas and stick to it. First port of call in an emergency is sharing gas with a team mate. Yet every diver in the team has a self contained pony bottle that contains enough gas for one diver to ascend from maximum depth, so they can either ascend independently if required, or donate the pony to a team member to allow them to do so.
Hope this helps.
EDIT: PS - you can calculate MGRs for a couple of depths, assuming tank size stays the same, then remember those. It means the whole thing comes down to applying the same rule of thumbs with no math. If you can remember the different MGRs, then you can easily do a multilevel dive in OW, i.e. when you hit your MGR for your first depth the ascend to your second and hang out there until you reach the MGR for that depth (assuming an all usable situation).
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derwoodwithasherwood
Contributor
Understood -- and agreed. And actually the process you describe is similar to the one I used to settle on a 19 for myself.
However, over the years the question of pony size seems to come up again and again. For one reason or another, not everyone seems prepared to delve into that level of detail. I was just trying to think of a shortcut 'rule-of-thumb'. Basing it on the size of the main tank seems to be a way of automatically sizing the pony as the size of the main tank limits the depth and duration of the dive, which in turn is the basis for the size of the pony.
Now, as you pointed out, the first step in the traditional dive planning stage calculates a minimum gas reserve (NASDS called it the "minimum turn pressure", but the meaning is clearly the same) for two divers to safely return (if necessary), ascend, and complete a safety stop. Similarly, application of the 'rule of thirds' is most often associated with technical diving evolutions that involve pairs or teams. So my suggestion may well be somewhat extravagant when applied to solo excursions in a rec diving environment (of course, where air is concerned I'm a firm believer that carrying too much is better than carrying too little <lol>)
Anyway, to put the application to the test, here are some numbers, assuming an RMV of .65 and a typical S80 (actually 77.8 cu ft). I'm allowing a 5 minute reserve at maximum depth to deal with an unexpected problem that necessitated an assent and a clear overhead, allowing a direct assent to the surface, and calculating for a solo diver -- ie, no reserve for a buddy. The SAC works out to about 25 psi/min.
At two atmospheres, 5 minutes burns 250 psi. Ascent to the safety stop uses another 25. The safety stop is 125, for a total of about 500 psi or 12.96 cu. ft. So for the person tooling around at 30 feet, a 13 cu. ft. pony is probably adequate.
At the other end of the rec spectrum, 4 atm, 5 minutes burns 750 psi. Ascent to the safety stop uses another 150 psi and then the 125 for the safety stop itself for a total of 1025 psi or approximately 26.8 cu. ft. That's not far off the 1/3 mark of the S80 main tank and it assumes the diver did not incur a deco obligation during that 5 minutes of dealing with the problem.
Now, obviously every situation is different. Big guys tend to use more air, out of shape people tend to use more air, stressed people tend to use more air and even just working harder (maybe during that 5 minutes dealing with the emergency?) will cause someone to use more air. But then too, people who use more air are apt to carry bigger main tanks. Smaller tanks tend to be popular with smaller individuals -- who tend to use less air. And not every dive is down to 100 feet.
So, while it is undoubtedly in a diver's best interest to fine tune the equations to suit their personal situation, saying that -- as a general rule of thumb to cover most situations -- a pony should be at least 1/3 the size of the main tank is not an unreasonable statement, IMHO. But that's just my 2¢.
I think I mis-judged your original post, sorry... for some reason I had the feeling you were looking for information, rather than simplifying things for others! My apologies.
I think a rule of thumb approach can work - 1/3 of main tank may be reasonable for many people most of the time.... I guess it's the exceptions that worry me.
Seeing as the question crops up time and time again, maybe it's worth having a sticky in this forum that puts a couple of calculations visible for people to work through??
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