on twin tanks, dir and manifolds

[Blargh]

Greetings....

I recently (last week-ish, "Equinox" on Ch4, if you're in the UK and curious) watched a very interesting program about the coelacanth, although that is not what my post is about

At the end of the program, it was mentioned that a group of "deep divers" in S.A. saw some live ones at depth of 140m or so - and that is when I noticed that said divers were carrying twins tanks without any manifold between them.

So... I wonder if the folks (dir-ers or otherwise) here on the board can give their opinions on diving twins without a manifold vs the "standard" (??) twin setup?

Just gimme a sec to get into the asbestos suit... ok. done. Shoot :0

 

[Lost Yooper]

They call that independant doubles. IMO, it's just about as foolish as it gets. Here's some reasons (from a DIR perspective):

1. You must constantly switch regs underwater to keep a decent balance thereby creating additional task loading.
2. There's an additional pressure guage and hose to contend with thereby creating additional failure points, task loading, and results in having a less streamline configuration.
3. Since there's no isolation manifold, if you have a free flow or have an O-ring go, you lose half your gas supply. With a manifold, you can isolate the problematic reg, and still have access to all the gas.

That's all I can think of right now, but there are other reasons not to dive independants. In an attempt to create a "safer" configuration, they end up creating more problems then they're trying to solve. It just doesn't make any sense to me.
The only thing I think I've seen that's worse than this system, is someone putting deco gasses on their back and the bottom mix slung under both arms. Talk about a mess and a disaster waiting to happen, but I actually met someone who dove this way. It's amazing what people will think up.

Mike

 

[Rick Murchison]

There are many reasons to use independent doubles - My suspicion is that on this particular dive (140M) the tanks held different gasses for different depths - useful for short stays deep and a friendlier gas for ascent. And for a 140M dive there are probably other tanks around you didn't see for deco (and possibly for descent, too)... to say nothing of the support team.
Another place where independent doubles are common is when side mounting for bedding planes in cave diving.
Independent doubles come in handy at places where you can't get manifolded doubles for rent, too - there are several rigs available to mount 'em.
They take extra training, extra management.. and there are some places where manifolded doubles are definitely better - but there is nothing inherently more dangerous in their use if done properly.
Rick

 

[Sir Barfalot]

Hi To all
I am a newbe to scuba board but I find this discussion interesting as i have been thinking of setting up a set of twins and have looked so far without success for information on the pro's and con's of both methods with or without manifold it seems that everyone has a different opinion HOW WHY WHEN WHERE your comments in this area would be appreciated

 

[TSandM]

Well, the advantage of manifolded doubles is that, in the face of a malfunction of either first or second stage, or a blown hose, you can stop the leak and retain access to all of your gas. The difficulty is that the valves are behind you, where you can't see them, and where reaching them can be an effort and shutdowns may be slow. And because the tanks are manifolded, it IS possible for a leak to lose you all your gas, if it is not handled properly. Damage to the manifold itself can do it, and the manifold is up where it's vulnerable to ceiling contact.

Using independent tanks makes it very unlikely that you could lose all your gas (it would require two catastrophic failures to do this). However, using back-mounted independent doubles still puts your valves behind you, where you can't see them and they are hard to reach. I have a difficult time understanding why someone would choose to do this, rather than sidemount the tanks, if they are going to go the independent route. At least, with sidemount, the valves are where you can see and readily manipulate them.

 

[aquaregia]

Can anyone comment on the relative drag of doubles vs single+stage? I'm thinking about rental/resort options.

Edit: With ID you have to be phenomenally stupid twice to accidentally go OOG.

 

[vtownsend]

Hi,
All of the above offer good advice and opinions.
I dive many configurations and I currently prefer my Inspiration CCR. In this instance manifolded twins would be better than independent. The reasons? - well like mentioned it gives an automatic balance to the cylinders (assuming all three valves are open) and it reduces workload in balancing the cylinders during use (5 minutes breathing each gas and regulator swapping). There are different shut down drills in the event of a leak etc.
If they were on independent twins at 140M then one can assume they have different bottom mixes in their twins and I assume they have their deco gases side slung. Now, at that depth and pressure a single cylinder will not last long thus reducing their bottom time. Added problems then bring into question their consumption rate and do they have enough gas. Diving at that depth with this set up will, for safety sake, should involve a back up team to supply differing gases at different planned depths.
Of course, diving at these depths is far better on a CCR where gas is supplied as it is metabolised thus giving a longer dive time, rather than blowing out through the regulator non metabolised gases during each exhale of breath.
Vince.

 

[DevonDiver]

Independant doubles. No great mystery. They're generally going out of popularity, but still in use for specific reasons with specific people.

I use that configuration when travelling abroad and local/resort dive centres don't have manifolded doubles for rent. Simply drop 2x AL80 into my cambands (backplate takes 2x cambands for direct mounting) and away I go.

How would a DIR zealot approach the issue of non-available manifolded doubles? Single and a slung? Cancel the dive? Ask their sponsors to increase their budget and ship the correct kit out to them?

 

[ajduplessis]

There are only a couple of spots for diving Coelacanth in South Africa. This fish species are very temperature specific and are to be found at a depth of 100-120m max. I know some of the divers that have escorted the National Geographic team and they all dive isolation manifolds (gas = hypoxic trimix blends – travel gas get clipped of somewhere around 60 meters). I am aware of a couple of mCCR divers on the same site and independent doubles divers, however this is very uncommon.

 

[elmer fudd]

Just a guess, but perhaps this was a not so well done dramatic reenactment.

In that case, using actors and rental gear, they might have rigged up the independent doubles just to get a tech look.