Planning a dive trip in a few months and wondering what I should do, bear with me. Twinsets or Stage/Pony Cylinder

[HenryUK]

You keep them in box

@HenryUK -- the great thing about this thread/discussion is it's getting you to think about the issues you're trying to mitigate. Like most things, they're hard at first, then become easier once you've done it a few times.


For your average depths... If you're planning ahead of time then you have to assume the worst case. For your wreck dive, you'll assume the 40m/132ft as a square profile for planning purposes. The reality will be different as you'll mooch around above the seabed, often around the deck or wheelhouse which could easily be 10m/33ft higher.

Groan; digs out the PADI RDP cards rather than using MultiDeco....

• From the RDP, a 30m/100ft dive on air is 20 mins NDL (No-Decompression Limit).
• A 40m/132ft dive on air is 9 mins NDL.

Thus if you go for depth, you've 11 mins less on the bottom phase than if you mooch around the top of the wreck.

28% Nitrox will give you a few more mins of bottom time -- The equivalent air depth EAD is 36m, so maybe 3 more mins - 12 mins at 40m.

Now your problem is this; your PLAN that you've (possibly) written on your slate is for a bottom time of 12 mins, but this assumes a square profile at 40m. You COULD have two plans, one for 40m and another for 25 mins at 30m. Or another for 35m. Where does it stop!

Therefore you'll use your computer. These are good at keeping a tally on your NDL. You know your bottom time will be somewhere between 12 mins and 25 mins; the computer will tell you when time's up.

BUT (this is Basic Scuba); you MUST keep an eye on your remaining gas pressure. Using the gas calculations further up this thread, you will know how much gas you'll need for the ascent from 40m to the stops and to the surface. Add to that the 50bar/750psi safety margin (you need to be on the boat with that left in the cylinder or suffer the wrath of the DiveMasters!


If this were me and I had to stick to the NDLs, I would more accurately work out the gas required for the ascent + stops + ascent. This would be a number X. To that I'll add 50bar/750psi and the result is my Minimum Gas. The dive would then be on the computer where the ascent is triggered at hitting EITHER Minimum Gas OR hitting the NDL on the computer.

You also need to factor in your buddies who will have worked out their figures in advance. With a reasonable 16 litre SAC you'll possibly find the others will be heading up before you -- and, as they're buddies, you must also leave with them.


As a complete aside and well beyond the scope of Basic Scuba... With Accelerated Decompression using oxygen rich gasses and the correct equipment (twinset + stage(s); rebreather) you could do an hour on the bottom with around 50 mins of decompression. We've a lot of dives like that around the UK. Lots more training required. And much more money!

Thanks for your reply, it’s been most helpful and has indeed prompted me to consider other variables and the risks these present.

I’ve been looking at the NOAA Nitrox tables (https://www.omao.noaa.gov/sites/default/files/documents/1.4 Nitrox Tables New ver 7 Oct 2016.pdf) which suggests a NDL of 15min at 130ft using 28% and I planned to keep well within this.

As you say, I won’t be spending 10-15min on the seabed, likely a couple of minutes before ascending to the deck and bridge; so I shouldn’t have too much trouble staying with NDLs so long as I closely monitor depth and time.

As suggested in another post, I’ve downloaded a dive planning app (Baltic lite - the free version) which provides a guide on multi-level dive run time, deco time, gas consumption and CNS which is very helpful.

Good advice on minimum gas, I’ll work that out, monitor gas and NDLs and let that dictate when to turn the dive - unless of course my buddy gets there first.

On twinsets, side mounts, stages and rebreathers - yes, I have it all to look forward to… better start saving now…

For UK diving - I am hoping to dive Scapa Flow at the end of this year or early next year once I’m trained to use twinsets and have done advanced Nitrox and accelerated decompression training, I will be in Portland during the summer and Lake District in spring with local (London) dives in between… lots of opportunities to get comfortable in my drysuit before progressing to more advanced diving.

Thanks again…

 

[HenryUK]

The mobile version of Subsurface doesn't have as much functionality as the desktop, including the lack of a planner. There are, of course, other mobile apps with planning capabilities. That might be another thread to find one that meets your needs (includes minimum gas and variable ascent rates).

FWIW, I would not use an 11 liter cylinder for an NDL/OW bailout from 40 m. I would use 10 m/min to the safety stop and then 3 m/min from there. Assuming 3 minutes on the bottom and your emergency SAC of 25 l/min, that takes 900 liters. (I doubt you'd maintain that rate throughout the entire ascent, but let's stay with the 900 liters.) An AL40 (5.5 l) has 1150 liters (27% more than you need). Your call, obviously, but you should try carrying whatever you think you need first.

