Is there a valid reason for a pony bottle

[Ryan Mcshane]

What does science say happens different at 101 feet than at 100 feet and everyone reacts the same to?

Nothing, 1ft is completely insignificant.

However, beyond 100ft is a good place to start introducing helium, as the further you get from 100ft the more gas density becomes an issue and the more difficult it becomes for your lungs to ventilate properly. You then start retaining co2, which is a fair bit more narcotic than nitrogen. See where this is going?

 

[saxman242]

See where this is going?

Yah, it sounds an awful lot like a depth based limit like 100 feet is arbitrary and in reality, depth limitation is also condition/person dependent. Not that it isn't a good rule of thumb to stick to, but that there are more considerations to keep in mind. The notion that a "thinking diver" mustn't ever exceed 100 feet or else because science is an unsubstantiated statement that is based in dogma.

 

[tursiops]

I don't remember seeing these mentioned in SDI Solo book. I remember them basically recommending nothing less than a 25 cf.

You'll find differences common between the course standards, the course materials, and the instructor's presentations.

I don't teach SDI SOLO anymore, since I think the PADI Self-Reliant course is a bit better, but my recommendation then and now is always 19 cuft or larger, which is good to 100 ft, allows not hurrying to the surface, and allows a safety stop. You can work out the numbers, allowing for some accelerated breathing rate, and see that a 13 cuft is marginal, if not inadequate from 100 ft; 60 ft is probably OK.

It is all about likelihood of some kind of failure, versus impact; you can't just look at one or the other. I suppose my cylinder might explode underwater, with rather large impact on me, but it is very unlikely. I suppose my reg-tank connection might seep some bubbles, but that is very little impact. What you are looking for is those failures that are "off the curve," either anomalously likely, or anomalously high impact. Anything to do with your breathing system tends to score a little higher on the "impact" scale, hence the reason to carry a pony. Anything to do with hose or o-ring failures tends to score a little higher on the "likelihood" scale, hence the need for pre-dive mitigation by inspection and periodic replacements, and during-dive mitigation by.....carrying a redundant gas system, which is what a pony is.

It is worth saying again: a larger cylinder is NOT a redundant gas system.

 

[Storker]

Yah, it sounds an awful lot like a depth based limit like 100 feet is arbitrary

It's just as arbitrary as NDLs. It's a fact that both narcosis and gas density issues start becoming significant between 30m/100' and 40m/130'. Both the 30m depth recommendation and the NDLs are bright lines drawn through a gray continuum, but you have to draw the line somewhere and then try to follow those guidelines.

I've set my personal depth limit at 30m, but I'm not going to freak out and think I'm going to die if I accidentally drop to 31m. And similarly, I choose to stay within the NDL, but I'm not going to freak out and think I'm going to die if I exceed the NDL by half a minute. But I still try to stay within those - in your apparent opinion "arbitrary" - limits. Because those are the limits I've chosen for my diving, and I think they are sensible limits which give me a comfortable safety margin to risks which I consider excessive for me.

The "what difference does 1 foot make" argument is a perfect example of walking the slippery slope.

 

[saxman242]

It's just as arbitrary as NDLs. It's a fact that nitrogen narcosis and gas density issues start becoming significant between 30m/100' and 40m/130'. Both the 30m depth recommendation and the NDLs are bright lines drawn through a gray continuum, but you have to draw the line somewhere and then try to follow those guidelines.

I've set my personal depth limit at 30m, but I'm not going to freak out and think I'm going to die if I accidentally drop to 31. And similarly, I choose to stay within NDL, but I'm not going to freak out and think I'm going to die if I exceed the NDL by a minute. But I still try to stay within those - in your apparent opinion "arbitrary" - limits. Because those are the limits I've chosen for my diving, and I think they are sensible limits which gives me a comfortable safety margin to risks which I consider excessive for me.

The "what difference does 1 foot make" argument is a perfect example of walking the slippery slope.

I don't have any qualms with 100 feet being a good rule of thumb to stick to and I agree with you on setting personal limits from an educated stand point. It's part of knowing the risks, understanding your personal physiology, and proper preplanning on a dive. That wasn't the point that I was trying to make, but I agree that there's a slippery slope there and that saying "well, what's one more foot" over and over can quickly get you in trouble.

