Single Cylinder vs. Doubles

[MikeS]

Scott,

How deep you dive is not the issue, the issues are:

Do you need/want the redundancy inherent to doubles?
Does the volume of air required for your dive plan make singles impractical?

I feel comfortable diving with a HP120 single to 120 feet on a no penetration, no decompression dive. That is based on the following:

I need to start up the anchor line with a reserve of 31.6 cubic feet.
This is based on ascent from 120 feet at 30 feet/min with a SAC of 2.0 (1.0 for me and 1.0 for my buddy) with a 3-minute safety stop at 17 feet.

That leaves 88.4 cubic feet. At 120 feet a SAC of 0.7 results in about 25 minutes of air time. This exceeds the NDL for EAN 30, which means I will be surfacing due to the NDL limit rather than gas supply. Experience has shown this to be the case.

So the dive plan would be:

Assuming that there is no current or swimming into the current, turn back to the anchor line at 2400 psi or ½ of the no decompression time which ever comes first for me or my buddy. Start up the anchor line at 1200 psi or NDL whichever comes first for me or my buddy.

Check out http://radawana.cg.tuwien.ac.at/~martinpi/units.html#TABLE . It has a calculator that lets you compare airtime to NDL. Unfortunately, it does not include Nitrox.

Mike

 

[boomx5]

Thanks alot, that was the type of answer I was looking for. The link was helpful to.

 

[The Iceni]

boomx5 once bubbled...
at what point do you find it necessary to use doubles.

With a manifolded twin set a diver has almost guaranteed access to all of his back gas, even with a first stage failure.

Not so even with a pony, which is why I always dive with my twin set.

Perhaps I am being ultra-cautious as I am no cave diver, but I always wear my seat belt when driving.

In addition, although possibly heavier a twin set is less cumbersome, more stable and better balanced.

At least it makes sense to me!

 

[MikeS]

Dr. Paul,

I find myself disagreeing with you. There are many pros and cons for ponies versus doubles. To the best of my knowledge there is no documented evidence that one is safer than the other and I suspect it is rather impossible to prove because there are so many variables, diving experience, training with specific equipment, conditions, etc. They both give you redundancy of SCUBA but I prefer a pony to doubles for the following reasons:

1. Although the possibility is slight, it is possible to damage the manifold and loose access to all of the air in doubles.

2. I can clearly see the pony, the reg, and SPG at all times because it is in front of me. Some would argue that with training manipulating valves behind you becomes automatic, while this may be true common sense tells you it is easier to work with something you can see right in front of you.

3. I can handoff the pony. This allows you to assist other divers without endangering yourself or you buddy.

4. Most importantly, the cost of the two sets of doubles required for two dive boat trips is cost prohibitive for most.

5. Most likely you can not travel by plane with doubles and typically you can not rent them. On the other hand you can take the valve off the pony and check it as baggage.

I agree that doubles are probably the only smart way to go for decompression or penetration diving (I’m not there yet), because of the shear volume of gas they allow. But for a no penetration, no decompression dives, I content that a pony, with a big caveat that it must be sized appropriately, is as safe or safer than doubles.

Mike

P.S.

I wear my seatbelt too!

 

[The Iceni]

MikeS once bubbled...
1. Although the possibility is slight, it is possible to damage the manifold and loose access to all of the air in doubles.

Yes indeed, there is a possibility but it is very, very slight.

2. I can clearly see the pony, the reg, and SPG at all times because it is in front of me. Some would argue that with training manipulating valves behind you becomes automatic, while this may be true common sense tells you it is easier to work with something you can see right in front of you.

Manipulating the valves does become automatic, but of course like all drills it must be practiced.

3. I can handoff the pony. This allows you to assist other divers without endangering yourself or you buddy.

FWIW I contend it is more difficult to unclip a pony than it is to donate a second stage on a long hose. (In fact, most pony setups I see have the pony clamped or banded to the main cylinder so it cannot be donated in any case). It is stage cylinders that are usually side slung.

4. Most importantly, the cost of the two sets of doubles required for two dive boat trips is cost prohibitive for most.

I accept that but many of my friends use their single twinset for two dives.

5. Most likely you can not travel by plane with doubles and typically you can not rent them. On the other hand you can take the valve off the pony and check it as baggage.

I also accept that. If only!

I agree that doubles are probably the only smart way to go for decompression or penetration diving.

Well, Mike. After all that it would seem you do agree with me then!

Cylinder rental is quite uncommon in the UK. Most of the more experienced divers I know nearly always dive with their own equipment.

Perhaps it would seem the UK and the US diving scenes are quite different?:buggy:

I think this discourse confirms why Roakey, quite rightly, said.

You're not going to get the simple answer you seek because there isn't one.

 

[MikeS]

Doc,

I hope you don’t think I being argumentative; but I love a good discussion based on logic and reason, that is the best way to learn (other than actually diving).

“Manipulating the valves does become automatic, but of course like all drills it must be practiced.”

Is this required training not a solution to a problem that doesn’t exist with a pony?

“Manipulating the valves does become automatic, but of course like all drills it must be practiced.” “FWIW I contend it is more difficult to unclip a pony than it is to donate a second stage on a long hose.”

That is why I carry it stage slung. It is no more difficult to detach the pony and hand it off than it is to hand off the reg. You also have the option of handing the pony reg first and then unclipping the pony. What you can not do donating your reg is hand it off and swim away. There was just a story in Rodale’s about a situation where a diver was able to pass a pony through the window of a wreck. If someone is trapped it gives you time to surface get help and come back. You can’t do that donating your primary reg.

“Perhaps it would seem the UK and the US diving scenes are quite different? ”

The US is a big place and things are quite different here from place to place. A good example is the New Jersey Divers I’ve seen on North Carolina wrecks with the really big reels. They dive in different conditions so have evolved differently. Checking out different equipment philosophies is part of the fun of diving. My arguments are more to convince myself than anyone else. Thanks for helping me understand another point of view.


Mike

 

[The Iceni]

Hi again Mike,

Possible I was the one being argumentative but, like you, I try to think of why we do many of the things we do and the reasons for them. I think this turned out to be a very useful thread because

boomx5 once bubbled...
I wondering how deep you will dive with a single cylinder. And at what point do you find it necessary to use doubles. If you could, please include the variables (cylinder size, gas mixture etc.).

I think that, quite possibly, boomx5 asked the wrong question, which the various contributers have pointed out.

It is not a question about singles vs. twin sets and depth in isolation. The question should have been in what circumstances are doubles preferable to singles and pony and what configuartion should be used.

Books have been written on the subject because the answer is "It depends . . . "

 

[JamesK]

I will dive as deep on singles as I will on doubles. Around 100feet. That is as deep as I go. I am not Trimix Certified yet.

 

[O-ring]

MikeS,
I know where you are coming from since we dive the same wrecks. I think that as long as true redundancy is achieved, that is the most important aspect of the decision to dive doubles or a pony within recreational limits. I think most of the NE wreckers I see on the boats should be in a single and pony and not doubles since most of them do not have a clue how to setup their gear and appear to not be able to reach their valves. In this scenario, I think doubles provide a false sense of security and are inherently dangerous. Doubles, like any other tool, have to be practiced with in order to be effective.

 

[The Iceni]

O-ring once bubbled...
. . . most . . . do not have a clue how to setup their gear and appear to not be able to reach their valves. In this scenario, I think doubles provide a false sense of security and are inherently dangerous.

I completely agree O-ring, in my opinion twinsets/doubles are not just a simple means to get more bottom time.