A pony should be unobtrusive so you won't be tempted to leave it behind.

Thanks, I’ve downloaded the Baltic dive planning app which is very helpful for calculating gas consumption for multi-level dives and variable ascent rates as well as providing additional information on deco time, CNS, etc.

Yes, I feel 11L bailout could be excessive and think 7L would be sufficient for a safe ascent.

Many thanks…

 

[HenryUK]

I dive with a 13 cf pony bottle using a shark mount to attach to my primary tank and have no trim issues.
I have trim pockets on my top cam strap and put 2 lbs in the one on the side of pony and 4 lbs on the other side.
That being said, I might want to get a larger pony. 13 CF is not enough to comfortably ascend and do a safety stop. In an air out situation, there's a possibility I'd have to skip the safety stop.

Have you considered sidemount diving? You also get redundancy.

Thanks for your reply, yes with time - but most likely when progressing into cave diving - I think I’ll go for twinsets first…

Thanks…

 

[NYCrecdiver]

Thanks for your reply, yes with time - but most likely when progressing into cave diving - I think I’ll go for twinsets first…

Thanks…

Curious since I'm thinking to go SM with time : why would you first go twin and then sidemount? Why not skip twin altogether then?

 

[HenryUK]

Curious since I'm thinking to go SM with time : why would you first go twin and then sidemount? Why not skip twin altogether then?

A number of reasons, first would be what configuration I am familiar with (which is back mount, albeit single tank) so adding another back mounted tank would be less of a change to moving onto side mount, this extends to regulator use as well, I’d rather breath off one regulator and not have to switch between two unless it’s an emergency… second, side mount is often a preferable configuration for confined environments (wreck penetration and some caves) so when I decide to do cave training or advanced wreck penetration, I’d consider side mount for better access, best tools for the job I suppose… finally, I’m interested in taking the GUE Fundies course which can be done on single or double back mount, but I don’t believe you can do it in side mount configuration…

I can imagine it would be a very comfortable and stable way to dive however, it just doesn’t fit with the diving or training I am doing and planning at the moment.

 

[NYCrecdiver]

A number of reasons, first would be what configuration I am familiar with (which is back mount, albeit single tank) so adding another back mounted tank would be less of a change to moving onto side mount, this extends to regulator use as well, I’d rather breath off one regulator and not have to switch between two unless it’s an emergency… second, side mount is often a preferable configuration for confined environments (wreck penetration and some caves) so when I decide to do cave training or advanced wreck penetration, I’d consider side mount for better access, best tools for the job I suppose… finally, I’m interested in taking the GUE Fundies course which can be done on single or double back mount, but I don’t believe you can do it in side mount configuration…

I can imagine it would be a very comfortable and stable way to dive however, it just doesn’t fit with the diving or training I am doing and planning at the moment.

I see. You obviously been thinking about it already. Valid reasons. For me, I think I'd still plan SM straight away since eventually I'd like to do a cave diving trip/training. I'd rather pay once to transition into new gear. I've been spending too much already! Haha.

 

[LFMarm]

Maybe an opportunity to learn sidemount. There are great instructors in Gozo. It’s the most travel friendly setup to have more gas and redundancy as you can find anywhere AL80s

 

[R.Chisholm]

I am planning a dive trip in a few months and wondering what I should do, bear with me.

I plan on diving to a maximum of 40m using 28% Nitrox with NO deco (using PPO2 1.4).

I have dived to 40m on a few occasions using 12L steel tanks (both air and Nitrox) filled to 220bar and typically get out of the water with 70-90bar (between 9min and 15min bottom time depending on gas mix & average depth).

At first (when I knew no better) I relied on being close to the DM in case of an out air situation, but I know that I regularly surface with more air than the DM and think I should be taking extra precautions.

Getting to my question, bearing in mind I am not planning to go into deco, would you suggest I am better off:

1. Diving twinsets for peace of mind (not because of air consumption but the possibility of failure) and redundancy
2. Diving 12L singles (or 15L but consumption isn’t my concern) and carrying a stage/pony cylinder

As I have mentioned, I am not planning for deco, but I want to avoid a situation where kit failure (free flow or other issue) could lead to an out of air situation without any redundant air source available.

