Redundancy with a Single Tank

[rjchandler]

Didn't somebody post once analyzing the time required to shut down a manifolded double vs. the time for a severed LP hose (or similar major breech) to empty the tanks and find that it was a fairly close race?

I can't find the post at the moment, but I recall it being fairly interesting in this context. Anybody else remember/find this?

There was a poll/survet about this (may have been on Deco Stop). Advanced Diver in its recent issue has some info on how long it takes a blown hose to empty a tank.

 

[SDAnderson]

How much redundancy I want is highly situational, so how rank is this:

1. Doubles with an isolation manifold and a skilled buddy.
2. Single with an H valve and a skilled buddy.
3. Single with a single valve and a skilled buddy.

To elaborate my feelings on what UP said, any option not involving a skilled buddy is solo diving. That's a topic for another other thread, however.

Pony bottles are a security blanket, not a solution. Some divers feel better carrying them but they aren't the best solution to any problem and they create new problems of their own. One complaint I have about them is that I see divers extending beyond what they have planned for, figuring that they can cut their margins closer since they have their little back-up bottle.

An H valve allows for a redundant first stage, which addresses one of my main concerns. Mike's point about the icing problem is serious - that's maybe the single biggest threat I face diving locally. Outside of a few months during the summer (and even then only in shallow water) every local dive I make must be planned with the thought that a freeflowing first stage is a distinct possibility.

As to the blown neck ring issue, I figure that the chance of that happening are about the same as the chance that my wife will catch me in bed with Julia Roberts. Pride prohibits me from saying that it can't happen but it would require a whole string of highly improbable events. Sometimes it's nice to be able to do an easy dive without the doubles, if the water is going to be cold an H valve goes a long way toward making it safer.

 

[Diver0001]

....snip....

An H valve allows for a redundant first stage, which addresses one of my main concerns. Mike's point about the icing problem is serious - that's maybe the single biggest threat I face diving locally. Outside of a few months during the summer (and even then only in shallow water) every local dive I make must be planned with the thought that a freeflowing first stage is a distinct possibility.

A lot of divers here use H and Y valves for this very reason too.

I actually prefer puddlestomping with a single 15 for the weight. Something about me doesn't like clambering up and down slippery dikes in gear that weighs any more than it has to....

Also, in the winter the water literally gets to the freezing point here and you'd need a hole in your head to ignore that fact. Aside from being able to turn off a frozen reg the double-valve means that your drysuit and BCD are not on the same reg and if you do need to share air then you've weighed the odds in your favor by splitting the load.

R..

 

[msandler]

the double-valve means that your drysuit and BCD are not on the same reg and...

FWIW. In an emergency you can always momentarily open and close the valve with the faulted reg. and still (most cases) get a blast of air for the BC, suit or even to breath.

 

[chrisch]

As reefraff points out a blown seal at the point where the valve enters the cylinder is so unlikley that it can be discounted. The O ring between the 1st stage and the valve does blow from time to time however. This can be cured by the use of DIN fittings.
In the case of a blown O ring the H valve would isolate that leak.

Manifolded twins may give more gas if they are bigger.

Independent twins are truly redundant, but bring with them a whole set of problems.

H valves have a place in recreational diving, but sadly many people are not trained in shut-down drills with type of gear.

Chris

 

[mania]

Here most of us not diving doubles or twins use single tank with H valve. The reason is simple - it's safer especially when diving in the cold water (and there is almost no warm waters in Poland) the chances of freezing the first stage are high. So redundancy of first stages that H valve allows is the safest way (once more - of course the safest are doubles or twins).
So if one first stage gets frozen we simply close this valve and switch to the back up. Anyway it's always the reason to finish the dive but in the safe way.
Mania

 

[jhelmuth]

On the rare occasions that I dive a single tank I use an h-valve. Mostly because I can but we dive cold water where a free flow is the most likely failure and when switching back and forth from doubles to a single I don't have to change over any regs. I just pull them off one and slap em on the other making both rigs exactly the same only one doesn't have the isolator.

In truth I have enough regs that I could just have a seperate one set up for a single tank but why?

I also like the way hoses rout off an h-valve better but it's not that big of a deal.

Another point, with regards to relying on a buddy as apposed to a redundant reg is that many of the shallow dives where I would dive a single tank are done with new divers or new buddies.

I'm not teaching now but I also got to like an h-valve for teaching. Shutting down a faulty reg and going to a backup rather than being tied to another diver would make controling a class while ending the dive easier even though I always have a buddy while teaching.

Ice dives are another time when I like an h-valve. I prefer doubles but if you have to walk a long way accross the ice to get to the hole they're a pain especially for the short shallow dives we usually do under the ice. Here again, we dive with 2 divers but the h-valve gives one more option in the case of the most likely failure...a free flow.

Nice post Mike. I was going to say something similar - but you did it so much better, and you are the man!
Listen up dudes... Mike speaks the truth.

 

[matt_unique]

Pony bottles are a security blanket, not a solution. Some divers feel better carrying them but they aren't the best solution to any problem and they create new problems of their own. One complaint I have about them is that I see divers extending beyond what they have planned for, figuring that they can cut their margins closer since they have their little back-up bottle.QUOTE]

Ha ha ha - hey - doubles are security blankets for divers who can't hold their breath.

The redundancy in doubles extends to a properly configured/utilized pony. If a diver uses and plans pony gas as part of his/her dive plan they are not using the pony correctly. The pony should never be touched except in case of emergency. If a pony is a security blanket, trust in a buddy to solve your gas/equipment problems is blind faith. It's like saying you do not need to wear a seatbelt because your friend is a good driver. I just assume have my own safety solutions in case another human being cannot provide them for me. How often is your buddy right beside you, within arms reach to his/her regulator, that you could grab with your eyes closed, during every second of a drive from splash to surface? I have never seen, heard about, or read about buddies who can offer this precise redundant-regulator-to-mouth availability as I described above offered by a pony (or doubles).

I dive doubles with isolator. I only use a single with pony when doing shore dives (winter) or while on vacation. It does not make sense to me to give up the redundancy I have in my doubles when I am diving in a situation calling for a single.

I agree that H/Y valves are a better safety solution compared to a standard single valve.

--Matt

 

[South Pacific Diver]

IMHO it seems to me that gas management and not violating the rule of 3rds accounts for emergency gas. Out here we dive doubles (isolator open) and BTFs (big #&$@ tanks) with H valves. Which one we use depends on the gas requirements for that paticular dive. If I know that I need 140 cubit feet of back gas then I carry 210. Then I always get out of the water with one third of my gas remaining unless there was an emergency that took time to deal with. Regardless of the type of setup being used, problem management and the ability to isolate loose air quickly is the key. As far as the tank o-ring going, well that could happen I guess but I could get hit by a meteor on ascent too.

 

[chrisch]

Regardless of the type of setup being used, problem management and the ability to isolate loose air quickly is the key.

Spot on.

Chris