Imported post
[b said:
Quote[/b] (sausagedog @ Oct. 19 2003,16:22)]
[b said:
Quote[/b] ]What is best?
a) completely independent twins
or
b) manifolded twins with isolation valve
As ever, you need to make an objective appraisal of the advantages and disadvantages of each system and then make a decision based on that information. There is a "risk based" decision making process which means that you end up with the system which presents the least "risk".
Now, assuming we are talking about UK ocean diving or clear water cave diving within a "team" environment (by that, I mean you have a similarly equipped buddy or team) you need to assess the additional risk of using a manifold against not using one.........
A manifold is a potential failure point. There is no doubt about that. There are additional o-rings in there. "Failure" can manifest itself in other ways too. For example, the isolator could be closed when filled with Nitrox or trimix, resulting in high O2 on one side of the isolator. Even if the isolator is open during filling, it could get inadvertantly closed after filling, resulting in the diver "running out of gas" when he still has 50% of his available gas remaining. A simple freeflow can empty both tanks if not correctly managed when a manifold/isolator is used. All of these failure modes are potentially life threatening and all of these failure modes are eliminated by using independents. That's the argument "for" independents.
The above risks can be mitigated as follows:
1. Closed isolator - keep the isolator open at all times and continually check, check, check even when in the water.
2. Freeflow - learn and practice how to shut the isolator. This needs to be a reflex action.
It's really a simple matter of learning how to properly manage a manifold/isolator.
The disadvantages of independents are as follows:
1. Gas management - you need to swap regs with independents. With a manifold/isolator you have access to all of your gas at all times, even if a regulator fails. With independents, you loose gas even in the event of a simple free flow.
2. Planning - we tend to plan dives so that, even in the event of a catastrophic loss of backgas, our buddy can donate enough gas to get us out of the cave or to our first deco gas. You can't do that with independents.
3. Failures - we have a system which will tolerate at least two independent failures. For example, if two regs fail, our buddy can get us out of the cave or to our first gas switch. You can't do that with independents.
4. With independents, you will need two pressure gauges. This adds an unwanted failure point. With a manifold/isolator, you only need one pressure gauge.
5. With independents, which cylinder do you use for BC inflation? With a manifold/isolator, you can use both.
I've gone on far too long and I think you will see which system I'm in favour of.
Hope this helps.
Bob