The pro’s and con’s I can see to each of these are:

Twinset pros:

1. Ample air supply
2. Redundancy in the event of failure

Twinset cons:

1. Cost to hire and fill tanks
2. Cost of training to learn shut downs etc.

Carrying stage/Pony cylinder:

Pros:

1. Limited additional cost (as I don’t plan on using it I only need to rent the tank and fill it once/*twice)
2. Limited additional training, ej. shut downs etc. (*I’d do some shallow dives to get used to the setup and switching regulators before diving to 40m)

Con’s:

1. Buoyancy and trim issues

I think twinsets would be the better option; but considering the additional cost, the fact I am comfortable with my air consumption and that I am NOT planning on going into deco, perhaps taking a stage/pony cylinder could be the better option.

I should also mention that I have recently started diving dry, which in itself is taking some getting used to (although I hope to get at least 20 dives in my drysuit before I go away, so hoping I’ll be used to it by then) and wonder if carrying a stage/pony cylinder could throw off my buoyancy and trim just as I’ve started getting used to my drysuit.

Any and all thoughts and suggestions welcome… I have even considered an advanced Nitrox/deco course - but as I am not planning a deco dive I think, at this stage, that would be unnecessary (if I was planning a deco dive, after having had proper training, I would only do so using twinsets and stages as necessary).

Looking forward to hearing your thoughts.

Hi,

Frankly if you have limited or no previous experience diving a twin set, I wouldn’t start on a trip unless you want to focus on learning to dive a twin set as part of some kind of “intro to tec” or similar course as taught by a good “tec” instructor.

If that is the case, I wouldn’t take the twin set to 40m either until you have 20 dives or so under your belt in this configuration.

The reasons for this are as follows:

A twin set will only really give you redundancy if you can quickly and efficiently operate the valves for a shut down. Most people require several practice dives to master this skill.

A twin set diving rig in a traditional OC DIR configuration (& yes – I know that BSAC divers can dive their twin sets with a Buddy Commander) will also require specific adjustments to the diver in terms of position of the weights, position of the d-rings, tank bands on the twin set and slack in the harness. Configured incorrectly the twin set will be uncomfortable and difficult to dive in trim.

Now if you had a lot of experience diving a twin set them you would be able to tell within one dive exactly what fine tuning was required – without this experience however it will be hard to pin-point where exactly adjustments need to be made.

Further skills you should aim to master in this configuration in the interest of yourself and your buddy include primary donate (& possibly – full deployment of the long hose) proper venting with the rear dump (or potentially going way out of trim when using the corrugated hose which is often short on a DIR rig to prevent trapping by a stage tank)

The whole situation will be further complicated by learning to dive a twin set at the same time as diving with a dry suit.

PADI (for example…) teaches that when diving a dry suit all buoyancy should be controlled using only the suit with the wing reserved for emergencies i.e. loss of buoyancy due to flooding of the suit. This isn’t practical with a 2x12l twin set since you will be ca 6.2 kg negative at the beginning of the dive which may exceed the lift capacity of the dry suit.

In view of the above you will need to be able to efficiently add and dump gas from both the wing and the dry suit throughout the dive while maintain correct buoyancy – again a skill that requires practice.

For similar reasons the PADI (for example…) method of determining correct weighting with a full tank of adding enough weight to sink on exhale and then adding the weight of the air in the full tank won’t work with a twin set either given that you are likely to be negative without weights with a large twin set at the beginning of the dive. If you can’t breathe the twin set down to empty without losing buoyancy you don’t have much redundancy either.

If you are diving in the Mediterranean a free flow short of a burst hose or torn 2nd stage diaphragm is unlikely. If you still feel you need a redundant air supply, I’d ask for a 7l stage tank or so rigged as a bottom or deco stage where you can operate the valve and see the stage SPG (i.e. not strapped to the back-gas tank(s).

regards,

Rory

 

[MarkA]

A twin set diving rig in a traditional OC DIR configuration (& yes – I know that BSAC divers can dive their twin sets with a Buddy Commander) will also require specific adjustments to the diver in terms of position of the weights, position of the d-rings, tank bands on the twin set and slack in the harness. Configured incorrectly the twin set will be uncomfortable and difficult to dive in trim.

You have met me?

This is what this wonderful setup looks like





A Cyklon coming over the right shoulder and also feeding the drysuit. A Cyklon coming over the left shoulder and feeding the Buddy Commando.

The twin 10l cylinders feel more balanced than a single 15l cylinder making it more pleasant to dive.

 

[inquis]

PADI (for example…) teaches that when diving a dry suit all buoyancy should be controlled using only the suit with the wing reserved for emergencies

Lots of good info for the OP regarding doubles. However, a correction to the quoted portion... PADI teaches BOTH approaches for single-tank (see the AOW materials) and then points out (as you have) that you must use the BC in their technical classes (see the TecDeep materials).