On using a small tank to increase bottom time and provide redundancy

[SouthernSharktoothDiver]

@mr_v I have no problem with your being honest about your opinion, but I would appreciate if you back your claim up with some reasoning. I'm open to changing what I'm doing, but I want a solid line of reasoning as to why I'm doing something wrong.
Other options: (From simplest to most involved in terms of equipment cost and general complexity.)

1. Dive a single tank (either 80 or larger), relying on CESA in the event of equipment failure.
Reason for rejecting: under the conditions I've discussed above, I dive very negative to remain on the riverbed in the face of steady current. With 16-20lbs of lead, no air, and a 40 ft ascent, I don't want to bet my life on CESA.

2. Dive a single tank, with a buddy on a tether to ensure a source of redundant air is nearby.
Reason for Rejecting: While I have a very good dive buddy, who is open to using a tether, it's not very practical, since down there in the dark with the logs and stuff, it makes for a significant entanglement issue (nothing dangerous, but very inconvenient), and additionally, it makes moving in search patterns more difficult. I can do it. I have done it. But it's a rather limiting solution, and I prefer to find a better way.

3. Dive a single tank, with a pony bottle held in reserve (barring emergency or the occasional practice/test scenario)
Reason for Rejecting: This is what I plan to default to if I am convinced my current plan is unsound. That being said, it seems silly to have the cost and burden/drag of carrying an extra 15 minutes of air and not use it if I can do so safely and effectively.

4. Dive a single tank, with a pony bottle to be used at the end of the second dive, while keeping a good reserve in the main tank.
Reason for Accepting: If I keep enough air in my main to make my ascent safely (600-700 psi, ascending from shallow depths, no safety stop required), I'm still fully redundant, since my gas plan is to finish the dive on the pony. If the main fails at any point, I either switch to or remain on the pony. If the pony fails at any point, I immediately either switch back to or remain on the main, and return to the surface. The only real risk is that whichever reg I am not using might free flow and lose my reserve gas, but I plan on positioning them where I would notice a free flow, and also, if I switch to the pony and it's empty, I would just switch back to my main. If my main has failed, and I switch to the pony to find it empty, I'm probably screwed, but no more so than I would be without it, and again, this requires a double equipment failure without me noticing the leaks, which I consider highly unlikely.

5. Dive independent (or manifolded) doubles
Reason for Rejecting: Fundamentally, the only difference between this plan and plan 4 is that both tanks are of the same size. I do not see how this affects the safety of my plan under the circumstance I have discussed in my other posts. Furthermore, quite frankly, while I think I physically could manage doubles, it seems like it would be quite the pain in the tuchas, and also, the cost of buying another set of 80's, a doubles wing, and the cam bands/metal bands would cost me a great deal more than my current plan or plan 3, and I don't really want to spend that amount of money for something that will also be physically much harder to use unnecessarily.

6. Dive sidemount doubles.
Reason for rejecting: In addition to the cost issues and relative lack of advantages discussed in plan 5, I don't know how to sidemount. No one I know knows how to sidemount. Suffice to say that, while someone might could dive under the conditions I'll be diving in, in sidemount, and it be much easier, I don't know where to find that guy, or how to convince him to teach me without spending a great deal of money.

Beyond these, there are definitely other things that could be done (a rebreather, Snuba, independent 40's, etc) but I think what I've discussed about are all the options that are really practical or feasible for the conditions. If there's any I've overlooked, feel free to bring them to my attention. A key thing to recognize here is that I want redundancy for the extra safety factor.

My reason for wanting to incorporate the pony into the gas plan is simply because, I'm carrying the gas, might as well use it, if for no reason other than practice switching regs and using the regulator regularly to prevent issues arising from disuse is probably a good thing, and because heck, 15 minutes added to two hours of dive time is not an insignificant amount. My biggest hang up, oddly, is the cost of VIP'ing the pony, which is making me consider defaulting to option 3 and transfill whip, but I think there might be a local who can get me a cheap VIP, so we'll wait and see.

