Why Double Small Tanks?

[herbdb]

As a recreational diver, I am curious about the concept of doubling smaller tanks. I get the usefulness of doubling up larger tanks for increased capacity, but why double tanks smaller tham 63 cu ft when single tanks with the same or larger capacity are so common?

It seems like the added gear like tank bands, manifold and second tank valve would make it heavier than a single tank with the same capacity. Yet, I see them discussed often on the board.

Do I have this wrong? Can someone explain please.

Thanks
Herb

 

[SuPrBuGmAn]

Redundancy


In a technical situation, you often have an overhead, literally, or by means of manditory decompression stops. If you lose one tank, you want another at your disposal to get you to the surface.

 

[Jimmer]

I have a set of double 45's that I use on typical "single tank" dives here in the great lakes. I have approx the same gas as my friends with single 100's, but I have dual regs in the event of a freeflow. That was my main reasoning for setting them up.

 

[herbdb]

I get what you are saying if you doubled 100's, but how much Cave can you explore on dual 50's if one is for backup? Likewise you would really need to be good on air to get into a deco situation with that setup if you were holding one of the 50's in reserve. (for Suprbgman)

I was not aware that doubles had two regs, you mean two 1st stages? (for Jimmer)

Sorry was answering two different posts

 

[Jimmer]

I was not aware that doubles had two regs, you mean two 1st stages? (for Jimmer)

Yes, if you do a little research into technical diving setups, you will see that we use two first stages, each with it's own second stage for redundancy. These are able to be shut off or isolated in the event of a freeflow or malfunction, meaning if my primary reg was to suddenly start blowing air down my throat on a cold dive, I shut it off and switch to my backup, still have access to the air in both tanks, and abort the dive.

 

[herbdb]

Thanks, the only set of doubles that I have seen did not appear to have double regs, just a manifold between the tank valves to a single 1st and second stage. It all makes sense now.

I'll try to pay more attention next time I see someone diving doubles. I was not looking at them very closely and probably missed something.

 

[DaleC]

You were probably looking at an older, center outlet, non isolating manifold. These were common in the past for doubling tanks when large, high pressure steel tanks did not exist (commonly). The above posters are talking about modern, isolating manifolds. They have two outlets.

 

[ajduplessis]

Redundancy


In a technical situation, you often have an overhead, literally, or by means of manditory decompression stops. If you lose one tank, you want another at your disposal to get you to the surface.

I agree to some extent. The tank sizes mentioned I believe is to small especially for deco (I assume you are talking deco and not safety stops), as half the gas (in the event of "losing" one tank) will not be enough to get you out. I think these size doubles in general is to small if used with rules of 1/3 in any case.

They will however provide you with redundancy and is a great way to get into twin tank diving. The down side is cost in relation to possible diving applications.

 

[DevonDiver]

Redundancy, without the need for greater air capacity.

In the UK, I notice a considerable number of divers using twin-7l tanks (isolated manifold) for recreational diving. This gives them a simular amount of gas as a single large cylinder (15l), but has the critical benefit of providing complete redundancy.

Many also prefer the stability of twin tanks on their back, compared to a single high capacity cylinder.

 

[SuPrBuGmAn]

I get what you are saying if you doubled 100's, but how much Cave can you explore on dual 50's if one is for backup? Likewise you would really need to be good on air to get into a deco situation with that setup if you were holding one of the 50's in reserve. (for Suprbgman)

Sometimes you can explore all of it with double 50s... Not all of them have miles upon miles of passage. Sometimes a bigger tank is just too big to get in, and small tanks must be utilized. Even if not the case, I'd hate to push into some cave, only to have the regulators fail on my single tank... and be screwed. All your eggs in one basket?

I agree to some extent. The tank sizes mentioned I believe is to small especially for deco (I assume you are talking deco and not safety stops), as half the gas (in the event of "losing" one tank) will not be enough to get you out. I think these size doubles in general is to small if used with rules of 1/3 in any case.

They will however provide you with redundancy and is a great way to get into twin tank diving. The down side is cost in relation to possible diving applications.

First off, you can get into deco on 1/3rds of smaller tanks(I've done it). I'm not talking about safety stops. Second, this isn't only about deco, this is about redundancy for technical applications, which can go beyond just a soft overhead(deco). It could mean an actual overhead. Either way, diving thirds with added conservatism if necessary should be enough to get you out, if you lose a tank.

Thats the entire point of diving thirds.

You should go ahead and agree with everything in my posts, cause its all fact