A DIR spin on an old question

[Dtaine]

Ok, so DIR divers don't dive with ponies, that I know.

Nevertheless, I've seen people on this board recommending to have air in a slung pony, even if the back-gas is Nitrox. I've been thinking about this for awhile, and I've never quite understood why if my buddy and I are breathing Nitrox I should put a less "efficient" gas in the slung bottle.

Is there any real reason to not put the same mix in the pony bottle and the back-gas?

Just to clarify to make this easier: 1) I'm not trying to accelerate deco on the safety stop 2) it is just for reserve on dives with questionable/insta buddies 3) I haven't dived with the tank yet as I haven't set it up to sling yet & 4) I'll be getting my Nitrox cert soon, I'm diving air now

So where's the DIR spin? Well, to be honest, I'm not looking for the exact DIR answer, but I'm looking for the advice of DIR-minded divers.

 

[ppo2_diver]

Since this is the DIR forum, a bottle is either a deco bottle or bottom stage. If you were going to use it at the safety stop, then it turns in to a deco bottle. Otherwise, you would normally start the dive breathing the bottom stage, then switching to back gas when the bottle gets spent. So the bottom stage would contain the same gas as back gas.

 

[Dtaine]

Since this is the DIR forum, a bottle is either a deco bottle or bottom stage. If you were going to use it at the safety stop, then it turns in to a deco bottle. Otherwise, you would normally start the dive breathing the bottom stage, then switching to back gas when the bottle gets spent. So the bottom stage would contain the same gas as back gas.

Well you could call it a deco bottle then, although I won't be putting anything other than the back-gas mix in it. I was thinking I'd breath it on the safety stop as that would be a good time to practice switching to it and getting used to working with it while maintaining trim and buoyancy.

So I take it that even if it was used as a bottom stage - which it won't be, there's really no reason for it to have a different mix then the back gas. Obviously a travel stage would be different, and either way I'm not doing that at this point. I also don't have any need to accelerate the deco or safety stop in any way, so no need for an enriched mix there.

 

[gcbryan]

There isn't a DIR answer to this question.

 

[Tortuga68]

Anyway, I've seen people on this board recommending to have air in the pony, I mean stage , even if the back-gas is Nitrox. I've been thinking about this for awhile, and I've never quite understood why if my buddy and I are breathing Nitrox I should put a less "efficient" gas in the slung bottle

I'm not sure that asking in the DIR forum about non-DIR advice given in other forums is the best way to the answer you want - you could always ask the OP why the said it? But anyway I'll have a go at an answer, and if the mods want to move the thread I guess they will

On a non-deco dive, nitrox is only more 'efficient' in extending your NDL

So if you're using a pony as backup (ie for use in the event of a failure of your primary gas source/s), then I don't see any real advantage to filling it with nitrox, since you'll be calling the dive when you have to switch to your backup, and the primary advantage of nitrox (extended NDLs) is moot

Also, nitrox fills are usually more expensive than air in my experience

You could also make a case based on air having a deeper MOD than nitrox, which could be advantageous if you had an incident and went deeper than your dive plan, meaning breathing your nitrox backgas would put you at risk of CNS problems

I'm not agreeing with the advice, just giving you my $0.02 on why it may have been given

 

[Dtaine]

There isn't a DIR answer to this question.

There's a reason I asked it here though. Every day since I discovered the philosophy of DIR diving I've been evaluating my equipment, learning ways to streamline my diving and make it easier, more relaxed, and to trump it all more safe. Never before have I seen a system where every gear decision and every aspect of the dive is evaluated and designed to be simple and efficient. In an ideal world I'd be diving single tanks with other divers who I know to be competent and safety minded, who dive as a team without the same day same ocean mindset. Unfortunately this isn't an ideal world and as such dives are done with people I've never seen before that day, with those who see no value in a buddy, and with those who take an all too carefree approach to diving. Because this is such I choose to carry a tank that otherwise I would see no need in having, and because I want to do it in the most efficient and most effective way I've asked those who follow what I see as the best approach to diving to give their opinion.

From everything I've read the DIR approach to diving involves being a thinking diver, adapting your equipment to dive in the most efficient manner possible; if I don't know who my buddy will be when I set out in the morning for a day of diving then I need to know that I have a equipment configuration that is setup to handle a buddy that doesn't care. If I approach that dive with a single-tank configuration and no redundant gas source other than my buddy's tank then I don't see how that's being a thinking diver, and I'm out of luck if I have a problem (yes I can self-rescue, but that could mean a CESA, which is less than ideal at 100-130 feet). If, however, I take the extra tank, my buddy bolts, and I have a problem, then I switch to the pony and turn the dive. Diving with an unknown buddy isn't DIR, but it's a situation I face all the time. The only way to solve this problem is to only dive with guaranteed buddies, or carry a pony, with the former method I'd be diving maybe once a month, with the latter I can dive once or twice per week.

As Doing It Right: The Fundamentals of Better Diving states, "A good SCUBA equipment configuration needs to carry through all diving; it should allow for the addition of items necessary to perform a specific dive (e.g., an ice dive, a cave dice or an open water dive) without interfering with or changing the existing configuration. Diving with the same configuration allows for the identical response to emergencies while reducing effective task loading due to familiarity. In other words, it not only helps solve problems, it prevents them." I will be diving with unfamiliar or possibly reckless buddies, which automatically makes those dives non-DIR, if I'm going to take a DIR approach to that problem, then adding a pony makes sense; it prevents a problem (I won't have to do a CESA, or I should say it becomes extremely unlikely), it adds very little complexity to the dive, and it doesn't interfere with anything. It's not DIR, but it's using the concepts of being a thinking, efficient diver, to manage an already non-DIR situation.