I'd appreciate any reasons why my thought processes are flawed, but if you're just going to say what I'm doing is stupid, or too complex, or "not the best way of doing things" with no further justification, save your breath. I like my criticisms to come with explanations, please and thank you.

 

[MaxBottomtime]

Equipment failure is extremely rare in diving. Adding additional failure points in the way of more O-rings, another reg, etc. just to get more bottom time is not as safe as just diving a larger tank and watching your spg.

 

[SouthernSharktoothDiver]

@MaxBottomtime This...makes no sense to me. The systems are fully separate, so while yes, I'm doubling the failure points (actually slightly less, since I'll still only have two second stages, one per first stage, but let's ignore that) if one fails, I still have the other. So, assuming I don't do something stupid (like not watching my SPG), we have the following:

Assuming the chance of a catastrophic failure is "p"

With one tank/reg: You have "p" chance of failure. In the event of failure, you have no redundancy. You either pull of a cesa (unlikely under the conditions I dive in, at least for me) or you die.

Ergo, you have a "p" chance of dying.

With a main tank and a pony bottle, both of which are always kept at sufficient gas levels to safely ascend:
You have "2p" chance of a failure. In the event of failure, you have redundancy. In the event of two failures, you die.
Ergo, you have a "2p"*"2p" chance of dying, which is say, 4p^2.

Now, as you've already said, the odds of failure are very, very low, thus p<<1; p>>4p^2.

Obviously, mistake can still be made to get you killed with a pony. You can't fix stupid with gear, after all. But your description that I'm "adding more failure points, just to get more bottom time" is simply incorrect. I'm adding redundancy as well, and that is clearly safer than just watching my SPG and hoping I'm not unlucky enough to get the catastrophic failure, or even something more mundane, like say, an SPG with needle drift (which is not so uncommon as all that).

As always, if you have a reasoned argument for why this way of think is wrong, I'd love to hear it, but thus far everything you've said to criticize what I'm doing has been on the assumption that I'm doing something other than what I'm doing. Essentially, what's happened quite a lot in this thread, is I say "I want to do xy and z) and somebody says "well, if you just do Y you could die. That's stupid" complete ignoring that I'm doing x and z to account for the issue of Y. (For example, how your above comment assumes I'm using this just for more bottom time, neglecting the fact I'm preserving redundancy throughout) You'll have to forgive me for not finding those critics too valid.

 

[lexvil]

Equipment failure is extremely rare in diving. Adding additional failure points in the way of more O-rings, another reg, etc. just to get more bottom time is not as safe as just diving a larger tank and watching your spg.

Some seek information and others affirmation, the trick is identifying one from the other…

 

[MaxBottomtime]

@MaxBottomtime But your description that I'm "adding more failure points, just to get more bottom time" is simply incorrect. I'm adding redundancy as well, and that is clearly safer than just watching my SPG and hoping I'm not unlucky enough to get the catastrophic failure, or even something more mundane, like say, an SPG with needle drift (which is not so uncommon as all that).

As always, if you have a reasoned argument for why this way of think is wrong, I'd love to hear it, but thus far everything you've said to criticize what I'm doing has been on the assumption that I'm doing something other than what I'm doing.

Nobody is criticizing you. You have less than 24 dives. You don't yet know what you don't know. Using the pony on purpose during your second dive is not redundant. Also, an equipment failure, even a rare one does not necessarily mean a CESA is required. A high pressure leak or blown hose will drain slowly, allowing a fairly normal ascent. Most failures (again, extremely rare) can be dealt with under water. Not watching your spg is the biggest avoidable failure.

 

[LI-er]

Nobody is criticizing you. You have less than 24 dives. You don't yet know what you don't know. Using the pony on purpose during your second dive is not redundant.

No one is criticizing him? That's laughable. The guy has a perfectly reasonable plan that provides redundancy during both dives and as he said he maintains the redundancy even at the end of dive 2 because he still has the typical main tank reserve.

