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

[snoozer]

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.

 

[snoozer]

Why not learn side mounted twinsets? Easy to learn and easy on our aging physiques. This also translates to other scuba and CCR modes if the mood strikes you. Enjoy!

 

[tursiops]

Another vote for a side-slung stage cylinder (aka Pony in the US**)

No, a pony is an emergency cylinder -- not part of the gas plan -- whereas a stage cylinder is part of the planned gas usage.

 

[Marie13]

I understand you’ll manage your profiles to avoid a decompression obligation.

However, if you have the liberty to extend your dive time, I’d choose twinset.

I have efficient consumption but remember always being a little frustrated at not being able to dive to my full ABT because of limited gas or because the charter started banging on the ladder at 60 minutes.

If others are waiting on you aboard the boat at the end of their single tank dive, then the twinset is probably overkill.

If not, then I’d dive a twinset.

Also, I encourage you to take that advanced Nitrox and decompression course. Not for depth so much but for extended exploration. You’ll graduate from pony bottle math and expand your exploratory profiles without even having to take a deco bottle.

He has no experience with twins. One should probably have some experience with them before diving them on a dive trip, hmmm?

 

[tursiops]

I have efficient consumption but remember always being a little frustrated at not being able to dive to my full ABT because .... the charter started banging on the ladder at 60 minutes.

How would a twinset have helped? There are three big things limiting a dive: NDL, gas, charter rules; whichever comes first causes the dive to end.

 

[Wibble]

Thanks for your reply, yes it looks like side slung cylinder is winning this one… I was thinking 7L filled to 210bar as follow:

7 x 210 =1470 litres for an ascent from 40m as follows:

2 or 3 min of faff time and then 40m to 20m @ 9m per minute 2.2min (3min) and then 3m per minute to the 5m (5min) with 5 min at 5m (3+3+5+5=16min) and a couple of minutes to surface… so planning for 20 min of redundancy gas.

My SAC rate is 16L per min, in an emergency this could increase to 25L so:

25L x 20min = 500L meaning I could surface with one other and have 470L to spare

Let me know if I’m missing anything…

Yes, 3L does seem quite pointless on a 40m dive, but (surprisingly) most UK diving is quite shallow - bad vis, strong currents and changeable weather - so if you’re not sharing (PONY ) I suppose it would be enough to get you back from 10-15m…

Thanks a lot for your reply…

@HenryUK

Sorry to 'disagree' with your above post. I don't think it's conservative enough.

When I do "back of a cigarette packet" calculations, I work out how deep I am for each minute in ATA (or Bar as the rest of the world knows it); sum up those minutes and multiply by your SAC. Always round high and use simple numbers (no decimals)

So for your 40m/130ft dive (which is 5ATA or 5Bar ) you'll have:

• 5ATA/Bar = 1 minute at 40m (faff1)
• 5 = 1 minute at 40m (faff2)
• 5 = 1 minute ascent from 40m to 30m -- use the deepest pressure
• 4 = 1 min ascent from 30m to 20m
• 3 = 1 min ascent from 20m to 10m
• 2 = 1 min ascent from 10m to 5m
• 2 = 1 min of 3 at 5m safety stop
• 2 = 1 min of 3 at 5m safety stop
• 2 = 1 min of 3 at 5m safety stop
• 2 = 1 min ascent from 5m to surface

Total = 5+5+5+4+3+2+2+2+2+2 = 32 ATA-mins
Multiply this by your SAC. This is your worst case SAC (panicked, stressed, worried, scared, generally not having a good time). In your case normal will be 16; in this bailout case you'd round it to 30 (or more for more conservatism)

So worst-case total gas in litres will be 32 ATA-mins x 30 = 960 litres.

You should use less, probably a lot less.

(Compare this rate with a normal ascent; there's no faff time so uses 22ATA-mins and use your own 16 litres/min SAC with will use 22 x 16 = 352 litres)

The general principle of bailout is you don't want to make life more difficult by adding the stress of very limited gas supplies.


(If anyone disagrees with this post -- especially as it's in Basic Scuba -- please let me know either here or by PM and I'll modify or remove it)

Edit: changed the units to "ATA per minute", ATA-min. This is the same as Bar-min

 

[HenryUK]

Slinging a 30 or 40 off your PB/W should be easy at a tech place. Sidemounting it will make it more compact, like a normal sling but tucked into the shoulder a bit with a bungee loop coming off the top side of the plate/back.

For future tech, small doubles are nice as an intro. Also you can dive any double as isolated doubles if you are not up on valve drills yet. I'd wait till after this trip.

Thanks for your reply and good advice, that’s exactly what I think I’ll do…

 

[HenryUK]

@HenryUK

Sorry to 'disagree' with your above post. I don't think it's conservative enough.

When I do "back of a cigarette packet" calculations, I work out how deep I am for each minute in ATA (or Bar as the rest of the world knows it); sum up those minutes and multiply by your SAC. Always round high and use simple numbers (no decimals)

So for your 40m/130ft dive (which is 5Bar or 5ATA ) you'll have:

• 5ATA/Bar = 1 minute at 40m (faff1)
• 5 = 1 minute at 40m (faff2)
• 5 = 1 minute ascent from 40m to 30m -- use the deepest pressure
• 4 = 1 min ascent from 30m to 20m
• 3 = 1 min ascent from 20m to 10m
• 2 = 1 min ascent from 10m to 5m
• 2 = 1 min of 3 at 5m safety stop
• 2 = 1 min of 3 at 5m safety stop
• 2 = 1 min of 3 at 5m safety stop
• 2 = 1 min ascent from 5m to surface

Total = 5+5+5+4+3+2+2+2+2+2 = 32 bar/mins
Multiplied by your SAC. This is your worst case SAC (panicked, stressed, worried, scared, generally not having a good time). In your case normal will be 16; in this case round it to 30 (or more for more conservatism)

So worst-case total gas in litres will be 32 bar/mins x 30 = 960 litres.