The point I was trying to make is that setting those limits is based on personal risk tolerance, understanding your personal physiology and proper preplanning of dive. The right number for you is 30m. For someone else, that number may be 20m, and for another it may be 40m. It's based on experience, conditions, who you are, etc. Are those guidelines established for a reason? Absolutely. It's much better to understand those reasons and act accordingly than to blindly follow a number that's been set and then justify it by non-nonsensical statements that only mean something at first glance.

 

[Storker]

I agree that there's a slippery slope there and that saying "well, what's one more foot" over and over can quickly get you in trouble.

The point I was trying to make is that setting those limits is based on personal risk tolerance, understanding your personal physiology and proper preplanning of dive.

Thank you for the clarification. Because that wasn't what I read from the post I quoted.

If that's what you really mean, then we agree pretty well. I have clubmates who - based on long experience - have set more liberal limits for their diving than I have set for my diving. I don't have an issue with that as long as they've made an informed choice and they're respecting my limits when we buddy up. OTOH, if one of my clubmates were a recently certified OW diver with hardly a double-digit number of logged dives and started arguing along the lines I read from your two previous posts, I'd probably take them aside and have a serious chat with them.

And this thread is located in Basic Scuba Discussions, a forum specifically for those who don't have decades of experience and a triple/quadruple-digit number of dives under their weightbelt.

 

[Ryan Mcshane]

I don't have any qualms with 100 feet being a good rule of thumb to stick to and I agree with you on setting personal limits from an educated stand point. It's part of knowing the risks, understanding your personal physiology, and proper preplanning on a dive. That wasn't the point that I was trying to make, but I agree that there's a slippery slope there and that saying "well, what's one more foot" over and over can quickly get you in trouble.

The point I was trying to make is that setting those limits is based on personal risk tolerance, understanding your personal physiology and proper preplanning of dive. The right number for you is 30m. For someone else, that number may be 20m, and for another it may be 40m. It's based on experience, conditions, who you are, etc. Are those guidelines established for a reason? Absolutely. It's much better to understand those reasons and act accordingly than to blindly follow a number that's been set and then justify it by non-nonsensical statements that only mean something at first glance.

Who is blindly following anything?

“Personal limits” have a big drawback. They dont work when it comes to diving in a team.

Beyond 30m/100ft just so happens to be where most people start getting a bit loopy without a bit of squeaky stuff, especially if they are working hard.

I’m at 30m on nitrox and I see something cool a few meters below, I’m gonna drop down and see it. It’s about using your noggin.

This “blindly following dogma” isn’t something that happens in the real world and is far from what is being taught in “Kool aid” classes.

 

[Storker]

“Personal limits” have a big drawback. They dont work when it comes to diving in a team.

They don't?

I'm not a "team" diver, but I'm definitely a buddy diver. If i don't know my buddy's limits before we take the pre-dive chat, I make damned sure to ask about them. And just as I'm perfectly willing to plan and conduct our dive within my buddy's limits, I'm expecting the same courtesy from my buddy. If I don't get that courtesy, we're probably not compatible and I'll prefer to buddy up with someone else instead.

How doesn't this work in a "team"?

 

[Ryan Mcshane]

They don't?

I'm not a "team" diver, but I'm definitely a buddy diver. If i don't know my buddy's limits before we take the pre-dive chat, I make damned sure to ask about them. And just as I'm perfectly willing to plan and conduct our dive within my buddy's limits, I'm expecting the same courtesy from my buddy. If I don't get that courtesy, we're probably not compatible and I'll prefer to buddy up with someone else instead.

How doesn't this work in a "team"?

Im not talking about the guy on the boat that doesn’t want to go past 20m. I totally agree with you on that. I’m talking about the “I’m good to 50m on air” macho men types.

 

[Storker]

I’m talking about the “I’m good to 50m on air” macho men types.

If the “I’m good to 50m on air” macho man respects the limits of the guy on the boat that doesn’t want to go past 20m, all is good, no?

As long the diver(s) with the most liberal personal limits respect the limits of the diver with the most conservative personal limits, personal limits definitely work also in team or buddy diving. It's when personal limits aren't respected that we have a problem. And that isn't a problem with the limits, the problem is with those who don't respect other divers' limits. If my buddy is fine with going to 56m on a single 12L filled with air, that's not a problem for me. Except if his SHTF while I'm there, because then I'm involved in a serious CF and will have to call emergency services. So if I'm the dive leader that day he won't be doing that on my watch. Otherwise, it's only a problem if he wants to take me with him to 56m on a single 12L with air.