If I'm flamed for this thinking I won't be surprised, and it won't affect my diving or my approach to it; all I'm looking for is feedback, I'm not looking to troll or offend, if the question's not welcome here I'll freely move it elsewhere.

-Dave

 

[Dtaine]

I'm not sure that asking in the DIR forum about non-DIR advice given in other forums is the best way to the answer you want - you could always ask the OP why the said it? But anyway I'll have a go at an answer, and if the mods want to move the thread I guess they will

See post #6. I do have a reason for asking it here.

On a non-deco dive, nitrox is only more 'efficient' in extending your NDL

So if you're using a pony as backup (ie for use in the event of a failure of your primary gas source/s), then I don't see any real advantage to filling it with nitrox, since you'll be calling the dive when you have to switch to your backup, and the primary advantage of nitrox (extended NDLs) is moot

Right, but breathing Nitrox decreases the nitrogen loading one gets during a dive and affects the final pressure group, by staying on Nitrox, rather than switching to air, it's much simpler to figure out what pressure group I'm at following the dive. Also, if I'm ascending from 100 feet sure breathing air for a few minutes won't be much of an issue, but staying on Nitrox keeps my equivalent air depth lower (81 feet @ 32%, at 100 feet), adding conservatism.

Also, nitrox fills are usually more expensive than air in my experience

True

You could also make a case based on air having a deeper MOD than nitrox, which could be advantageous if you had an incident and went deeper than your dive plan, meaning breathing your nitrox backgas would put you at risk of CNS problems

True, but having a competent buddy, and a completely DIR dive without a pony, on Nitrox, would also subject my dive to the same MOD concerns. I don't see a reason why a failure would cause me to exceed the MOD, I wouldn't be pushing the limits of the mix anyway. I understand this issue though, it is a risk.

I'm not agreeing with the advice, just giving you my $0.02 on why it may have been given

I'll thank you for that.

 

[Tortuga68]

by staying on Nitrox, rather than switching to air, it's much simpler to figure out what pressure group I'm at following the dive

Yes, but if you still have nitrox available, why would you be switching to your pony?

I don't see a reason why a failure would cause me to exceed the MOD, I wouldn't be pushing the limits of the mix anyway

No, because if you're not diving close to the MOD, then you're not using the best mix


These are two examples of why this isn't the right forum for you to get the answers you're looking for - it seems to me that you're trying to apply rules you don't fully understand to a scenario that the rules weren't designed to cover

DIR is designed as an all-or-nothing approach, not take some bits and ignore the others. By all means adopt some DIR practices to your rec. diving, lots of people do (long primary hose, BP/W, bungied secondary etc), but there's really no point asking for DIR validation of it

 

[John_B]

Look, what you're missing in your "what are the DIR rules" approach to this is that DIR is primarily about team diving.

There is almost no end to the number of problems with slinging a pony with air in it from the perspective of team diving. Because you don't have a DIR buddy? Because you haven't properly calculated "rock bottom" (aka: minimum gas) for a 100'+ dive? Because its a cheaper way to redundancy (vs. doubles)? Because you can't afford an EAN 32 fill? You are planning to use air at 130' to "solve" an OOG situation (for you or someone else)? They used to use the term "nickel rocketry" to describe the thought process of skirting safety protocols to save money, maybe do a google search with site:scubaboard.com in the search terms.

Maybe you just have to realize that its not possible to do some of the above dives as DIR dives. Its pretty hard to do a DIR dive without a DIR buddy (I struggle with this locally where there is nobody else with a DIR mindset). We haven't even opened the discussion of doing the second dive on a 40 minute SI....

The case can be argued whether you could reasonably dive air on a relaxing 30' warm water reef dive, but I don't think you can make a similar argument for 130' on a single tank with no DIR buddy and an air in a pony bottle in place of redundancy. They used to say, "There are no DIR divers, only DIR dives." I don't think like the above example sounds anything remotely like a DIR dive. Maybe the dives you are doing today with the buddy/training/skills/equipment you have today aren't compatible with DIR. That's a tough thing for some people to digest and it drives some people away from DIR. Its not for everyone, n'est-ce pas?

 

[Web Monkey]

Is there any real reason to not put the same mix in the pony bottle and the back-gas?

Sure, because it's extra "stuff" and doesn't get you anything.

Just to clarify to make this easier: 1) I'm not trying to accelerate deco on the safety stop 2) it is just for reserve on dives with questionable/insta buddies 3) I haven't dived with the tank yet as I haven't set it up to sling yet & 4) I'll be getting my Nitrox cert soon, I'm diving air now

You're really looking for a DIR answer to a non-DIR question. It's like asking "What kind of cheese can I put on my bacon cheeseburger to make it kosher?" The "DIR" answer would be "Don't dive with insta-buddies"

If you dive with someone you trust, and have both done the necessary gas calculations and are both ready and able to share gas if needed, you won't need the pony because the person you're diving with, by definition would have reserved all the gas you need and would be right there next to you in case you have a problem (not just OOG).

I used to dive with a pony all the time, however after spending the summer doing Adv Nitrox/Deco training with my buddy, I've realized that it's much safer having him around than an extra tank. A pony won't have a line cutter out when you send up a lift bag (in case you get caught) or have a regulator right in front of your face, ready and waiting if you have some sort of air delivery problem, or stop you when you're about to do something dumb.

If you're diving with someone you don't trust, it's by definition not a DIR dive, since you're now essentially a solo diver with the extra liability of someone who may run out of air, not maintain your reserve gas or do other weird stuff at any time.

Although this will probably sound elitist, my solution to insta-buddies is to not dive with them on non-trivial dives. It keeps my stress level down.

Terry