As usual on ScubaBoard if you don't do it the way another diver does it then you're wrong even if the only thing they can come up with is the bizarre statement that "2 tanks are more complicated than 1". Tell that to those who dive doubles or who carry up to 6 tanks on a dive. Or a rebreather diver who carries a bailout hottie.."That's too many tanks it's uneccessarily complicated you better not do that something can go wrong!"

@SouthernSharktoothDiver's only mistake is thinking he will change someone's mind on ScubaBoard by presenting a clever and innovative idea.

Note: as I've posted previously I do a similar thing only I don't typically draw a single breath from the pony. I confirm it's full during the dive via a second wireless computer/transmitter and "use" it on every dive on which it is carried, as a backup to the primary which allows me to draw the main tank much lower than usual.

That way I don't need to keep refilling the pony bottle.

 

[LI-er]

Sometimes I wish we did not have TOS so we could speak freely.

To stay PG-13. OP, sorry, but this is the dumbest idea I've heard today. You introduce complexities by avoiding a simple solution. What other options have you considered?

He has redundancy throughout both dives. He always maintains sufficient reserve in both tanks to ascend from his dives that average 40'. With a second tank as backup he is in far better shape than a typical single tank diver that is also responsible for getting a second diver to the surface.

You say it's dumb and not surprisingly have nothing to say about what's wrong with the idea. The naysayers who do give a reason say they're afraid of the complexity of adding a second tank, somehow conveniently ignoring the vast population of tech divers who use multiple tanks. Or they say there's no redundancy once he starts to draw down the pony even though he clearly said he keeps a typical reserve in the primary tank.

 

[Tom_Ivan]

Equipment failure is extremely rare in diving. Adding additional failure points in the way of more O-rings, another reg, etc. just to get more bottom time is not as safe as just diving a larger tank and watching your spg.

This comment is the most ridiculous!
By this logic cave, wreck, mine divers should only have one cylinder to reduce failure points.

He is in 40ft. He is retaining enough gas in each cyliner to reach the surface on either. His chances of total requipment failure has REDUCED!
Your guys only problem is that he used the word "pony" and it's gotten you all worked up!


@SouthernSharktoothDiver some advice;
1. Make sure that you have two VERY different SPGs so that there is no mistaking which cylinder you are checking. If you can, label the one attached to your pony so it is obvious. You will need aprox 4X the pressure in the pony to have the same volume of gas as your main. If you get these mixed up then you could be in a world of hurt.
2. On you first dive with your new cylinder dont use it unill you ascend. Make a note of how much gas you use so that you know how much to save on following dives.

Also,

... ...
I dive very negative to remain on the riverbed in the face of steady current. With 16-20lbs of lead, no air, and a 40 ft ascent, I don't want to bet my life on CESA.
... ...

I do have a BIG problem with this comment.
Current is no reason to be overweighted! A correctly weighted diver is negative throughout the entire dive anyway. Having additional weight only adds more gas to your BCD and makes boyancy control much harder!
Correct weighting is fundamental. Far more important than if you are taking a few gulps of your pony bottle now and again.
GET CORRECTLY WEIGHTED.

 

[happy-diver]

Nah mate he is correctly weighted for what he wants to do so he doesn't get sucked off the bottom

Did you read the formula

thus p<<1; p>>4p^2.

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How are your thoughts affecting your happiness

Go diving!

 

[LI-er]

Your guys only problem is that he used the word "pony" and it's gotten you all worked up!

Excellent post on all points. This one in particular.

Whenever I've posted that I keep the reserve in my pony and draw my primary tank low, thus extending my dive I used to get a whole bunch of backlash. That's not how you do it, you're in severe danger, you're going to die!

Until someone recommended I refer to the extra cylinder as a "stage" bottle and not use the term "pony".

In fact a new phrase was coined as a result, which may even make it into the next PADI AOW update.

Stony Bottle = Pony Bottle + Stage Bottle

Who knew one word would be powerful enough to shut up even the most arrogant on ScubaBoard?

Info - Are Pony Bottles Dangerous?

A pony bottle can be dangerous and I have had the experience to prove it. But I still think it would be more dangerous to have a full primary failure at depth with no pony...... and most definitely the case if diving solo...

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