You should use less, probably a lot less.

(Compare this rate with a normal ascent; no faff time so uses 22bar/mins and use your own 16 litres/min SAC with will use 22 x 16 = 352 litres)

My general principle of bailout is you don't want to make life more difficult by adding the stress of very limited gas supplies.


(If anyone disagrees with this post -- especially as it's in Basic Scuba -- please let me know either here or by PM and I'll modify or remove it)

Thanks Wibble, very much appreciated and absolutely no need to apologise (I’m not look for agreement, only advice)… this is a very good point, of course more gas will be used at depth.

I have switched from ‘fag box’ calculations to a quick spreadsheet and have come out with the following (which is according to my preferred ascent rate):

40m (5ATA) 1min - faff 1
40m (5ATA) 1min - faff 2
40m to 30m (5ATA) 1min - 10m per min ascent (ideally 9m)
30m to 20m (4ATA) 1min - 10m per min ascent (ideally 9m)
20m to 17m (3ATA) 1min - 3m per min ascent
17m to 14m (3ATA) 1min - 3m per min ascent
14m to 11m (3ATA) 1min - 3m per min ascent
11m to 8m (3ATA) 1min - 3m per min ascent
8m to 5m (2ATA) 1min - 3m per min ascent
5m (2*5=10ATA) 5min - safety stop
5m to surface (2ATA) 1min

Total ATA: 45
SAC rate: 30
Total consumption: 1350 litres

If a 7L pony at 210bar carries 1470 litres I’d have 120 litres remaining, or 17bar - not a lot.

I appreciate this VERY conservative, but wonder if I should consider an 11L bailout as being conservative never killed anyone, running out of gas has…

I have based my preferred ascent rate on half my maximum depth at 9m per min (10m in this case for ease of calculation) and 3m per minute to the safety stop which I have increased to 5 minutes - again its conservative, but it doesn’t hurt to ascend slowly if you can.

Then the question becomes one of diving with an 11L bailout vs using twinsets.… something to seriously consider for later in the year I think.

For this dive, I am thinking (and happy to be corrected) that I’d be best using 15L steel tanks filled to 220bar planned as follows:

9min @ 40m max (max time & depth, likely between 5min and 7min)
2min @ 40m to 30m
2min @ 30m to 20m
2min @ 20m to 15m
15min @ 15m to 10m (a nice reef to explore on the way back)
10min @ 10m to 5m (same reef)
5min @ 5m (extended safety stop)

Dive time 45min
15L tank at 220bar = 3300 litres
Gas consumption = 2140 litres / 143bar
Gas remaining = 1160 litres / 77bar

One question on this, how do I workout my average depth? I am using an average of 25m for these calculations but that’s taking information from a similar dive at a similar depth and adding 5min - is there a more accurate way of calculating this?

Finally, and sorry for the incredibly long response, where would this best be post - I thought as a non-deco dive this should be post in the ‘basic scuba’ section.

Thanks, Henry

 

[HenryUK]

I understand you’ll manage your profiles to avoid a decompression obligation.

However, if you have the liberty to extend your dive time, I’d choose twinset.

I have efficient consumption but remember always being a little frustrated at not being able to dive to my full ABT because of limited gas or because the charter started banging on the ladder at 60 minutes.

If others are waiting on you aboard the boat at the end of their single tank dive, then the twinset is probably overkill.

If not, then I’d dive a twinset.

Also, I encourage you to take that advanced Nitrox and decompression course. Not for depth so much but for extended exploration. You’ll graduate from pony bottle math and expand your exploratory profiles without even having to take a deco bottle.

Thanks for your reply, yes - I do plan on learning twinset by the end of the year, but not before summer when this trip is planned… I think I’ll use a 15L / 220bar back tank and carry a 7L or 11L bailout tank just in case.

Yes, I’m certainly considering advanced Nitrox and decomposition courses - just a question of when; hopefully this year, but if not then next year I’d expect (once I have done my twinset course and can make better use of the knowledge - for now I’m staying out if deco)…

Thanks for your reply,

 

[HenryUK]

My “back of a cigarette packet” (international version?) calcs came to 990 litre so the same as @Wibble. I think you have missed the pressure (depth) in your calcs.
Example. Faff time of 3 mins at 40m at 25l/min = 3 (mins) x 25 (SAC) x 5 (bar) = 375 litres.

I prefer a twinset to a pony.

P.S. There is a lot of very pleasant UK diving that is not “quite shallow”.

Yes, some great dives in the UK - I’m looking forward to seeing them... ill leave the deeper stuff until I’m on twins with more knowledge of advanced gas mixes and decompression planning… but lots to see shallower too…

Yes, I didn’t allow for depth (stupid me) in my bailout gas calculation; I’ve posted a lengthly reply to Wibble and welcome additional feedback.

Many thanks for your reply