Diving with a 100cf, uh, pony?

[fuzzybabybunny]

Edit: I posted in the wrong forum ><

Say that I wanted to extend bottom time to several hours underwater. Could I dive with a second slung 100cf tank as a pony?

The idea would be dive my primary single tank until I'm at about 700psi, then switch to my 100cf pony and dive that to 700psi.

Should the primary tank fail, I've got the 100cf pony as redundancy. Should the pony fail, I've still got 700psi in my primary tank to make an ascent at that point.

This would also save money over getting a doubles rig because:

1. I wouldn't have to purchase a new BP/W.
2. I wouldn't have to buy 2 tanks, 2 regulator sets, and hoses dedicated to JUST diving doubles.
3. I wouldn't have to buy hardware to set up a doubles rig.
4. I could easily dive a single tank when I want to, and just lug the second 100cf with me whenever I want to extend bottom time.

This would be for recreational, not deco diving.

 

[Dr Wu]

Search "Independent Doubles".

 

[DevonDiver]

It could be done.

There'd be all sorts of issues with your trim and 'balance' during the dive. It would be very cumbersome. It would add unnecessary task loading.

You wouldn't get any of the redundancy benefits associated with an isolated manifold.

In theory, yes. In practice... get the right gear for the dives you intend to do.

 

[Kern]

How deep do you want to go? What depth gives you an NDL of several hours?

 

[fuzzybabybunny]

It could be done.

There'd be all sorts of issues with your trim and 'balance' during the dive. It would be very cumbersome. It would add unnecessary task loading.

You wouldn't get any of the redundancy benefits associated with an isolated manifold.

In theory, yes. In practice... get the right gear for the dives you intend to do.

I'm curious - when tech divers dive with two tanks at their sides, how large are those tanks and how badly do those tanks affect their trim and balance?

I was thinking I could even do something like carry two smaller 60s on each side of me if balance was a real issue.

Which redundancies would I not have that an isolation manifold would give me?

 

[fuzzybabybunny]

How deep do you want to go? What depth gives you an NDL of several hours?

50ft? Obviously longer if diving with nitrox. And several hours was just an example. Basically having longer bottom time than a single 100 could offer me.

 

[aquaregia]

This might not be as much work as you might think.

1. I wouldn't have to purchase a new BP/W.

Older doubles wings are often quite cheap. Backplates aren't particularly expensive anyway. Additionally, I thought the Express Tech supported twinsets, so you can probably stick with your BC.

2. I wouldn't have to buy 2 tanks, 2 regulator sets, and hoses dedicated to JUST diving doubles.

You'll need the same number of regulators and hoses either way. My doubles setup is just my singles setup with one additional first stage. To dive independent cylinders (either backmounted or slung) you'll need at least that, plus another SPG, plus potentially another second stage depending on how you're considering your stage/pony/twin in terms of gas management.

3. I wouldn't have to buy hardware to set up a doubles rig.

Independent doubles are unpopular around Monterey, but appear to be well regarded by many northeast divers. In that case, you wouldn't use a manifold but just band the tanks together. You can use the steel bands, or you can get travel bands that are basically just two single-tank cambands and an adapter.

4. I could easily dive a single tank when I want to, and just lug the second 100cf with me whenever I want to extend bottom time.

Independent twins and travel bands with your current BC would allow you to switch around your tanks quite easily with minimum commitment.

As a disclaimer, I'm not at all experienced in these areas. I think a more appropriate bottle to sling would be an AL80. I find that slinging an AL80 is sort of a pain for the first few hundred psi until it's positive because it sort of gets near my knees and limits how close to the bottom I can get by a good 10" or so. Once it starts to float alongside me it's much easier to manage. The HP100 will be negative the entire time.

If you're interested, one of my friends has a pair of 80's Luxfer AL80s that neither of us can be bothered with. If you're interested in them, I think she'd let them go cheap. They could be used as either stages or doubles. I also have some decent 7.25" bands that I haven't used that I might let go for what I paid for them.

 

[fuzzybabybunny]

Ohhhh.... interesting. I basically had this idea after talking to someone who dove doubles around here.

1. You buy a second tank, second regulator set, manifold, steel band, wing, and other accessories. You then put it all together and never take it apart because it's a total bitch to do so.

2. You have a dedicated singles setup. So a third tank, third set of regs, third BC, third computer, etc.

I guess just diving with doubles with no manifolds would be best, huh? Or is it actually easier to dive with a slung tank?

 

[aquaregia]

I'm curious - when tech divers dive with two tanks at their sides, how large are those tanks and how badly do those tanks affect their trim and balance?

I was thinking I could even do something like carry two smaller 60s on each side of me if balance was a real issue.

My diving often gives me free access to AL80s, and my SAC is extremely poor, so I've been known to sling an AL80 and wear one on my back. With my 36# BP/W, I don't really notice any balance problems, but I can definitely feel drag from the slung tank much more than I do from my double 72s (disclaimer: I probably have less than 5hrs on each setup).

Which redundancies would I not have that an isolation manifold would give me?

If you have a reg freeflow, you lose all of the gas in the cylinder that's freeflowing. A manifold gives access to both tanks from either regulator. Compared with a single tank / single reg setup, you're in the same position you were before, so it's not worse (unless you've used that extra gas to get yourself into deco), it's just not any better.

 

[aquaregia]

Ohhhh.... interesting. I basically had this idea after talking to someone who dove doubles around here.

They would know. I wouldn't. I'm not trained in diving either with doubles or a stage, and I've only done a few dives with either.
I'm responding on this thread only because I've had similar thoughts and been talked out of a lot of it.

1. You buy a second tank, second regulator set, manifold, steel band, wing, and other accessories. You then put it all together and never take it apart because it's a total bitch to do so.

2. You have a dedicated singles setup. So a third tank, third set of regs, third BC, third computer, etc.

I guess just diving with doubles with no manifolds would be best, huh? Or is it actually easier to dive with a slung tank?

Well, say you want a manifolded doubles. The tanks, bands and manifold will run $500-$1200 depending on the tanks and whether they're used or not. You add a wing that's $100-$500 (new vs used, etc). At that point, you're say close to $1500 into this thing, so adding a backplate and webbing costs you what, $80 more? Why the hell not. Now you need a new first stage, which costs a bunch more. If you're traveling with a singles setup, you probably have a yoke reg. It'll cost you $100 to swap that to DIN and buy a yoke adapter for when you travel, so you might decide just to get two regulators while you're at it.

Doing things the right way might cost money, but I think there's probably a limited number of shortcuts you can take before stuff gets real dangerous.

Whatever you choose to do, make sure you have emergency procedures that can cope with it. Consider air sharing even if you're diving solo. Read DCBC's Mari-Bahn story if you aren't convinced.