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

[Tom_Ivan]

... ... I keep the reserve in my pony and draw my primary tank low, thus extending my dive......

I like this.
I assume when you leave bottom you have enough gas in the main to reach surface and not much more? Any trouble on the ascent you switch to the "stony".

Much more elegant than partially breathing the pony each time. Reduces the chance of SPG confusion that I was worried about.

 

[OMyMyOHellYes]

Regs shouldn't be bubbling through a dive since they are either in your mouth or under your chin.

Or bungeed to the side of the pony, comfortably in view, slung from the BC.

 

[LI-er]

I like this.
I assume when you leave bottom you have enough gas in the main to reach surface and not much more?

I've usually got 250-300 psi at my safety stop and less than 100 psi back on the boat. Once or twice I've taken my last breath on the ladder and told the crew they've got a tank to tumble when they get back to the shop.

 

[SouthernSharktoothDiver]

@MaxBottomtime I'm using the more old fashioned definition of criticism (like a movie or art critic). I'm not saying I'm being insulted or attack (some people have, but I don't mind), I'm saying people are making comments about the quality of my plan, which is exactly what I came here for. My only complaint is the lack of reasoning accompanying many such comments and the fact that some are responding to a plan that is not mine. Other than that, why does using the pony make it not redundant? As far as I'm aware, I'm redundant as long as I have two tanks/regs that work, with enough gas in them to turn the dive and surface. But, as you said, I don't know what I don't know. Is there a reason using my pony (in exactly the same way a twinset diver would use independent twins, just smaller) makes it not redundant?

@LI-er I appreciate the support, and good job summing up what I'm doing. There seems to be some confusion. Question about the way you do things: Is it not potentially bad for a tank to drain it below 500 psi? I heard that it was, and so leave a fairly padded reserve in mine, but people say lots of things, and I'm not certain of the science/reasoning behind it. Would draining a tank to 200 psi be worse than draining it to 500, for the tank's lifespan/longevity?

@Tom_Ivan Good advice on the spg, hadn't thought about that. My main spg is a three gauge console, and I'll probably put a 2 inch SPG single gauge on the pony, so that should do the job, don't you think? And yeah, I actually planned to take the pony out in a nearby lake (still water, land within easy swimming distance, diving off a buddy's dock) to test how fast I drain the pony, then definitely give it a wide margin of error until I'm certain about consumption rates.

As for my weighting habits...I dive neutral/slightly negative when doing "sightseeing dives" like when I dove a wreck in a local lake, but for river dives, my goal is to remain securely on the bottom throughout the entire dive, until ascending. My bcd is completely empty from the time I go under until I'm ready to surface, and then I add a small amount of air (perhaps a total of 1 second pressing the inflator, in short bursts) to make me less negative, and then fin my way to the surface. I usually reach the surface still negative, and fully inflate there. I don't love being so heavily weighted (it's definitely a pain on the surface, if nothing else) but I find the current becomes hard to manage without the extra weight, since it will sweep you off the bottom. Thoughts/advice, given all that?

@happy-diver ...I'm not even sure what the joke is here. And perhaps you didn't understand the math, but the point of that formula was to say, if there's a 5% (random number I made up) chance of failure, with one tank your chance of dying is 0.05, which with two tanks/regs, it's 4*0.05*0.05=0.01. This was in response to another diver's comment about two tanks being a higher chance of failure. I'm willing to hear anybody out, but if you continue to add nothing of value to the conversation, I'm going to stop responding.

 

[LI-er]

Would draining a tank to 200 psi be worse than draining it to 500, for the tank's lifespan/longevity?

I figure it won't make any difference. Although from what I've read dropping below 100 psi at depth might result in water entering the tank. So I try to stay above that but I'm not always successful.

 

[mr_v]

@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.

Conservative diving with a well maintained tank/regulators is straightforward, simple, and safe.

Sudden mechanical regulator or tank failures are uncommon. Proper maintenance and gear checks may significantly reduce the failure likelihood. Also, when modern regulators fail, they flow, so you will have access to air as you ascend (given your dive profile). If that's not enough, consider a Y-valve with two 1st and 2nd stage combos.

I know I am going to get shut down for the following advice, but do consider diving with two sidemount baby bottles. Scuba John's in Lexington (SC) can help you, and the small investment in additional gear will be cheaper than a funeral.

 

[SouthernSharktoothDiver]

@mr_v can you please explain why diving say, two sidemount 40’s would be safer/better than what I have in mind? I see very little difference between the plans in terms of function, just that one is lopsided, sizewise, and the other use two equal size bottles. Well, that and backmount vs sidemount. What in that makes sidemount the difference between safe diving and paying for a funeral? I’m not trying to shut you down or be argumentative, but I would like an answer.

And yeah, I’m aware regs tend to fail open, and in all likelihood I could ascend safely off a freeflow, but why not give myself the extra redundancy and bottom time by doing what I’m planning here? Other than an increase in drag/burden, and cost what’s the drawback compared to any of my other options?

 

[MaxBottomtime]

@MaxBottomtimeIs there a reason using my pony (in exactly the same way a twinset diver would use independent twins, just smaller) makes it not redundant?

In the event the pony leaks (it happens) or you forgot to fill/check it before the dive (it happens a lot) you would be in for a big surprise when you switch regs and find an empty tank. I'm not saying you're going to die, just that surfacing with enough air in a single tank is safer, cheaper, and chances of a boo boo lessened. A diver using independent twins switches regs twice during a dive and has plenty of time to get to the surface in case of a boo boo. They don't switch during the safety stop.

 

[mr_v]

@mr_v can you please explain why diving say, two sidemount 40’s would be safer/better than what I have in mind? I see very little difference between the plans in terms of function, just that one is lopsided, sizewise, and the other use two equal size bottles. Well, that and backmount vs sidemount. What in that makes sidemount the difference between safe diving and paying for a funeral? I’m not trying to shut you down or be argumentative, but I would like an answer.

And yeah, I’m aware regs tend to fail open, and in all likelihood I could ascend safely off a freeflow, but why not give myself the extra redundancy and bottom time by doing what I’m planning here? Other than an increase in drag/burden, and cost what’s the drawback compared to any of my other options?

Given your location and the mentioned conditions, I presume you dive Cooper River for treasures. That’s a challenging dive given the current and the visibility, yet you are not comfortable with making a CESA. You also claim that there is no need for a safety stop, and that is unfortunate, as the difference between 30 and 0ft halves the pressure; you should treat the last 30’ before the surface with great care.

These facts, combined with a more-complex-than-necessary approach to redundancy, are red flags. That's why many have suggested straightforward approaches, like diving with a bigger tank or investing in two fairly-sized tanks.

Proper sidemount training will give you better trim, longer run times, and good breathing-down habits, so that you stay in trim and not lopsided. Sidemount also forces you to continuously pay attention to gas consumption and perform several regulator switches—all these practices will double-ensure that your gear is working all the time. You can do that with doubles, but given your dive profiles, I think sidemount will be easier.

 

[NothingClever]

Logic and best practices proffered by more seasoned and more highly trained practitioners are no match against an entrenched mind. I think the principal problem in this thread is an insincere attitude, not an equipment configuration.