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

[tursiops]

Pony

Con’s:

1. Buoyancy and trim issues

Nope. You'll hardly notice a 2-4 litre AL pony, and even a 5-6 litre you will get used to very quickly.
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[HenryUK]

Pony


Nope. You'll hardly notice a 2-4 litre AL pony, and even a 5-6 litreyou will get used to very quickly.
.

Thanks, that was my main concern about this option…

 

[tursiops]

Thanks, that was my main concern about this option…

My comments are for a slung pony. If it is mounted to your back cylinder it can be aggravating.

 

[Wibble]

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

Side slung is: easy to use, manipulate valves; check pressures; tidy up hoses; hand off to some other diver or hand up to someone on a RIB.

This would need to contain enough AIR to: faff around on the bottom (40m/130ft) for a couple of minutes; do the ascent from 40m/130ft to 5m/15ft; do a three minute safety stop at 5m; ascend to the surface; and still have some minimum reserve gas left so you could do another three minutes at 5m (contingency). That multiplied by your ELEVATED SAC rate.

My "back of a cigarette packet" calculation is 32 x SAC, circa 960 litres for a relaxed emergency ascent. You must do your own gas calculations — assume these are wrong!

The cylinder would probably be an aluminium 5.5 litre/40cf (containing 5.5x200 = 1100 litres) or an ali7 (containing 7x200 = 1400 litres). Ali80s are heavy but do contain a lot of gas (11x200 = 2200 litres)

(My preference would be the ali7 — nobody died from too much gas!)


** In the UK a Pony normally refers to a steel 3 litre cylinder strapped to the steel 12 or 15 litre cylinder. Was popular in certain circles, now more common to use a twinset. The nickname is PONY — Piss Off Not Yours. 3 litres at 220 bar is 660 litres of gas. Not enough for a stress free ascent with stops (see gas requirements above). There’s also a ton of other challenges for a backmounted PONY, e.g. checking pressure, stowing hoses, valve twiddling. They need regular practice.

Another post on Ponies

 

[inquis]

Pony. It will hardly impact trim, and that's easily corrected by shifting lead to your right side if it bothers you. (The pony is normally slung on the left.) A full AL40+reg is negatively buoyant by 1 kg.

Doubles are quite different than a single tank, both procedures and weight distribution (trim). Like the drysuit, you'll want experience with them first.

 

[Searcaigh]

Another suggestion to use a pony, and if not used you don't need to refill it after every dive either.

 

[HenryUK]

My comments are for a slung pony. If it is mounted to your back cylinder it can be aggravating.

Yes, that’s what I meant too… left shoulder to left hip as described here (Decompression Stage Cylinder - Using one Properly - Wreck and Cave)

Thanks for your reply

 

[HenryUK]

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

Side slung is: easy to use, manipulate valves; check pressures; tidy up hoses; hand off to some other diver or hand up to someone on a RIB.

This would need to contain enough AIR to: faff around on the bottom (40m/130ft) for a couple of minutes; do the ascent from 40m/130ft to 5m/15ft; do a three minute safety stop at 5m; ascend to the surface; and still have some minimum reserve gas left so you could do another three minutes at 5m (contingency). That multiplied by your ELEVATED SAC rate.

My "back of a cigarette packet" calculation is 32 x SAC, circa 960 litres for a relaxed emergency ascent. You must do your own gas calculations — assume these are wrong!

The cylinder would probably be an aluminium 5.5 litre/40cf (containing 5.5x200 = 1100 litres) or an ali7 (containing 7x200 = 1400 litres). Ali80s are heavy but do contain a lot of gas (11x200 = 2200 litres)

(My preference would be the ali7 — nobody died from too much gas!)


** In the UK a Pony normally refers to a steel 3 litre cylinder strapped to the steel 12 or 15 litre cylinder. Was popular in certain circles, now more common to use a twinset. The nickname is PONY — Piss Off Not Yours. 3 litres at 220 bar is 660 litres of gas. Not enough for a stress free ascent with stops. There’s also a ton of other challenges for a backmounted PONY, e.g. checking pressure, stowing hoses, valve twiddling. They need regular practice.

Another post on Ponies

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…

 

[NothingClever]

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.

 

[MarkA]

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…

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”.