confused about redundancy

[etowndiver]

Could someone please clear this up for me.

Not that I am anywhere near diving double tanks, but I am curious about it. How is a double tank setup redundant gear? Once the first tank is empty, you become a single tank diver again, right? It seems to me that the only way to have a redundant air source is to carry another self-contained air souce with you.


Thanks in advance,

joe

 

[metridium]

It's gear redundant in that you're using two first-stage regs with one second-stage reg each.

In terms of breathing supply redundancy, if you're using a manifold with isolator, you breathe from both tanks simultaneously. If you have a problem with one tank (blown tank o-ring, etc.), you isolate that tank, call the dive, breathe it down, then switch to your unaffected tank and surface.

Is that clear as mud?

 

[MASS-Diver]

Most people dive doubles connected to each other with an isolation manifold, so that you are breathing from both tanks at once. If you have a problem you can shut down the post that is giving you a problem and still breath from both tanks, if you crack a neck or somthing you can close the iso vale and isolate the two tanks. Most people count doubles as redundant, but, some people will drag along another bailout bottle for true redundancy.

 

[Walter]

Twins are set up with a manifold which allows you to draw air from both tanks at once. There are also independent double set ups as well, but they are not as common. The manifold often has an isolator valve. The isolator valve allows the diver to separate the systems. This would be useful in the event of a neck O-ring or burst disk failure. With those very rare exceptions, manifolds are identical, except there is an additional failure point in the isolation manifold.

If there is a regulator failure, the diver shuts off the valve with the faulty regulator and breaths off the good regulator. The system is redundant. The isolator manifold is totally redundant and the non isolator in all but two very rare possibilities. When you have such a failure, you use the back up system to safely abort the dive.

Any time you are using a redundant system and one part fails (regardless of what type of system) you no longer have a working back up. To eliminate this, you need three independent systems.

 

[diveski01]

When I need redundant air supply I carry a 30cuft stage bottle when diving a single tank DIR configuration. I effectively have 2 first stages and 3 second stages with this system.

I do not count the 30 cu ft as part of my gas when calculating rock bottom or turn pressures. It's there solely in case of an emergency situation that my single tank rig can not handle.

It's easier for me to deal with since I can carry the stage bottle to the water before I get into the rest of my gear if it's an arduous walk.

Doubles are just too much weight for me and I'm not going into Cave or Tech diving so they are not required for my recreational diving. When we dive "deep" (>60ft, 130 ft max) in a cold dark quarry, then I have a redundant air source, for sure.